A three-way switch allows you to turn a light on or off from two different switches. Three way switches are useful for large rooms with multiple entrances, but require additional wiring than a standard single pole switch. The wiring system depends on whether the current can get to the switch or the light bulb first.
Steps
Part 1 of 2: Prepare for Installation
Step 1. Deactivate the system
Make sure the circuit of the room you are working in has been turned off. This will prevent any accidental electric shock and reduce the risk of fire.
- In most cases, the main switches are located in the garage or basement, but the location varies from house to house.
- When you find the breaker box, locate the one that controls the power supply of the room you are working in. Put it in the off position to cut power in that sector.
Step 2. Use a tester to make sure that power is no longer reaching the room
Step 3. Decide if you want power to go to the lamp or switch first
This will affect how you go about installing. You can determine if current is reaching the switch by removing the switch panel. If there are two black wires going into the switch box, then the current goes to the switch first. If there is only one black wire, the current passes through the lamp.
Step 4. Examine the three-way switches
Before starting the installation, take a look at your new three-way switch to identify all the terminations you are going to connect the wires to.
- Power Wire Terminals - These are located on each side of the switch on the top of the switch.
- Ground wire terminal - older switches may not have it, but all new switches do. This is located on the top or bottom of the switch, mounted on the frame.
- Normal wire screw - This is located on the left side of the switch, and is a different color than the two power terminals.
Step 5. Install larger electrical boxes
Larger boxes than already exist are likely to be needed if you are replacing a normal single pole switch. Three-way switches have more cables, so you will need a little more space to work.
Step 6. Connect a three-wire cable to the two boxes
Choose a 14-3 or 12-3 NM cable, depending on the switch. A 14 gauge cable needs a 15 amp switch, while a 12 gauge cable requires a 20 amp switch. This type of cable must be used to ensure correct tension and correct number of wires.
- See this guide to learn how to pull wires inside a wall.
- The first number indicates the gauge and the second number is the number of wires carrying current. A three-way cable will have three insulated wires: one black, one red, and one white.
Part 2 of 2: Connecting the Switches
Power the Switch
Step 1. Connect the terminals of each switch
Connect the black wire from the power source to the first switch terminal. Connect the black wire from the new terminal wire to the second switch.
Step 2. Connect the white cables
Connect the white wire, both ends, to the power terminal on the right side of the switch. Mark this wire with black tape to indicate that it is a "hot" or connected wire (white wires usually aren't).
Step 3. Connect the red cables
Connect the red wires to the left terminals of each switch.
Step 4. Connect the ground wire of the second switch
Use the ground wire from the new cable.
Step 5. Connect the old white wire to the existing cable connected to the lamp
When you took the old switch apart, there was a white wire attached. Connect this white wire to the white wire that is connected to the lamp. Use a connector to connect them.
Step 6. Connect the black wire from the lamp to the new wire
This black wire was connected to the old switch, and is connected directly to the lamp. Connect it to the black wire in the new cable, connected to the first end of the switch. Use a connector to connect them.
Step 7. Connect the ground of the first switch
Connect the ground wire from the power source, the ground wire to the lamp, and the ground wire from the new wire to the ground terminal of the first switch. Use one connector to connect them all.
Step 8. Install the switches
Now that both switches are wired, you can install the electrical boxes in the wall. Flip the switch to insert it, and then check if it works.
Connect the power
Step 1. Connect the power to the new cable
Since the current comes from the lamp, it is necessary to connect the second switch. Connect the black wire from the lamp to the black wire from the new cable in the original switch box. Use a connector to connect them.
Step 2. Connect the power terminals
Connect the white wire of the lamp to the terminal of the first switch. Mark the wire with electrical tape to indicate it is connected. Connect the black wire of the new cable to the terminal of the second switch.
Step 3. Connect the terminals
Connect the white wire, both ends, of the new cable to the remaining terminals of each switch. Mark them with electrical tape to indicate they are connected. Connect the red wire, both ends, of the new cable, to the terminals on the right of each switch.
Step 4. Connect the ground of the switches
Connect the ground wire of the new wire to the ground terminal of the second switch. Connect the ground wire of the new wire and the lamp power wire with connectors. Then connect them to the ground terminal of the first switch.
Step 5. Install the switches
Now that both switches are wired, you can mount the electrical boxes in the wall. Flip the switch to insert it, and then check if it works.
Warnings
- Always make sure the power in the area where you are working is turned off to avoid electric shock.
- Call an electrician if your home is made of aluminum. The inside of the liner will be light gray rather than copper. You shouldn't be working on it alone.
