How to Replace Air Conditioning Fuses
Why Do Air Conditioners Break on Hot Days?
"The A/C quit working and it's a hundred degrees outside. It's almost like the air conditioner knows how much we need it and breaks down on the hottest days of the year."
This is a statement I've heard many times in my 22+ year HVAC career, and it's funny because it's true. Although the air conditioner doesn't have the ability to "know" anything, you're right, your A/C is more likely to break down on the hottest days of the year. This holds true for nearly any electrical appliance.
Heat is one of the worst things for electrical equipment like an A/C. Increased temperatures cause electrical components to run hotter than recommended, and this causes them to fail. When it comes to air conditioning, it is the capacitor and fuses that are most likely to "feel the heat." There is very little air circulation to cool these components, increasing their chance of failure. I'm not saying that they will blow, just that the odds are better on a hot day.
Here, we will focus on the fuses and how simple it is to fix this problem. Below, you'll find detailed, step-by-step instructions on how to troubleshoot the problem by...
- checking the fuses
- checking the voltage
- removing and replacing the fuses.
Let's get that A/C up and running again without breaking the bank.
What Do Fuses Do?
Think of a fuse like a secret service agent. It takes the shot. If more amps than the air conditioner can handle try to reach the unit, the fuse will blow to protect the condenser. The fuse is designed to handle a limited amount of amperage based on the maximum amount that the A/C is rated for. If you use a fuse that is too small, the fuse is likely to blow more often but if you use one that is too big it could allow more than the rated amount of amperage reach the unit and cause fatal damage to the system. The same is true of your breakers. Kind of cool, eh?
How to Check if Your A/C Fuses are Blown
If you suspect your fuses may be blown, the first thing you'll notice is that the A/C unit outside is not doing anything. You may hear a slight humming, but that's all. The best way to check the fuse is by using a voltmeter. Let's go through this process first and then, for those of you who don't have a voltmeter or are uncomfortable testing voltage, I will give you another method for checking the fuses.
Why is the A/C Humming With the Power Off?
The stat tells the furnace to tell the A/C to run. The furnace sends 24v to the A/C contactor which then connects the power and let's the A/C run and that makes a humming noise. Obviously we don't have any power but the furnace doesn't know that and will continue to ask the A/C to run.
REMEMBER! You are working with live electricity here. We cannot test fuses and voltage with the power off or the fuses removed. You should not attempt to work with live electricity if you are not a skilled electrician. In this case, we are taking a simple voltage reading but must still be careful and confident in what we are doing.
- Locate your disconnect. This is the usually the grey box mounted to your home near the outdoor condenser part of your A/C system. See the images below for what it looks like.
- Open the disconnect. Simply lift or swing open the cover. There may be a small tab on the door that you'll have to apply a bit of pressure to so that it will open.
- Expose the wiring in the disconnect. There should be another cover inside the disconnect that is protecting the wiring (or protecting you, depending on how you look at it). This should easily pop out or it may be held in by just a single screw.
NOTE: Not all disconnects have fuses in them. Though it is most common that they do, and it's code in many states, some do not. If that is the case, they would not be your problem.
How to Test the Voltage on an Air Conditioner
- With the wiring exposed, you should be able to locate the incoming and outgoing wires. The wires will be labeled the "line" (which is the incoming power) and "load" (the outgoing power). This wiring is not like that of your light switch or receptacle (which runs on 110/120 volts), so listen closely: Both wires carry 110 volts, not just one of them.
- First, set your meter to the voltage (V) setting and make sure that the display reads "0" volts (or infinity).
- The positive (+) and negative (-) leads need to be placed on the lugs of the "line" side of the circuit or fuses. This means that the red lead from your meter goes the lug of the black "line" wire and the black lead from your meter goes on the lug for the white "line" wire.
- Your meter should now read voltage in the range of 220 to 240, give or take a few volts. If you see voltage in this range, you've confirmed that you have power coming into the disconnect to your fuses. On the other hand, if there is no reading, then the problem is happening at the breaker panel where a breaker might have been tripped, the fuses are likely not your problem, and it may be time to call an electrician if resetting the breaker doesn't work.
- If you did see voltage during step #4, then run the same test on the "load" side of the fuses. Again, you hope to see voltage in the range of 220 to 240. This tells you the power is making it through the fuses (like it should). If you read voltage on the line side but not the load side, this means your fuses are blown and need to be replaced.
What Are Leads and Lugs?
The "leads" are the voltmeter's wires. They have hard metal on the end with insulated grips that make the test safe.
The "lugs" are the screws on the disconnect that are housed in a metal block. They are the screws that hold the wires down. One should say "line" and the other should read "load."
When you test the voltage, you press the metal end of the lead down on the lugs as directed above.
Testing for Voltage
Do you know how to test for voltage using a voltmeter?
What If I Don't Have a Voltmeter?
For those of you who don't have a voltmeter, it's really not a big deal. You should be able to find the size of fuses you need, buy them at your local "big box" retail store, and put them in to test if they were the problem. Worst case scenario is you'll have an extra set of fuses that you should have anyway and will have only spent a few dollars in the process.
How to Remove Fuses
- It's possible that your fuses are in the same location as the wiring, fully exposed. However, many disconnects have fuses in the handle itself, so you will have to pull the "T" handle out to find the fuses on this type.
- Either way, you need to pull out the handle to stop the power from running through the fuses while you work. This DOES NOT stop power from coming to the disconnect and fuses on the line side. You should go back and shut down the breaker to your air conditioner to insure maximum safety.
- Now you can remove the fuses either by popping them out with your hands if they are in the handle itself or, if not, by grabbing them with a pair of pliers with insulated handles. Never use bare metal handles when working with electricity.
So this may all sound like a lot but honestly, it only takes a few minutes to do. Just pop the fuses back in where you found them and turn everything back on before you sweat to death!
Air Conditioner Breakers
Most residential air conditioners are wired into a 25 - 50 amp circuit breaker based on the unit size and maximum amp rating for your specific unit. This will be a 2 pole (double switch) breaker and rated 208/230 volt.
Different Types of A/C Fuses
What Kind of Fuse Does A/C Use?
It is important that you buy the right size fuses for your air conditioning system. In a pinch, you could use a fuse that is rated for a lower amperage than the ones you need, but never more. As stated earlier, allowing too many amps to flow to your condenser will cause it to break down and possibly be fatal to the unit. At worst, smaller fuses will blow more easily and overprotect your air conditioner. This means you'll be having this problem again soon.
Many times, the fuse size and type (typically a "TR" type) are written right on the fuse itself. If it isn't, then you should be able to get the amp rating of the air conditioner off of the rating plate on the unit. Sometimes these are located inside the access panel on the air conditioner. It should be rather easy to find.
If you can't find any amp rating on the fuse or rating plate, you could try and go with a 20 amp fuse (since that is about the lowest rating for any A/C unit) and see if you can get your unit running temporarily while you find out. (Never go higher than the breaker amperage rating.) That should allow some cooling but if it blows as soon as you turn it on, stop there. It's either rated for higher amperage and we don't want to guess or there is a short in the wiring and you may want to contact an electrician or an HVAC technician to locate the problem. Simply shut down the breaker to the A/C and turn the thermostat to "off".
If the old fuses are bigger in size than the new fuses (we're talking about physical size here, not amperage), you will also need to buy what is called a fuse reducer. These will fit onto each end of your smaller fuses and make up the difference of the physical size.
Never insert anything in the disconnect to bypass the fuse. This is not only a great way to cause major damage to your unit but can also be a danger to you and your home. Trying to put copper tubing or aluminum wire in where the fuse goes to try and get the A/C working is a VERY bad idea. (Yes, people do this and it never ends well.)
Short Video on Replacing A/C Fuses
Why Does My Air Conditioner Keep Blowing Fuses?
If your A/C unit keeps tripping the breaker or blowing fuses, don't just keep replacing the fuse. When a fuse blows, that's sometimes your A/C's way of communicating that there's a problem. If the fuse keeps blowing then there's probably a bigger problem with your cooling system and it's time for you to figure out what it is.
There are many possible causes:
- It's a problem with your circuit, fuse box, or power supply. Breakers trip and fuses blow when too there's too much amperage in the line. You might try to convince yourself that the solution is to get a larger fuse, but that would be an extremely dangerous mistake.
- There's something wrong with the capacitor, which is the part in the condenser that helps regulate electrical current.
- Hot weather + a dirty filter (or a dirty condenser coil) = disaster. The unit is struggling so hard to push cool air through a clogged filter that it overheats and causes the fuse to blow. You should always thoroughly clean your unit at the start of the warm season.
- The filter is clogged or the motor is blocked. When the individual parts aren't working as they should, the whole system has to work harder to compensate.
- The electrical connections are loose. Temperature changes can wreak havoc on connections.
- It's a faulty condenser fan. If the system can't cool itself effectively, it may blow a fuse.
- It's a faulty compressor.
- The levels of refrigerant are too high (or too low).
- The unit is just too old.
It might be time to hire a skilled HVAC technician or buy a new A/C.
Get the Air Conditioning Working Again!
I hope this has resolved your air conditioning problem, but remember, there are a few things that could be wrong, and if what you find in your system doesn't match up to what I've explained here, don't guess. Contact a professional.
Stay cool, my friends.
~We're all in this together ~
Questions & Answers
What happens when the fuse is hot and the a.c. unit will not run?
I recommend you try some of the steps in the article to see if the fuses are the issue. Fuses can get hot sometimes so that by itself doesn't mean they are the problem.
Can I bypass the fuse boxes to run my AC? My roommate took the fuses.
The answer is yes, but I cannot in good conscious tell you how because it's a bad idea and fuses are so cheap, so why risk it? The unit should say right on what the "max fuse size" or "max amperage" is, and you can go buy the fuses. They are probably just a TR type fuse.
How expensive are the fuses?
They are very cheap. It should not cost you more than $10-15 for standard residential sized fuses. (That's for both.)
© 2012 Dan Robbins