How to Hook Up a Three-Way Electrical Switch
Controlling a single fixture with two switches is accomplished with a three-way circuit. The configuration takes its name from the fact that the switches have three connecting terminals rather than the traditional two. Three-way switches are convenient when you want to control a single fixture from different locations. Installing a three-way circuit is no more difficult than a normal circuit, provided you keep track of the many wires involved and which connect to which.
Cut the openings for the switch boxes using the utility knife or keyhole saw. Attach the switch boxes directly to the drywall.
Cut the opening for the fixture junction box. If the box will be in the ceiling, the hole should be large enough to let you attach the box to the bar that will support the fixture between joists.
Cut access holes above the switch box holes where the wall and ceiling meet to make feeding the wires easier. Save the drywall pieces for patching later.
Feed the fish tape through the access hole above Switch 1 and across the ceiling to the junction box. Attach the three-wire NM cable to the fish tape and pull it back toward you. Drop the wire behind the wall to the switch box. Repeat this process between Switch 2 and the junction box.
Run two-wire NM cable from the power source to Switch 1. Switch 1 will now have one red wire, two black wires, two white wires and two green wires. Switch 2 will have one red, one black, one white and one green wire. At the fixture you will have two red wires, two green wires, two black wires and two white wires, not including the wires attached to the fixture itself.
Make the connections at Switch 1 using plastic electrical connectors. Connect the black wire from the power source to the common terminal (it is labeled "COM") on the switch. Connect the red wire to one of the remaining terminals and the black wire leading to the fixture to the final terminal. Connect the two white wires together. Make a pigtail (see Section 2) with the ground wires and connect the pigtail to the green ground terminal.
Make the connections at Switch 2. Connect the black wire to the common terminal and the white and red wires to the remaining terminals. Connect the ground wire to the ground terminal. Wrap a piece of electrical tape around the white wire to indicate that it is carrying current.
Make the connections at the fixture. Connect the red wires together. Connect all of the green ground wires together. Connect the white wire from Switch 1 to the white wire on the fixture. Connect the white wire from switch 2 to the black wire from Switch 1. Wrap electrical tape around the white wire to indicate it is hot. Connect the black wire from Switch 2 to the black wire on the fixture.
Patch the access holes with the drywall patch kit.
Cut a small piece of appropriately colored wire about 1-1/2 to 2 inches long. This is the pigtail.
Connect the appropriate wires together with the pigtail using plastic electrical connectors.
Connect the pigtail to the appropriate terminal.