How to Wire a Three-Way Switch
Wiring a three-way switch allows you to control a light fixture or other device from two separate locations. For example, you might have a light at the top and bottom of a staircase that you want to be able to turn on or off from either location. Three-way switches require a bit more wiring than a standard single-pole switch, but the job can be easily accomplished by the average do-it-yourselfer with some basic electrical knowledge. In this guide, I'll walk you through the complete process of wiring a three-way switch step-by-step.
What You'll Need
Before getting started, make sure you have the following materials:
Two three-way switches - You'll need a pair of three-way switches for each set of lights you want to control. These allow on/off control from two locations.
Electrical box and mounting hardware - You'll need a box to house each three-way switch, along with screws and clamps to secure the boxes.
NM-B electrical cable - This is the standard cable used for wiring lights and switches. Make sure you have enough length to run cables between the two switches and the light fixture.
Wire connectors - To join the wires together securely.
Wire strippers - For stripping the insulation off the ends of the wires so they can be connected.
Voltage tester - To make sure power is off before working.
Phillips screwdriver - For installing the switches and securing them to the boxes.
How a 3-Way Switch Works
Before jumping into the wiring, it helps to understand how a three-way switch works.
A three-way switch has three terminal screws - the common (COM or black screw), the traveler (red screw), and the ground (green screw). The common terminal is connected to the hot power source. The traveler terminals allow the switches to communicate and alternate turning the light on and off.
The key thing to understand is that the switches don't directly control the light. Instead, they switch the traveler wires on and off which alternates the power flow to the light.
With the materials gathered and a basic understanding of how three-way switches work, you're ready to install them. I'll go through each step of the process in detail:
1. Turn Off Power
The first step is to turn off power to the circuit at the main breaker panel. Verify power is off by using a non-contact voltage tester. Working with live wiring can result in a nasty shock!
2. Install Switch Boxes
Install a box for each three-way switch following standard electrical box mounting procedures. One box will contain the "line" switch that connects to the hot power source. The other box houses the "load" switch that connects to the light fixture.
3. Run Cables
Run cables between the two switch boxes and to the light fixture. You'll need 3-wire NM-B cables with a black, red, white and ground wire. The black and red wires serve as the travelers.
4. Connect Switches and Lights
With the cables run, you can now connect the switches. Follow these steps carefully:
Line Switch - Connect the black hot wire to the common terminal. Connect the red and white wires from the cable to the light fixture to the traveler terminals. Cap off the white wire (it will not be used).
Load Switch - Connect the black and red traveler wires from the line switch cable to the traveler terminals. Connect the black wire from the light fixture cable to the common terminal.
Light - Connect the black wire from the cable to the hot terminal on the light. Connect the white wire to the neutral.
The ground wires are all connected at each box for safety.
5. Install Switches and Test
With all wires securely connected, install the switches in their boxes and secure them with the included screws. Turn the power back on and test that the switches work to control the lights!
Three-Way Switch Wiring Diagram
Here is a wiring diagram that summarizes the key connections for installing a pair of three-way switches to control a light fixture:
Installing three-way switches is more involved than standard single-pole switches but certainly doable for the dedicated DIYer. The key is understanding how the travelers operate between the pair of switches. Take it step-by-step and double-check all connections before turning the power back on. With a little care, you'll have three-way control wired up in no time!