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Watts, Amps and Volts, Kilowatt Hours (kWh) and Electrical Appliances - Basic Electricity Explained

Updated on October 19, 2017
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Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.

Watts, Amps and Volts and How to Understand Electricity


What Are Volts, Amps and Watts?

"Voltage is a measure of pressure in an electrical circuit, amps is a measure of the current flowing and watts is a measurement of power or the rate at which energy is used."

In this article you'll learn all about volts, watts, amps, ohms, current, power, resistance and kilowatt hours (kWh). The equations are really quite simple and you'll find some examples on how to apply them to home appliances.
Want to test yourself? See how you perform in Quiz A, B and C at the end of each section.

Power Consumption of Household Appliances

If you just want to know the power rating of, and how much it costs to run electrical appliances in the home, you can take a detour to this related hub which gives a comprehensive list What is the Cost of Running Electrical Appliances?

Current is a Flow of Electrons in a Conductor

All matter is made from basic building blocks called atoms. A simplistic model of an atom, known as the Rutherford–Bohr model or Bohr model or Bohr diagram has a central nucleus made up of particles called protons and neutrons. The nucleus is surrounded by orbitals containing electrons. In some materials such as metals, electrons are bound loosely to the nucleus so they can detach and move when a voltage is applied. These materials are known as conductors and can conduct electricity. The flow of electrons is called a current.

Electrons Surrounding the Nucleus of an Atom

Conceptual image of atom with protons and neutrons in the central nucleus and electrons in outer orbitals
Conceptual image of atom with protons and neutrons in the central nucleus and electrons in outer orbitals | Source

Electron Flow in a Conductor

Electrons with a negative charge flowing through a conductor
Electrons with a negative charge flowing through a conductor | Source

Definitions of Volts, Amps and Watts

Like any discipline, electrical engineering has jargon or specialized terminology. Voltage and current are like water pressure and water flow rate respectively, and reference is often made to pumps and water pipes as an analogy to explain electrical circuitry.


This is the pressure in a circuit and is measured in volts. Think of a pump in a water pipe. The greater the pressure and the force which the pump exerts, the greater will be the flow of water through the pipe. Similarly a voltage source in a circuit is like a pump and pushes electrons around the circuit. The higher the voltage applied to a circuit, the greater the current which will be forced through it.


This is the device connected to a voltage source. It could be a motor, bulb, heater, LED, or an electronic resistor.


An electric current is due to the movement of electrons through a conductor and load and is measured in amps. High current means lots of electrons flowing through the circuit. The water analogy is water flow rate in gallons per minute.


The resistance of the load is measured in ohms. Every electrical device or load has resistance. Resistance is like a restriction to the flow of electrons and electricity is dissipated as heat energy in a resistance. For a given voltage, the higher the resistance, the lower the current. Going back to the water analogy, when you stand on a hose, you increase the resistance and restrict the flow. The only way to restore the flow is by getting the pump to pump harder, and force water through the restriction, i.e. the pump needs to have a higher pressure. Alternatively if you take your foot off the hose, you increase the diameter and lower the resistance and more water can be forced through. In an electrical circuit, if the voltage is increased, more current is forced through the resistance. If the resistance is lowered, more current will flow even if the voltage doesn't change.
Even connecting wires in a circuit have resistance so when higher currents need to be carried by a cable, thicker gage cable must be used to avoid overheating.


This is the rate at which energy is consumed and is measured in watts. A kilowatt is 1000 watts, also abbreviated to kW.

kWh or kilowatt hours

This is a measure of energy consumption. KWh are sometimes called units and are what you pay for on your electricity bill. A 1 kilowatt (1000 watt) appliance uses a kilowatt hour of electricity in one hour. Similarly a 500 watt device uses a kilowatt hour of electricity in 2 hours.


For an AC supply, this is the number of times per second that the current changes direction, measured in cycles per second or hertz. Electricity is distributed to homes at 50 or 60 hertz.

What's the Difference Between Volts, Watts and Amps? - A Simple Circuit

In the photo below, an AA cell powers a torch bulb. Current first flows out the top of the battery, through the wire and bulb and then returns via the bottom wire.

We can represent this circuit in a simple manner using a schematic or circuit diagram. Looking at the schematic below, a voltage source V will force a current I around the circuit through the load (the bulb in this case) whose resistance is R.
In a real life circuit, the voltage source could be the 120 or 240 volts coming out of a socket outlet, a 12 volt car battery, or an AA cell and the resistance would be an appliance or component in an electronic circuit. The lines joining the source to the resistance would be the connecting wires inside an appliance or power flex, or tracks on a printed circuit board.

Note: Conventionally we think of current flowing out the positive terminal of a source such as a battery. However current is a flow of sub-atomic particles called electrons which are negatively charged, so current actually flows the other way, from the negative terminal of the battery

A Simple Circuit

An AA cell forces current through the wires and lights up a bulb
An AA cell forces current through the wires and lights up a bulb | Source

Schematic of a Simple Circuit

Current in a circuit
Current in a circuit | Source

What are Some Commonly Used Voltages ?

Voltage Source
AA or AAA cell
1.5 volts
Mains supply in the home
Nominally 120 or 240 volts
Car battery
12 volts
Truck battery
24 volts
Voltage input to transformer supplying home
16kV (kilo volts)
High voltage transmission lines
Up to 1.2 MV (Mega volts)

The Voltage Supply to a House

In general the voltage supply to your home is nominally 230 or 120 volts. Voltage in the USA is 120 volts, but two "hots" are supplied to homes so that a 240 volt supply is also available. The higher voltage is used for high powered appliances such as washers, driers, kitchen ranges (cookers) and air conditioning. 120 volts is used for lower power and portable devices. It is also safer because in the event of an electrical shock, less current flows through the body so there a lesser risk of electrocution.
In countries where 230 volts is standard, generators or step down isolating transformers are used to provide a 110 volt supply for power tools. This is normally mandatory on construction sites. Again the idea of the lower voltage is to lessen the danger of electrocution, if for example a power flex is inadvertently cut, or a tool gets wet.

Supply Voltages of the World

Utility voltage by country
Utility voltage by country | Source

Watts, Amps and Volts Equation

Relationship between watts, amps and volts
Relationship between watts, amps and volts | Source

How to Convert Between Volts, Amps and Watts

We will consider Ohm's law later, but first let's examine the quantities which are usually of interest when dealing with appliances, such as volts, amps and watts and how to convert between them. If you look at the casing of an appliance (see photo below) you can usually find a specification label or panel which indicates the voltage supply, frequency, wattage and possibly current. On some appliances e.g. TVs and washing machines, this panel may be mounted at the back of the device.

So here are three simple equations for converting between volts, watts and amps:

Watts = Volts x Amps

e.g. A 120 volt appliance takes 2 amps, what is the power?

Power in watts = 120 x 2 = 240 watts

Amps = Watts / Volts

e.g. A 240 volt appliance consumes 480 watts of power, How much current does it draw?

Current in amps = 480 / 240 = 2 amps

Volts = Watts / Amps

e.g. A 720 watt appliance draws 3 amps, What voltage is it running on?

Voltage in volts = 720 / 3 = 240 volts

So it's really that simple. Notice I have chosen values in the examples so that everything works out nicely. You only really need to remember the first equation and if you know basic algebra you can rearrange to give the other two equations. However as you can see, you always need to know two of the quantities before you can work out the third quantity. From looking at the Google Analytics statistics and the questions which land people on this webpage, I often see questions asked such as "how many watts are in 480 volts?", which obviously makes no sense!

For high powered appliances, power is often specified in kilowatts ( abbreviated to kw)

1 kilowatt = 1000 watts

Test Yourself! - Quiz A

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Information Labels/ Specifications on Electrical Appliances

Typical electrical appliance labels/panels
Typical electrical appliance labels/panels | Source

What is a Digital Multimeter?

A multimeter is an instrument which can measure voltage, current, resistance and possibly additional parameters. If you don't know how to use one, read How to Use a Digital Multimeter (DMM) to Measure Voltage, Current, and Resistance. Multimeters normally have a continuity range also, and this comes in useful for checking breaks in cables, fuses and loose connections.

Digital Multimeters From Amazon

These multimeters are useful for measuring voltage, current, fuses and continuity of wires and connections. If you want a quality meter, Fluke, a US instrument manufacturer, specifically recommend the 113 model for general purpose home or car maintenance.

Fluke recommend the 113 meter for general purpose home use
Fluke recommend the 113 meter for general purpose home use | Source

What is a kWh? - Calculating Power and Energy Use of Appliances

Power is the rate at which a device uses energy. So for instance an air conditioning unit, shower or powerful floodlight uses electrical energy much faster than a light bulb

Energy used = Power x Time

So to figure out the energy usage of an appliance, you multiply its power rating by the time period for which it is running. The standard unit of energy is the joule or calorie, but generally energy used in the home is measured in kWh, also known as "units". To work out the number of kwh, you divide the power in watts by 1000 to convert to kilowatt (kW) and then multiply by time in hours to give kWh.


kWh = Watts / 1000 x time in hours

Kilowatt hours, kWh or units are what you pay for on your bill. Your electricity meter counts and displays the number of units used by all the appliances and lighting in your home.

e.g. A 2500 watt drier runs for 3 hours a day, how many kWh does it consume and if electricity costs 12c per unit, what is the cost of running it?

kWh = watts/1000 x time = 2500 / 1000 x 3 = 7.5 kWh or units

Cost = 7.5 x 12c = 90 cents

Some appliances don't run continuously. Examples are devices controlled by a thermostat such as refrigerators, freezers, ovens in cookers and air conditioning systems. The time for which the appliance is powered on and consuming power is called the duty cycle and it is often quoted as a percentage. So for instance a fridge which stays on half of the time has a duty cycle of 50%.

This article gives a comprehensive list of domestic appliances and their energy use: What is the Cost of Running Electrical Appliances?

A kilowatt hour meter counts the number of units of energy you've used
A kilowatt hour meter counts the number of units of energy you've used | Source

How to Convert Horsepower to Watts

Horsepower is a measure of....you guessed it!..... power!

Just as an engine's mechanical output can be measured in horsepower, so can the mechanical output of an electric motor.

1 horsepower = 746 watts

E.g. A fractional horsepower motor in a washing machine is rated at 1/2 horsepower

So the power output of the motor = 746 watts x 0.5 = 373 watts

A motor is not 100 % efficient, in other words not all the electrical power input is converted into mechanical power at the output shaft, some being wasted as heat in the windings.

Test Yourself! - Quiz B

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Power consumption monitoring adapter
Power consumption monitoring adapter | Source

What is an Electricity Usage Monitor?

An electricity usage monitor or tracker tells you everything you want to know about your appliance behavior. The parameters are displayed on an LCD and include voltage, current, power consumption, kwh used, cost of running and run time of appliance. The latter is useful for troubleshooting fridges, freezers, air conditioners etc which are controlled by a thermostat and switch on and off. A failed thermostat or waterlogged insulation can cause an appliance to run constantly, so this problem can be identified.

You can read about these devices here: Tracking the Power Consumption of Your Appliances

What Does it Mean to Consume Electricity?

What happens when an appliance is powered from electricity? Scientists tell us that energy cannot be destroyed, it just changes from one form to another. This process happens all the time - on Earth and throughout the Universe. For instance a rock on the edge of a cliff has potential energy, because of its altitude above the ground. If it falls over the edge of the cliff, it starts to pick up velocity, i.e. gains kinetic energy (motion energy) while losing potential energy. When it hits the ground, this energy is dissipated as heat (think of the heat produced by an asteroid impact). Similarly when an appliance is plugged in, the electricity doesn't get wasted or "consumed", in the sense of being destroyed, it simply changes form. So in the case of a lamp, it ends up as light energy or as heat energy when a heater is used. Electrical energy can also be converted to sound in a loudspeaker or electromagnetic radiation (microwave oven or radio transmitter), all forms of energy. Electrical energy can also be converted to kinetic energy in an electric motor or to potential energy when an elevator is raised in a building.
Power is a measure of the rate at which energy is used. So for instance a 1000 watt heater or high powered hvac air conditioning system uses energy at a higher rate than a 60 watt light bulb.

Electricity Can be Converted to Other Forms of Energy

Energy Type
Incandescent light bulb, LED, Fluorescent lamp
Electric heater, Incandescent light bulb
Electromagnetic radiation
Radio transmitter, Microwave oven, Radar
Loudspeaker, Thunder
Motor spinning a shaft or driving a vehicle
Winch or lift raising a load, Electromagnet tensioning a spring
Air compressor

Ohms's Law and Electrical Resistance

In the circuit above, a voltage V pushes a current I around the circuit and through the load. As you may remember, this could be a device such as a bulb, electrical heater, motor, LED or other electrical appliance. The load resists the flow of current and the magnitude of its resistance R is measured in ohms. Electronic components called resistors have precise values of resistance so that they can be used to control the value of current flowing in an electronic circuit.


I = V / R


R = V / I

This is known as Ohm's law and basically says that the current is proportional to the voltage and inversely proportional to the resistance (As resistance increases, current decreases and vice versa) Remember the resistance measured in ohms is just a measure of how the load or appliance in the circuit "resists" the flow of current.

An example:

The resistance in a circuit is 100 ohms, a voltage of 120 volts is applied, what is the current?

Current = 120 / 100 = 1.2 amps

Electrical resistance and conductors

A conductor is a physical medium which carries an electric current. This could be a power cable, prongs on a plug, a liquid such as water, battery acid or ionized gas in a discharge lamp (e.g. fluorescent or sodium lamp).

In the case of a solid conductor such as copper wire, the electrical resistance is proportional to the length of the conductor and inversely proportional to its cross-sectional area. In effect this means that the longer a piece of wire, the higher its resistance. Similarly the greater the diameter of the wire, the lower its resistance. This has implications for conductors used in appliances and power transmission. For example, the gauge of wire used in an extension lead is important, if the wire is too thin, the resistance will be high and the cable can overheat. If a power cable is very long, its resistance may be too high if not properly rated, resulting in an unacceptable voltage drop at the end of the cable (because of the resistance).

For a conductor with cross sectional area A and length l, the resistance R can be calculated using the equation:

R = ρl / A

ρ (Greek letter "rho") is a constant known as the resistivity. The lower the resistivity of a material, the lower will be the resistance of the conductor.

Copper has the lowest resistivity of most common materials and this is why it is widely used in the manufacture of cables. Silver has a lower resistivity than copper, but it is much more expensive. Aluminium is generally used for overhead cables and although it has a higher resistivity than copper, it is lighter. Gold has a resistivity about 1.5 times that of copper, however it is unreactive and doesn't oxidize (tarnish). A tarnish coating on a conductor increases contact resistance, so this is why gold is often used as a coating on audio / video connectors. Gold is also used for the miniature connecting wires in integrated circuits.


An electrical insulator is a material which has a very high resistance because there are no free electrons to carry current. For all practical purposes an insulator can be considered to have infinite resistance. Because resistance is infinite (infinity is represented by the symbol ∞), then current through an insulator is:

Current = Voltage / resistance = voltage / ∞ = 0

Insulators are used to prevent current flow between two electrical points with differing voltage e.g. insulation on the individual cores of a power cable or glass/ceramic insulators on power lines, and also to prevent high voltage from causing electric shock. Typical insulators used for electrical purposes are various types of polymers (plastic), ceramic, glass, glass epoxy (used for PCBs) and Bakelite (an older style thermosetting plastic)


When certain materials are subjected to very low temperatures, their resistance falls to zero.

Since V = IR, if R is zero, then V becomes 0 even if I is non zero

The consequences of this are that a current can flow even if the voltage source is removed. Because resistance is zero, and no heat is dissipated, huge currents can be carried by thin cables. Superconductors are used for example in MRI machines to carry the high currents required by powerful magnets.

Resistivities of Various Materials

1.59×10−8 Ωm
1.68×10−8 Ωm
2.44×10−8 Ωm
9.71×10−8 Ωm
1.06×10−7 Ωm
Nichrome (used in heating elements)
1.10×10−6 Ωm
1.00×1011 to 1.00×1015 Ωm
Hard Rubber
1.00×1013 Ωm
Materials with increasing resistivity
A load could be an electronic resistor like this one, or an electrical appliance
A load could be an electronic resistor like this one, or an electrical appliance | Source
Detail of the insulator string (the vertical string of discs) on a 275,000 volt suspension tower near Thornbury, South Gloucestershire, England
Detail of the insulator string (the vertical string of discs) on a 275,000 volt suspension tower near Thornbury, South Gloucestershire, England | Source
PVC insulation on the cores of a power flex
PVC insulation on the cores of a power flex | Source
The insulating black shrouds on the pins of this plug prevent contact with the pins during insertion/removal
The insulating black shrouds on the pins of this plug prevent contact with the pins during insertion/removal | Source
Superconducting cables
Superconducting cables | Source

Alternative Way of Working Out Power

Remember watts = volts x amps? Another way to work out power is from the resistance in ohms:

So if I is the current in amps, V is the voltage, R is the resistance in ohms and P is the power in watts,


I = V / R from Ohm's law

But also P =VI

So substituting the expression I =V/R into P = VI gives:

P = VI = V(V/R) = V2/ R


P = VI =(IR)I = I2R

It's unlikely when dealing with appliances in the home to need to use the last two equations. However here is an example.

A 240 volt supply is connected to a load of 100 ohms. What is the power consumption of the load?

Power = V2/ R = (240)2 / 100 = 576 watts

Test Yourself! - Quiz C

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What is AC and DC?

The current produced by a power source can take one of two forms, AC or DC. The power source could be a battery, electrical generator, power transmitted along service cables to your home or the output of a signal generator, a device used in laboratories or by test personnel when testing or designing electronic systems.


This stands for direct current so the current provided by the source only flows one way. A DC source will have a nominal value voltage level and this voltage will fall as the source is loaded and outputs more current. This drop is due to inherent internal resistance within the source. The resistance is not due to an actual resistor, but can be modelled as such, and is composed of actual resistance of conductors, electronic components, chemicals etc.
Examples of DC sources are batteries, DC generators known as dynamos, solar cells and thermocouples.


This stands for "alternating current" and means that the current "alternates" or changes direction. So current flows one way, reaches a peak, falls to zero, changes direction, reaches a peak and then falls back to zero again before the whole cycle is repeated. The number of times this cycle happens per second is called the frequency. In the U.S. the frequency is 60 Hertz (Hz) or cycles per second. In other countries it is 50 Hz. The electricity supply in your home is AC.
The advantage of AC is the ease by which it can be transformed from one voltage level to another by a device known as a transformer.
AC sources include the electrical supply to your home, generators in power stations, transformers, DC to AC inverters (allowing you to power appliances from the cigarette lighter in your car), signal generators and variable frequency drives for controlling the speed of motors. The alternator in a vehicle generates electricity as AC before it is rectified and converted to DC. New generation brushless, cordless drills convert the DC voltage of the battery to AC for driving the motor.

Reducing Costs of Transmitting Electricity over the Grid

Because AC can so easily be transformed from one voltage to another, it is more advantageous for power transmission over the electricity grid. Generators in power stations output a relatively low voltage, typically 10,000 volts. Transformers can then step this up to a higher voltage, 200,000, 400,000 volts or higher for transmission through the country. A step up transformer, converts the input power to a higher voltage, lower current output. Now this decrease in current is the desired effect for two reasons. Firstly, voltage drop is reduced in the transmission lines because of the lower current flowing through the resistance of cables (since V = IR). Secondly, reducing current reduces power loss as current flows through the resistance of the distribution cables (remember power = I2R in the equations above?). Power is wasted as heat in transmission cables, which obviously isn't wanted. If current is halved, power loss becomes a quarter of what it was previously (because of the squared term in the equation for power), If current is made 10 times smaller, power loss is 1% of what it was, and so on.

AC waveform is a sine wave
AC waveform is a sine wave | Source
Transformer in an electrical sub-station. The function of a transformer is to either increase or decrease voltage
Transformer in an electrical sub-station. The function of a transformer is to either increase or decrease voltage | Source

3 Phase Voltage

Very long distance transmission lines may use DC to reduce losses, however power is normally distributed nationwide using a 3 phase system. Each phase is a sinusoidal AC voltage and each of the phases is separated by 120 degrees. So in the graph below, phase 1 is a sine wave, phase 2 lags by 120 degrees and phase 3 lags by 240 degrees (or leads by 120 degrees). Only 3 wires are needed to transmit power because it turns out that no current flows in the neutral (for a balanced load). The transformer supplying your home, has 3 phase lines as input and the output is a star source so it provides 3 phase lines plus neutral. In countries such as the UK, homes are fed by one of the phases plus a neutral. In the US, one of the phases is split to provide the two 'hot' legs of the supply.

Why is 3 Phase Used?

  • More power can be transmitted using just 1.5 times the number of wires
  • Motors powered by 3 phase are smaller than a similar single phase motor of the same power
  • Evening of output torque smooths operation and results in less vibration of motors powered by 3 phase
  • Neutral conductor can be reduced in size because of lower current flow
  • Neutral is unnecessary for transmitting power between substations and transformers

3 Phase voltages. Each phase is sinusoidal with a phase difference of 120 degrees
3 Phase voltages. Each phase is sinusoidal with a phase difference of 120 degrees | Source

Delta - Star Transformer

A Delta - star (also known as delta - wye or delta Y) transformer is often used for producing a 3 phase, or single phase and neutral supply to homes and industry. The incoming supply is typically 11kv and output phase voltage is 230 volts (in countries which use this voltage)

Delta-Star(Wye) transformer which can supply single or 3-phase supply
Delta-Star(Wye) transformer which can supply single or 3-phase supply | Source
3 phase power lines
3 phase power lines | Source

How to Measure Voltage, Current and Resistance

As explained above, a multimeter is an instrument for measuring voltage in volts, current in amps and resistance in ohms. Each function usually has several ranges to allow large and small values to be measured. A multimeter has two probe leads which are connected to the circuit being tested, the measurement is then displayed on an LCD display.

Check out this hub: How to use a multimeter

Magnetic field lines around a conductor
Magnetic field lines around a conductor | Source

What Are Other Effects When a Current Flows?

As mentioned above, when current flows through the resistance of a load, it gets hot. This is sometimes the desired effect, e.g. an electrical heater. However it is an unwanted effect in lamps, because the desired function of the device is to convert electricity to light, and not produce heat as a byproduct. Excessive current in power cables during an overload can potentially cause a fire if protective devices such as fuses or MCBs (Miniature Circuit Breakers) aren't included in line with the cable.
So what else happens when current flows through a conductor? One effect is that a magnetic field is produced. This phenomenon is used in a device called a solenoid or electromagnet which is basically like a spool or coil of wire through which a current flows. Electromagnets are used in the old style, non-electronic, door and phone bells, water inlet valves on washing machines, relays (a switch operated by an electromagnet), starter motors on vehicles and in salvage for lifting iron and steel.

Current flowing through a conductor also produces an electric field. An extreme example of this is the high intensity field produced under a high voltage power line which is sufficient to illuminate a fluorescent tube held in the hand.

The electric field under a high voltage power line is sufficient to produce an electric discharge in a fluorescent tube
The electric field under a high voltage power line is sufficient to produce an electric discharge in a fluorescent tube | Source

How Do Switches Work and What Are Sparks?

As you've discovered, if resistance is increased in a circuit, current decreases. If you just break the conductor in a circuit and create an air gap, the magnitude of the resistance for all practical purposes is infinite because air is a good insulator and no current will flow. I.e.

Current = Voltage / Resistance = Voltage / = 0

So this is how a switch works. Two contacts, usually made of brass in a domestic switch, are pressed together when the switch is on and closed. When the switch is turned off, the contacts rapidly separate and interrupt current.

What are Sparks?

Imagine two electrodes or points in a circuit separated by an air gap (e.g. the gap in an automotive spark plug). If voltage is high enough, the air between the two points becomes so stressed by the electric field that it becomes ionized, i.e. atoms have their electrons ripped off. These electrons are then able to traverse the gap, attracted by the positive electrode and in doing so, collide with other gas molecules and release more electrons. Eventually an avalanche of electrons occurs (all of this happening in a split second) and the result is called a spark or spark discharge A spark produces a flash of visible light, heat, UV radiation and sound and it's temperature can be about 5000 deg C, hotter than the surface of the sun. The voltage required to produce a spark is about 3000 volts per mm between rounded electrodes in air.
Sparks can be small, e.g. automotive spark plug or gas lighter, or much larger.

An example of a large spark is lightning. When clouds get charged up, voltage becomes so high that a spark jumps from cloud to cloud or cloud to ground. The sound we call thunder is caused by the explosive heating and expansion of air by the electrical discharge.

Sparks occur in an air gap when voltage exceeds the breakdown voltage of the gap. When two electrodes are separated, current tends to continue to flow and heating of the metal electrodes causes material to vaporise and also ionise the air. This results is a continuous spark discharge called an arc which is similar to a spark. If the electrodes are separated sufficiently, the arc won't be sustained and will stop abruptly. Arc welding makes use of an arc between two electrodes to melt metal. Switches must also be designed so that their contacts separate sufficiently apart and quickly enough so that arcs are rapidly quenched and reduce damage to the contacts. In substations, large air gaps or oil filled circuit breakers are necessary to quench the high current arcs which occur when high voltage is switched.

Arc Between Switch Contacts at a Substation

Summary of Equations For an Electric Circuit

V =
I =
R =
P =
I =
V =
P =
P =

© 2012 Eugene Brennan


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    • profile image

      yuvaneshwaran 11 days ago

      how to calculate capcitor value from the circuit

    • eugbug profile image

      Eugene Brennan 3 weeks ago from Ireland

      Hi Gijs,

      The meter seems to indicate line voltage (phase to phase). One of the line to line voltages is reduced presumably because of the extra loading on one of the phases.

      You're using single phase loads which I presume are 230 volt rated and connected between each phase and neutral? (or are they connected phase to phase?). If they are connected phase to neutral, and the meter indicates the current flowing from phase to neutral through each load, then the real power for a balanced load is just the sum of the powers for each load. I.e. :

      3 x (V/√3)I CosΘ = √3VICosΘ

      where V is the line voltage (approx 400 volts), I is the phase current and CosΘ is the power factor.

      230 volts corresponds to a line voltage of 400 volts, so 412 to 419 volts on your display might indicate that your phase voltage could be higher than 230 volts.

      (edit: Just saw the second image which shows a higher voltage)

      Anyway if the meter indicates phase to neutral current, real power would be (using the values in the second photo):

      ((239 x 46) + (239 x 20) + (244 x 13)) x 0.9 /1000 = 17kw approx.

      So you just sum the power for each load.

      Your equation B) might be good enough for rough calculations if you just want to estimate energy usage from the currents.

      Now that assumes the power factor is 0.9, but it could be different for the 3 loads?

      Maybe if you have someone qualified for the task, you could check current through one of the appliances with a clamp meter to see if it tallies with what's on the display? Probably easier said than done because it might be difficult to access a neutral core. The phase voltage also seems to be somewhat high, which isn't the best if you are running 230 volt appliances?

    • profile image

      Gijs 3 weeks ago

      Hi Eugene,

      Good to find this page. Perhaps you could assist with my below questions:

      We are having a 65kVA 3-phase generator and only single phase appliances are connected to the generator. The generator display (F.G. Wilson Power Wizard 1.1) gives me the following output:

      V 412 417 419

      A 48 25 19

      Now I want to know how much kW the output of the generator approximately is (assuming a power factor of 0.9) to estimate the kWh production.

      Would this be

      A) (48+25+19) * 0.9 * 415V = 34kW


      B) (48+25+19) * 0.9 * 230V = 19kW


      C) another calculation

      Any feedback would be much appreciated!

      Pictures of the generator display can be found here:


      Best, Gijs

    • profile image

      RichHI 4 weeks ago

      Thanks. It is a new appliance that comes from Japan. All their products like many Japanese manufacturers come with polarized 2 pin plugs. My problem is that nothing worth buying is available in UK as they are focused on Europe and Japanese. Korean, Chinese stuff is not main stream. The only Japanese version I can buy in UK is a restaurant version which is total overkill. Far too big and runs on its own 30 Amp circuit. I have tried to find a double isolated transformation but again they all seem aimed at commercial budgets. Does this mean my Windows computers are illegal in the UK? My Mac has a two pin connector on power supply with an extensions lead which has a grounded plug, is that illegal too? And my iphone, ipad, my other cell and my electric razor? I knew the EU would be involved. I thought US equipment grounded through neutral back to the sub station? I guess UK works on different standards as everything is wired with hot at 230 v not two taps of 120 v which are only used on the high current stuff. Long time since I did any electrical stuff, I remember we used 440 volt triple phase which had three taps. Maybe I wait for Britain to leave the EU and hope this is one of the bits that is dropped. Though I am not supplying anything as I am using it myself. Thanks for info, I think I will investigate double isolated transformers more.

    • eugbug profile image

      Eugene Brennan 4 weeks ago from Ireland

      Hi RicHI,

      The transformer should have adequate capacity, but is the appliance doubly insulated? If it isn't, it may be an older type class 0 appliance without a ground which may have been permissible in the US, but prohibited in the UK since 1989:


      Also if you are using an autotransformer, there is a connection between the primary and secondary, so the secondary is not isolated. If a fault occurs and the appliance isn't grounded, you can get a shock or potentially be electrocuted. A GFCI will hopefully detect the current flowing through the casing and your body to earth and rapidly shut-off power preventing this scenario. When an appliance is grounded, this acts as a secondary backup safety feature, causing a surge current to blow the fuse or trip the breaker.

      Ideally you should be using a more modern and safe, grounded appliance.

      I suggest you consult an electrician to get further info.

      (An option could be to use the appliance with a step down isolating transformer, the type used for powering tools on construction sites)

    • profile image

      RichHI 4 weeks ago

      I am specifying a step down transformer so I can run a US cooking device on UK power. It has 13 amp glass fuse on 230 v side which is grounded and transformer has 3 pin socket but cooker is 2 pin polarized. 120 volt side has a reset button.

      q Specifying transformer am I correct (assuming 110 v 1350 w and a recommendation to use 15 amp 110volt circuit) :110 volt 1350 watts is 12.3 amps the Volt/Amps is 1650vA or 1800 vA if you use 120 volt. So a Transformer Rated 3000 vA on a UK 13 amp 230 volt Circuit will work safely and give enough spare capacity for safety?

      q Can I use a GPFI on wall socket or is that unsafe on an autotransformer with an ungrounded appliance?


    • eugbug profile image

      Eugene Brennan 5 weeks ago from Ireland

      Hi Raghu,

      If the current is 'i', the power is 12i

      You would need to measure current or it may say it on the device.

    • profile image

      Raghu 5 weeks ago


      12v dc input energizer.

      What its power (watts)

    • eugbug profile image

      Eugene Brennan 5 weeks ago from Ireland

      Hi thy,

      It's not as simple as just adding up wattage of appliances to calculate total load, Factors have to be taken into account such as demand factor, starting wattage etc. A generator can source surge power from anything between 1.5 to 2.8 times the continuous VA rating, but voltage drop could be an issue for some motor equipment, which if it doesn't go from a start to a run phase can trip out.

      I don't have enough experience of this, so I'm not going to advise you. I suggest you contact a professional in this field who can help.

    • profile image

      thy keang 5 weeks ago

      How can I calculate generator power need ? exp: I consume all equipment in watt 77300 watt and devise 1000 equate 773 Kw so do I need to devise by 3 phase? if the generator is 3 phase?

      Thank you for your answer :)

    • eugbug profile image

      Eugene Brennan 2 months ago from Ireland

      Hi Zendee,

      Multiply voltage by current to get power~:

      So 200 x 20 = 2000 watt

      1horse power = 746 watts

      So 2000 w = 2000/746 = 2.68 hp

      This is the input electrical power, however output mechanical power or shaft output power will be somewhat less because a motor isn't 100% efficient in converting electrical energy to mechanical energy. The waste energy ends up as heat.

      Have a look at this:


    • profile image

      zendee Argallon 2 months ago

      hai i have a question . what is the rating of a motor in horsepower if the current is 20 Ampere and the voltage is 200 Voltage .. Pleasee .Answer this. Pls thankyou

    • eugbug profile image

      Eugene Brennan 2 months ago from Ireland

      You can do this yourself Fida.

      Find the model number online of the appliances, check the power rating and add up all the values.

      Alternatively if you have the appliances, check the details printed on the label on the appliances or embossed into the casing. The wattage should be indicated.

    • profile image

      fida khan 2 months ago

      Please calculate me electric watts.

      8 energy savers

      8 ceiling fans

      1 computer

      1 lcd tv

      1 Water pump

      2 pedestal fans

    • eugbug profile image

      Eugene Brennan 2 months ago from Ireland

      Hi Ken,

      The load would be designed to be fed on single, double or three phase supplies. Load is specified for a given voltage.

    • eugbug profile image

      Eugene Brennan 3 months ago from Ireland

      A charger would typically consume 5 to 10 watt while charging and much less when the phone is charged because only a trickle charge current would flow into the battery to maintain charge.

      Current is a flow of electrons which flow into the battery. Metal electrodes in the battery gain or lose electrons and the metal combines with an electrolyte in a chemical reaction.

      Tesla coils are a source of very high voltage, high frequency electricity sourcing low current. A phone needs low voltage and a higher current for charging, so a Tesla coil wouldn't be appropriate for the task.

    • eugbug profile image

      Eugene Brennan 3 months ago from Ireland

      The current would be from line to line with no current to neutral, so the kwh meters wouldn't register the energy used.

    • profile image

      Zapped 3 months ago

      So, if a three phase delta plus N + E incoming line feeds a 3 phase supply and three single phase supplies, to a street distribution system (so that an industry therein connected can obtain 400v 3phase supply, whilst three houses can each obtain a single one phase supply and a fourth house obtains two single (different) phases - what kwh "poaer" is measured on either of it's two kwh meters, if a load is directly connected (NOT ACROSS EITHER PHASE AND N but) across two of the line phases, with out a n or e connection.

      In other words obtaining a 400 v load source, using two single 230v line feeds - what power in kwh is measured and which single phase meter measures that power used, or is any power used measured at all?

      The reason I ask is very simple.

      Is it a possibility to obtain a 400 v supply / load power supply, from two different line voltages, in a two phase supplied building, when there IS no 400v kwh meter installed, just two single phase line/n meters connected.

      As the load across the two lines IS NOT directly (or indirectly) connected to either meter's neutral wire.

      Thus could either meter even see the 400 v load across the two incoming phases.?

    • Vinyasi profile image

      Vinyasi 4 months ago from southern California

      Thank you, Eugene, for your response. I cherish answers, dearly.

      The only way I can presume that a simulator can give diametrically opposite results from apparently two different reference frames might be on account of divergence cropping up during its calculations? Namely, two answers which -- as far as the simulator can tell -- are both valid at the same moment in time?

      This assumption, if true, doesn't shock me since divergence is common in many of my simulated experiments of replicating an electrical surge to try and better understand this little understood phenomenon.

      This sort of answer coming from a simulator also doesn't shock me from a realistic standpoint since complex/imaginary numbers can also produce multiple results during a lengthy course of calculations. The trick, I think, is to not interfere by throwing out one or the other result without considering the other option that: both results may in fact be true since two reference frames for a singular phenomenon may be created by that phenomenon creating two different references: one for energy and the other for time. Not to imply that time reversal is occurring, but that time and energy are not two different phenomena, but one and the same phenomenon looked at from two different perspectives. That's my take on this, at any rate.

      Thanks, again.

    • eugbug profile image

      Eugene Brennan 4 months ago from Ireland

      Well the amperage as you call it, or current, can't be out of phase with itself. 180 degrees phase shift means the current is flowing in the opposite direction, so maybe you're monitoring the current wrt 2 different reference points which would give you two traces the inverse of each other.

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      kamlesh 4 months ago

      Thank you very much.

    • eugbug profile image

      Eugene Brennan 4 months ago from Ireland

      Hi Kamlesh,

      Short-circuit current rating (SCCR) is the maximum short-

      circuit current a component or assembly can safely withstand

      when protected by a specific overcurrent protective device(s)

      or for a specified time.

      The topic is outside my range of knowledge, but take a look at these links.



    • profile image

      KAMLESH 4 months ago



    • eugbug profile image

      Eugene Brennan 5 months ago from Ireland

      Hi Rakesh,

      These are the basic equations:

      Watts = Amps x Volts

      Amps = Watts / Volts

      Volts = Watts / Amps

      Horsepower = Watts / 746

      If you have any more questions, just let me know!

    • profile image

      Rakesh 5 months ago

      Hi Eugene, I have a lot of electrical work to do on daily basis. But Im a non technical person. If you just tell me how watts, voltage, am pier and horse power related to each other, it will be a great help to me. I need just simple formulas and calculations. Hope u will reply

    • profile image

      kawser ahmed 5 months ago

      very nice

    • eugbug profile image

      Eugene Brennan 6 months ago from Ireland

      Hi Alok,

      It depends on what type of voltage regulator you use.

      There are two types of regulator, the linear regulator and switching regulator.

      A linear regulator is a semiconductor device, but effectively works as a controlled dropper resistor in series between the input supply and the regulator output. So it drops voltage from eg 12 volts to 5 volts. The regulator monitors its output voltage and if the load tries to take more current and op voltage tries to fall, the the resistance of the pass transistor is reduced so that it drops less voltage in order to maintain the output at a constant 5 volts. Similarly if the load takes less current, the resistance increases. A linear regulator is a classic negative feedback control system (like the governor on an engine, keeping speed constant as the load increases/decreases). So since the regulator is in series with the load, the current supply from the source is the same as that supplied to the load. However since voltage is dropped by the regulator, power is wasted as heat in the device. The higher the input voltage, the greater the wastage since P = VI, where V is the drop across the regulator. The lower the input voltage the better, and a small or large heatsink may be needed, depending on the ambient temperature and voltage drop. Basic regulators need about a 2 volt difference between input and output voltages to work, but low dropout regulators are available which can work with a smaller difference between IP and OP.

      A switching regulator on the other hand works differently. Unlike a linear regulator which can be very inefficient and waste power as heat, switching regulators can be up to 95% efficient. In buck mode (reducing voltage), they work by chopping the input voltage to the regulator into a pulsed waveform and applying this to a capacitor/inductor which effectively works as a tank, smoothing the chopped waveform (analogous to the way an engine flywheel smooths the pulsed intermittent power from the cylinders). The duty cycle (how long the pulse is on) of the switching waveform is varied depending on the demand of the load in order to keep the op voltage constant. As the load current demand increases, the duty cycle of the pulse waveform increases. To answer your question, if the regulator is near 100%efficient, power in = power out. But op voltage is less, so current in must be less. Also much less heat sinking will be needed because there is less heat produced.

      Google switching regulators online, here's a link to get you started:


      The LT1076 switching regulator from Linear Technologies/Analog Devices is probably just what you need. Check it out here:


    • profile image

      Alok Pradhan 6 months ago

      Thanks for all this information you have put here it was really easy to understand. My question is... Suppose i want to build a solar charger for my phone by using a pre-built voltage regulator which outputs 5v and 2A... And takes 5-12V input... So since my charge gives me 5x2 =10 watts of power... When building the charger do I need to supply the required current as well? Or I could supply higher voltage and the deficient current will be compensated by the high voltage as the overall power is still the same.. In other words can we compensate for voltage with current or current with voltage? I don't know if that made sense but i hope you get it hahaha

    • eugbug profile image

      Eugene Brennan 6 months ago from Ireland

      In response to an email query:

      Voltage may be nominally 220 volts but if lots of people turn on high powered appliances (e.g. power showers, electric heaters, cookers (ranges)) simultaneously, the voltage output of the transformer supplying your home can drop substantially if it is underrated. If you live in an urban area, usually the transformer is quite "large" and has a high capacity but in a rural area, the transformer can be a small one on a pole and only capable of supplying a few homes. Sometimes the utility company will upgrade the transformer if more homes are connected and voltage drops excessively. Utility companies are supposed to maintain their supply voltage within +- 5 to 10% of the nominal voltage.

      When voltage decreases, current decreases also because current = voltage / resistance. So because P = VI , P is now less. So you get less power and pay less for the resultant electrical energy. Your electricity meter measures both voltage and current and effectively "multiplies" the two parameters to get a figure for power. So you're not getting less value when the voltage is less. Notice also that both V and I are lower in the equation P = VI. So for example if V drops by 10%, this causes a 10% drop in current and the new power is 0.9 x 0.9 = 0.81 or 81 % of what it was before.

    • profile image

      baesex 8 months ago

      Hey mate, just touching base, I used power meters to measure all my equipment, and from pumping it as loud as I could manage, I got three of the speakers to pull a peak of 800w .. so am guessing that 5A measurement is indeed for the 110V.. or that they are capable of earth shattering sound pressure movement if one were to put say a synthesiser on max volume and hold all the keys...

    • eugbug profile image

      Eugene Brennan 8 months ago from Ireland

      It depends on your voltage supply Pawan. If the equipment is using the full 7kW, then divide 7000 by the voltage to get the current. Also don't think of "amp used in one hour". If you think of the water analogy, current is like gallons per minute or litres per minute. You wouldn't think "how many gallons per minute do I use in one hour?", just "how many gallons do I use?" You can only think of energy used in an hour, that is, kilowatt-hours (kWh).

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      pawan kumat 8 months ago

      I have 7 kwt equipment so what amp used in one hour

    • profile image

      baesex 9 months ago

      Mr. Brennan you are an absolute legend and your efforts and knowledge is hugely appreciated.


    • eugbug profile image

      Eugene Brennan 9 months ago from Ireland

      I found this datasheet,


      Not sure whether it's for your specific model but on the panel diagram it shows the power input is 600W 5A. However the speaker is 3-Way 1500 W (3 x 500W, I wonder is this for the woofer, squaker and tweeter?). So it sounds as if 600 W is probably the average or RMS power and 1500 watt is peak. Anyway I forwarded your question to JBL Professional's support email address and I'll let you know if they respond with any more info!

    • profile image

      baesex 9 months ago

      Thank you! All the spec sheets I've been through haven't been very helpful, the speakers are indeed rated at 1500W output, so the 5A @ 230V does seem like a correct draw? But the 120V/230V 5A figure does confuse me..

      edit: the speakers are JBL PRX 635 but I didn't want to cause you extra time and hassle

    • eugbug profile image

      Eugene Brennan 9 months ago from Ireland

      Hi Baesex,

      If you could find the manual/datasheet for the speakers it would be great or if you have the model number I can try and search for it. I don't know whether 5 A would be the max or peak draw, its likely the RMS value which is quoted for the power supply in the speakers. However if the power supply is fairly efficient (i.e. power doesn't end up as heat in the supply/power amplifier and becomes sound power), is 5A x 230 volt or 5A x 120 volt close to the power rating quoted for the speaker? That may give you the answer as to which voltage the 5A refers to. In any case, the speakers will take much less than 5A when they are not outputting full sound power.

    • profile image

      baesex 9 months ago

      Hi, thank you very much for this explanation! Well paced and easier to follow than other examples I've tried to digest.

      I have a query that someone may be able to help with-

      I'm trying to work out the max power draw for some powered speakers, the rear says "120V/230V 5A" -they are on 230V mode however AFAIK that doesn't mean they will draw twice the power as 120V mode.. I'm assuming they just wrote "5A" because that's the max, and easier than writing "5A FOR 120V, 2.5A FOR 230V" -does that sound right?

    • eugbug profile image

      Eugene Brennan 9 months ago from Ireland

      Yes, that's true.

      x /1000 converts watts to kilowatts, and x/1000 kilowatts uses x/1000 units or kwh per hour

    • profile image

      chandan 9 months ago

      X watt device take X/1000 unit per hour

    • eugbug profile image

      Eugene Brennan 9 months ago from Ireland

      Hi Rishi,

      As you know, power is the product of voltage and current. So although voltage may be negative and current negative at the same time, the product is positive. If you take one phase on the graph of phase voltage (line to neutral) versus time and multiply voltage by current for the phase, you get a positive result and this is the instantaneous power at that instant of time for that phase (assuming the load is real and power factor is 1.0). If you graphed the power versus time, the result would be in the form of a sin² graph with all values positive. The total power for the 3 phases can then be obtained by adding together. For a delta load the result is a little more complicated..

    • profile image

      rishi 9 months ago

      In 3 phase system at any instant sum of voltage is zero know and current also is zero so power will be zero know... But it isn't true.so what is the write answer

    • eugbug profile image

      Eugene Brennan 9 months ago from Ireland

      Hi Tony.

      If you measure current I and voltage V and multiply them together, the result is the VA of the load. However current and voltage may not be in phase and actual real power may be less than this figure (as in your example). This is the case with loads which have inductive or capacitive components, e.g.motors or lighting. Capacitors are commonly added to electrical equipment to correct power factor or reduce it to near unity, i.e θ = 0 and cos (θ) = 1. If power factor isn't corrected, excessive current can flow in a load which not only doesn't contribute to power used, but results in higher current flow in distribution cables. Power companies don't like this because it puts a higher demand on their transformers.

      Real power measured in watts = VICos(θ), where is the phase angle between voltage and current

      Cos(θ) is known as the power factor of the load.

      Some power adaptors will actually display the power factor of the load for you.

    • profile image

      tony keo 9 months ago

      measuring P V I

      P=50w , I=0.69A , V=222v

      P'=VI=0.69 x 222=153w

      why P'greater P explain

      thank you i wait you

    • eugbug profile image

      Eugene Brennan 10 months ago from Ireland

      You can also use a power monitoring adapter to see the effects of voltage variation on current demand, power demand and run time of appliances.

      See: https://dengarden.com/misc/Tracking-the-Power-Cons...

    • eugbug profile image

      Eugene Brennan 10 months ago from Ireland

      Hi Shedrick,

      Induction motors as used in freezers and fridges take more current when voltage is reduced. The increase in current is roughly proportional to the reduction in voltage. So for your example, a 27% decrease in voltage from 220 to 160 volts would cause a 27% increase in current to maintain power output of the motor. This could cause overheating of the motor and shortening of its life if the current goes over the rated current.

      Starting torque, pull-up torque, and pull-out torque of induction motors, all change by a factor of the voltage squared. So if voltage drops by 27%, new voltage is 100% - 27% = 73% or 0.73 times what it was before. So all these torque parameters are 0.73 x 0.73 = 0.53 times what they were before. This could lead to the motor not starting at all.

      So to answer your questions, power input should be the same so cost of running should be the same.

      I don't know whether it would take longer for the freezer to freeze with lower voltages. Lower voltages cause a higher current in the motor windings. So although power input from the supply would be constant, more power would be dissipated in the resistance of the windings and less mechanical power would be available for running the compressor. So it seems this would lengthen the time needed to freeze. The best thing would be to experiment. So buy a cheap freezer thermometer, put it into a fixed quantity of water and time how long it takes for temperature to drop, both when voltage is normal, and when it is low.

      Hope this helps!

      Have a look at this link which discusses how voltage variations affect induction motors:


    • profile image

      shedrick thomas 10 months ago

      thanks for the education.

      i am from Liberia West Africa, the cost of unit of electricity is 0.55c/kwh

      i have notice over the time that my unit run faster with a low voltage so i have come to suspect some things which i want to be correct about.

      my mini freezer voltage is 220-240v and in recent time the voltage from the transformer to the house has drop to 160v Yet the freezer still come on. but i am noticing that it take a longer time to freezed and the unit run faster than when the transformer was not sending this low volt.

      1. so than my question is: does the unit run out equally with a low volt the same way it run out with a normal or higher volt?

      2. if at a 220v my frezer take 2 hrs to freezez water to 0 degree assuming that the consumption of the freezer is 1kwper hour.

      will it take the same 2hours to freeze to 0 degree at 160v?

      3. if it take 4 hour at 160 volt to freeze to 0 degree; will the cost be the same as when the voltage was 220?

      i am trying to make a comparison with the water the author make mention of.

      if the water pressure is high, it take a shorter time to fill a five gallon from the pump.

      when the water pressure is low, it take a longer time to fill a five gallon but what is clear is that only five gallons of water the the meter will read and and if the cost is 10c per gallon the five gallon will cost 50c: in that case the time it take to fill the gallon has nothing with the cost.

      i understand this water issue but need some clarity with the low voltage issue. thanks

    • eugbug profile image

      Eugene Brennan 10 months ago from Ireland

      The unit of electric power is the watt. Kilowatts or megawatts are terms commonly used when greater amounts of power are involved.

    • profile image

      narayanan 10 months ago

      16. The unit for measuring electric power is the

      A. ampere.

      B. watt.

      C. volt.

      D. ohm.

    • profile image

      Janet 12 months ago

      I'm purchasing an RV and trying to sort out all of the electrical basics. Your information answers my questions about electricity and is easy to understand and presented in a logical order. Thanks so much!

    • eugbug profile image

      Eugene Brennan 13 months ago from Ireland

      Hi Arthur,

      12 gage wire has a resistance of 0.001588 ohms/foot.

      For a 290 foot run, total length of current path is 2 x 290 = 580 feet.

      So resistance of the power line = 580 x 0.001588 = 0.919 ohms

      A 1.5 H motor uses 1.5 x 746 = 1119 watts of power

      So on full load it draws 1119/230 = 4.87 amps

      If a motor was a linear device like a resistor, its resistance could be worked out from the wattage and voltage equation

      So from equation P = V²/R then R = V²/P

      Then you could just add the two resistances and total power would be less and = V² / (Rcable + Rmotor)

      However the current drawn by an induction motor increases as voltage decreases for a fixed mechanical load. This is proportional so e.g. a 5% decrease in voltage causes a 5% increase in current. It would really be necessary to draw out a load line and voltage / current characteristic for the motor to calculate the voltage actually reaching the motor terminals and the current input. There are probably tables for working out on the Net, but couldn't find anything specific to motors. However the power drawn by your motor would stay the same.

      The current in the supply cable is 4.87 amps, power dissipation is I²R = 4.87² x 0.919 ohms = 22 watts approx.

      Your meter at 90 feet from the motor wouldn't register the power dissipated in the 200 feet upstream of the meter to the power source. So it would under-read by about 200/290 x 22 = 15 watts approx.

      Voltage drop should be IR = 4.87 x 0.919 = 4.47 or 2% of supply

      However because the motor draws more current because of the drop in voltage, current would end up being greater than this.

      The only thing I would be concerned about is whether the voltage drop could cause difficulties on motor startup. Startup torque depends on voltage squared so for instance if voltage drops by 10 %, voltage is now 0.9 times what it was before and torque is 0.9 x 0.9 =0.81 times what it was. Also as I mentioned above, reduction in voltage causes an increase in current, which compounded with a heavy load could push the current drawn by the motor above its rated value. You could use a current clamp to measure this to make sure it's within limits.

      This is an interesting discussion on the topic:


      and also:


      Try the EEWeb forum aswell:


      It's over 30 years since I studied all this sort of stuff and I haven't actually worked specifically in the field of electrical engineering so I can't guarantee the calculations above are 100% correct. However the power un-measured by the meter (about 15 watts) is definitely much lower than the 1119 watts used by the motor.

      Hope this helps!

    • profile image

      Arthur 13 months ago

      what i need to know is, how much more electicity am i using by having 290 feet of #12 copper wire on a 1.5 hp 230 volts motor? and am I getting a true reading from an installed meter at only 90 feet from the motor, of all of the electricity been total used?

    • eugbug profile image

      Eugene Brennan 13 months ago from Ireland

      If the alarm is powered by a plug in power adaptor, you need to know the wattage/VA rating which should be printed on the adaptor. This would be the max rating of the adaptor though, and the alarm could be taking less power than this. Alternatively if you could find out the model number of the alarm, the specification would indicate the current drawn by the electronics.

      I doubt whether the alarm consumes a lot of energy. So for instance if it uses 10 watts, then:

      Units used in a day would be 10/1000 x 24 = 0.24 units

      In a month units used = 0.24 x 31 = 7.4

      If a unit costs 10 cent. Then cost = 7.4 x 10 = 74 cent

    • profile image

      Bebe 13 months ago

      hello i have a question how much current does a 110v plug in cost a month for a ADT alaram system

    • eugbug profile image

      Eugene Brennan 13 months ago from Ireland

      Hi Maynal, I don't understand your question, can you rephrase it?

      A constant 12kw load for a day is equivalent to 12 x 24 = 288 units.

      In one month (eg 31 days), total number of units is 288 x 31 = 8921 units.

      If you work the other way, 4000 units for a month is on average 4000/31 = 129 units per day or 129 /24 = 5.37 kw average load

    • profile image

      maynal 13 months ago

      If I have just EB consumption reading suppose 4000 unit a month ,permission load 12 kW load in ampear 23 amps

      Now how can find the EB availability in a day


    • eugbug profile image

      Eugene Brennan 14 months ago from Ireland

      Hi Amit, is 300A the model of the battery? I'm not familiar with audio/speaker systems so I can't really advise, but if 50-60 Hz is quoted on the speaker, it sounds as if it is an active speaker requiring a mains supply. Your battery would then need an inverter to drive the speaker. However maybe the speaker has a 12 volt power input? The speaker has a 70W output so the power input requirement from the supply would be greater than this. You would be pushing it a bit with only 12 x 5 = 60 watt maximum available from the accessory o/p of the battery, if the speaker was driven to its maximum level.

      What's the make and model of the speaker?

    • profile image

      Amit 14 months ago

      I have speaker of 100V/70W/50-60Hz.

      I have portable battery 300A with DC Accessory Outlet of 12VDC,5A and USB Outlet of 5VDC,2A.

      Thought to ask you if I can play speaker using this battery.

    • eugbug profile image

      Eugene Brennan 14 months ago from Ireland

      Brushless motors are like an AC induction motor and use an electronic commutation system to spin the magnetic field of the stator.

      See http://electronics.howstuffworks.com/brushless-mot...

      Your other battery is the same voltage but lower capacity (10 AH), so it would provide the same power to the motor but run out quicker. Because it has a lower capacity, it may also be less capable of supplying the current and power requirements of the motor and could possibly overheat.

      Unlike controlling a DC motor, you can't simply reduce input voltage to reduce speed/torque/power because of the intervening electronic control of the motor. Maybe if you contact the supplier they may advise you on whether there is a setting in the motor which limits power. However it seems you may have to change the motor.

    • profile image

      Kyle High 14 months ago

      I've purchased a Front 36V 800W Electric Bicycle Hub Motor Brushless Conversion Kit 26" and 36v 20ah LiFePO4 Battery 5A Charger BMS E Bike Rechargeable Powerful USE 800W to converse my regular bike to an electric one. But now I learn that the 800W is not acceptable in the electric bike laws in my city. It's only 500W or less. I have a battery that is 36V 10Ah 350W at home. Can I use this battery for the bicycle hub motor mentioned above? Would like your answer please. Thank you very much.

    • eugbug profile image

      Eugene Brennan 15 months ago from Ireland

      Wow Ed, what a lot of questions, and by the way there are no dumb questions, just questions!

      Q: Is 110V single phase electric a sine wave that oscillates between +55V, -55V; or +110V, -0 volts?

      A: No, the peak voltage for a sinusoidal waveform is √2 times the RMS voltage. The RMS voltage in your example is 110 so the peak is √2 x 110 = 156 approx. So voltage ranges between -156 to +156

      Q: I read that at a 90 degree phase angle difference between current and voltage the two would cancel each other out?

      A: For a purely resistive load, voltage and current are in phase. For a purely capacitive or inductive load, voltage and current will be 90 degrees out of phase. Power dissipated is VICos(ɸ) where ɸ is the phase angle between current and voltage and power is zero. If two voltages are 180 degrees out of phase (as is the case with the two hot legs of a US supply), the voltages don't cancel each other out, in fact the voltage is doubled (which is where the 220/240 volt supply is derived from).

      Q: What would happen if your circuit theoretically had 0 ohms resistance at 110V

      A: In theory current flowing would be infinite (but infinity isn't actually a number!). In practice current flow in a real circuit would be limited by the resistance of the circuit cables but could potentially be thousands or tens of thousands of amps for a split second. This is why fuses should never be replaced by glass types which don't have a high rupturing capacity. Ceramic types must be used.

      Q: Does a common household 110V circuit have a nominal resistance of 5.5 Ohms?

      A: Well it depends on the length of the circuit cables and their gauge.

      Q: Is 220V really 2-110V legs out of phase with each other, pulsing the power to twice as many end points.

      A: One hot is 110 volts wrt neutral. The other hot is also 110 volts wrt neutral but 180 degrees out of phase (a diagram would be nice but think of the sine wave flipped on its head). So the 220 volt supply is derived from the difference between the two voltages. It's basically like putting 2 cells in series. Think of the point where they join as neutral and the total battery voltage is double the individual cell voltages. The supply transformer in the street is centre tapped and the centre tap is the neutral. The frequency never changes.

      Q: In Europe my understanding is that 240 IS single phase but in the U.S. the two 110 legs alternate being each other's grounds or returns so no ground/neutral connection is required?

      A: No ground is needed for the 220 volt supply. You just use the voltage between the two hot legs.

      Q: Why is 220 in the U.S. called single phase?

      A: Not sure if it's called single phase, but there is only a single phase supply between the two hot legs. The two legs can be thought of as split phase. This differentiates it from a 3 phase supply which has 3 wires 120 degrees out of phase with each other. No neutral is used for distribution of power between transformers (delta system), but a neutral connection is created at the secondary of the supply transformer for supplying homes (star). This is the way it is here (Ireland). I'm not totally au fait with the setup in the US and how a transformer secondary can supply a three phase supply and also two hot legs. Maybe one of the phases is centre tapped?

      Q: Finally, does either amperage or voltage have a greater effect on field or is a joint relationship where as voltage decreases amperage increases, to deliver the same wattage and that is what is measured by an electric meter?

      A: When voltage decreases, current decreases and wattage decreases (and visa versa). The meter measures both current and voltage and the product is what determines the speed of the disk in the older style meters. So if the supply voltage to your home is low, the meter runs slower and you're not being cheated!

      Q: Does a 220V motor driven appliance that will operate down to 197V slow down and deliver less power or does it need more amps and use the same amount of watts?

      A: Universal motors, (the noisy ones with the brushes used in vacuum cleaners, power drills etc) are voltage dependant and will slow down and use less power when voltage drops. AC induction motors (the silent ones in fridges, freezers, washing machines) are less sensitive to variations in voltage. The speed of these motors is controlled by varying the frequency of the supply.

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      ed.scherer@gmail.com 15 months ago

      SIMPLE questions. Is 110V single phase electric a sine wave that oscillates between +55V, -55V; or +110V, -0 volts? Also, I read that at a 90 degree phase angle difference between current and voltage the two would cancel each other out. I thought this happened at 180 degree intervals, where the VOLTAGE is a mirror of the current, thereby cancelling it and resulting in O power. What am I not seeing? You don't have to oversimplify the answer. I am just thinking in a unit circle that at 0 degrees sin=0, 90 degrees sin=1, 180 degrees sin=0, 270 degrees sin=-1 and at 360 degrees sine =0 again. Two waves 180 degrees out of phase mirror each other and so cancel (or am I mistaken), just as the relationship of sin to its mirror happens every 180 degrees (0,180; 90,270; 180,360; etc.). I also learned that watts=voltsXamps. What would happen if your circuit theorhetically had 0 ohms resistance at 110V (besides tripping the breaker because of a dead short, if there was no limiter on it)? Does a common household 110V circuit have a nominal resistance of 5.5 Ohms? Is a 110V circuit +/- 55V, +110V,-0V or another set of values? Is 220V really 2-110V legs out of phase with each other, pulsing the power to twice as many end points, effectively increasing the frequency to a nominal 120Hz, each leg serving as the other's return or ground, and if so, what is the phase angle relationship (e.g. 180 (my thought), or 90))? If 220 represents 2 different phases, why is 220 in the U.S. called single phase? In Europe my understanding is that 240 IS single phase but in the U.S. the two 110 legs alternate being each other's grounds or returns so no ground/neutral connection is required. Finally, does either amperage or voltage have a greater effect on field or is a joint relationship where as voltage decreases amperage increases, to deliver the same wattage and that is what is measured by an electric meter? If current had a greater effect then the meter would spin faster at lower voltages to deliver the same wattage. Does a 220V motor driven appliance that will operate down to 197V slow down and deliver less power or does it need more amps and use the same amount of watts? I should remember these things from physics E/M but that was over 40 years ago when I was 16 or 17. Sorry to ask the dumb questions! Thank you!! I am a minor geek and electricity has always fascinated me. Ed S.