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How to Wire a 3-Way Switch: Wiring Diagram

Dan has been a licensed journey-level electrician for 17 years. He has extensive experience in most areas of the electrical trade.

A "3-way switch" is really two switches that both control one light. This illustration makes it look simple, but this article explains the intricacies of wiring a 3-way switch.

A "3-way switch" is really two switches that both control one light. This illustration makes it look simple, but this article explains the intricacies of wiring a 3-way switch.

3-Way Switch Wiring

Wiring a 3-way light switch is not a difficult task... there are only three connections to be made, after all. Making them at the proper place is a little more difficult, but still within the capabilities of most homeowners if someone shows them how. That's where understanding a wiring diagram can help.

First, what is a three-way switch?

When you want to be able to control a light from two different locations (for example, you want to be able to turn the stair lights on from both upstairs and downstairs), this is what electricians call a "three-way switch."

Is it hard to wire a 3-way switch?

To replace a switch is not difficult at all: Simply watch how you disconnect the old one and then put the wires back on the new light switch in the same position. Problems can arise when an extra switch is being added or if you forget which wire went where. That's when it becomes necessary to understand a little more about how a 3-way switch works and how to read a wiring diagram.

What do I need to know before I begin?

If you know what the purpose of each wire is, the task will become much simpler. This article will explain everything you'll need to know in order to wire a 3-way switch, with wiring diagrams and common wiring methods explained.

What about 4-way switches?

Read How to Wire a 4-Way Switch for instructions and wiring diagrams for wiring four-way switches.

How to Wire a Three-Way Switch

  1. Not all 3-way switches are the same. Choose which configuration you want to follow by looking at the diagrams provided below. If you're starting from scratch, Diagram #3 might be the best place to start, but these methods can be used interchangeably in old work. They merely indicate different ways to run the necessary cables.
    Diagram #1 works when several light fixtures share one common breaker and the switches are both on the same wall.
    Diagram #2 works best when power is available in the ceiling but the switch boxes are on opposite walls—it is often easier to run the cable up into the ceiling light box instead of between switches.
    Diagram #3 works best for cases with multiple switches in the same box, as other switches then have power available and can operate other lights without having to have a separate power-in line run to them.
    Diagram #4 can be useful when the light is near the first switch box. It results in lots of wires, so installing a larger box may be necessary.
  2. Turn off the power at your electrical panel before you begin working.
  3. Make sure you understand which screw terminals and which wires serve which purpose. Below, you'll find full descriptions to guide you.
  4. Have plenty of 14-3 type NM cable on hand, which has three insulated wires—white, black, and red—plus a bare copper ground wire. If you're connecting to 12-gauge wire, or the breaker is 20 amp, you'll use 12-3 instead. Most home lighting circuits are 15 amp, which only requires 14 gauge wire.
  5. Follow the diagram to connect the wires (see instructions below) to the new three-way switch.
  6. All white wires used as travelers between the 3-way switches should have their ends wrapped with black electrical tape or in a plastic wire nut.

How a 3-Way Switch Works: Identifying the Terminal Screws

  • There are three screw terminals on the sides of the switch and one on the end. Every switch has these same three terminals, but older switches might be missing the fourth ground terminal.
  • The small, green screw terminal on the end is the ground terminal. It is usually painted green, although the picture does not show that color well. It can often be recognized as the screw that is part of the metal framework of the switch and is not insulated from other metal parts.
  • The green or uninsulated ground wire always goes to this ground terminal. Older switches often did not have this ground terminal screw, but those are no longer legal to use. Now, all light switches must have a ground terminal screw to attach the ground wire to.
  • One of the three other terminals is a different color, usually darker, and is called the common terminal. Mechanically and electrically, this common terminal is connected internally to one of the other two brass screws called the traveler terminals. When the switch is flipped the other way, that connection is broken, and the common terminal is then connected, internally, to the other traveler terminal.
  • The common terminal is always connected internally to one (but only one) of the traveler terminals. Which one is dependent on whether the switch is up or down.
  • It should perhaps be noted that the traveler terminals are essentially interchangeable. Given that each one is to have a traveler wire attached to it, and there are two traveler wires and terminals, it doesn't matter which traveler wire goes to which traveler terminal.

Identifying the Ground, Common, and Traveler Terminals in a 3-Way Switch

The common terminal is on the top in this view, with a traveler on the lower end.  The ground terminal screw is showing up as silver-colored at the very bottom.

The common terminal is on the top in this view, with a traveler on the lower end. The ground terminal screw is showing up as silver-colored at the very bottom.

An old switch. This switch has no ground terminal/screw and is no longer legal to use.  Make sure your switch has a ground terminal.

An old switch. This switch has no ground terminal/screw and is no longer legal to use. Make sure your switch has a ground terminal.

Identifying the Screw Terminals by Color

Here is where to find the different screw terminals.

What is the green terminal screw?

The small, green screw terminal on the bottom is the ground terminal. All new switches must have a ground, but some older ones don't.

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What is the darker screw terminal?

One of the three screw terminals will be a different color, usually darker. This is the common terminal.

What are the brass screws?

The two brass screw terminals are the traveler terminals.

Identifying the Wires by Color

Here is how to identify the different wires.

What is the green wire?

The green or uninsulated (copper) ground wire always goes to the ground terminal.

What is the white wire?

The white wire is the neutral. You'll bundle all the neutrals together with a "wire nut" or a twist-on plastic wire connector.

What is the black wire?

The black wire is "hot" at all times unless the entire circuit is turned off at the circuit breaker panel.

A note about wire colors: The National Electric Code requires that every neutral wire be colored white, and that ground wires be colored green. Only neutral wires may be white in color, but the code makes an exception for white wires in a cable that are not being used for a neutral. These wires should be colored black using a magic marker or some other method. Many electricians will do this, but many will not, and it can make troubleshooting in the future difficult and can be a safety hazard to anyone else working on the system. I encourage you to take the few seconds necessary to color these non-neutral wires.

The colors shown in these wiring diagrams are common color usages only. Not all electricians use the same color code (except for neutrals and grounds), so the wires could be different colors.

Identifying All the Parts of a 3-Way Light Switch

The terms "traveler" and "common" have already been explained, but there are other terms that will be used in this article that also need some explanation.

  • Cable. The term "cable" refers to a combination of two or more wires, bundled together, usually in a sheath of insulating material. Each wire is insulated separately, with the possible exception of the ground wire. The ground wire may be insulated with a green color or left bare (copper), without insulation.
  • Power-in. The power-in cable is that cable that eventually ends in the circuit breaker panel or fuse box. It is the cable that provides the power to the lighting system.
  • Neutral. This is the white wire contained in the power-in cable. It does not terminate at or connect to any switch, although it may be present in a switch box and ended with a wire nut that connects it to another neutral wire.
  • Ground. The grounded wire in each switch or light fixture box. It is either colored green or left bare of insulation (copper).
  • Hot wire. This is the second, black wire, contained in the power-in cable. It is "hot" at all times unless the entire circuit is turned off at the circuit breaker panel.
  • Circuit breaker panel. Commonly called a "fuse box," it may contain either circuit breakers or fuses. This panel controls all the power in the building and it is where that power may be shut off.
  • Two rope. "Two rope" is the designation given to a cable that has two individual wires, plus a ground wire. These wires will be colored white and black, with a green or bare (copper) ground.
  • Three rope. Three rope is a cable with three wires, plus a ground. Normally the colors are white, black, and red with an additional green or bare (copper) ground.

Understanding a Wiring Diagram

Each diagram will show the two 3-way switches (but not the wall box they are contained in), the various cables and wires used in the configuration being discussed, and the light box and light fixture.

How does the electricity flow through the switch?

To understand the wiring diagram, you must know that the electrical current enters the system on the black wire in the power-in cable, passes through the switches, through the light fixture, and returns to the white wire in the power-in cable. If the circuit is broken anywhere (a switch turned the wrong way, a broken wire, or a bad light bulb), the current will not flow and the bulb will not light. For discussion purposes, each 3-way switch will be considered to have the common terminal connected to the right-hand traveler terminal when in the "up" position and connected to the left-hand terminal when in the "down" position. This is not necessarily true, however, it's simply helpful for discussion purposes.

Read the descriptions carefully and compare them to the diagrams to understand the diagrams. Each diagram will have a description of how the current travels in order to light the lamp.