Expert Answers: Install a Three-Way Switch

Need to hook up multiple switches to one light fixture? HANDY has the answer.

Q: I just purchased a new home, and I would like to be able to turn on my entryway lights from both the top and bottom of the staircase. How can I wire the necessary switches, and what materials will I need? - Al Roberts, Fayetteville, NC

A: Master electrician Rex Cauldwell says that to wire a three-way switching system (two switches that control a single outlet, usually a light) you'll need three-conductor cable (two insulated hot conductors - red and black - along with a neutral and ground wire), a power (LINE) cable (usually NM/2 with ground) and a LOAD cable (also NM/2). The three-conductor cable provides the conductors for the back-and-forth switching, the LINE cable brings the power from the source, and the LOAD cable takes the power to the light.

Three-way switch

There are many ways to interconnect the cables in a three-way switch system. Whichever method you use, always follow a repeatable order for running the cables.

Here's the simplest (and easiest to troubleshoot) method of installing three-way switch:

  • Mount the two boxes for the three-way switches and the light outlet box. If there is more than one light, use one light outlet box as your main box and feed the rest of the lights off of that one.
  • Run three-way switch cable from one three-way switch box to the other three-way switch box.
  • Bring power cable into one of the switch boxes - never the light outlet box. The latter will work if wired properly, but it is more complicated.
  • Bring the cable from the light outlet box into either three-way switch box.

Now all you have to do is wire the boxes. Once the cabling has been run, wiring a three-way switch is not hard if you understand how it operates. It has three screws: one input (the dark screw, called a common or tongue) and two alternating outputs. The common (COM) terminal can connect to only one output screw at a time. The incoming power is attached to the common pole and is transferred to one of the other two screws, depending on which way the switch is thrown. This type of switch has no ON/OFF indicated on its front.

Find the power cable that you brought into the first box. It will have two insulated wires (black and white) and a ground wire. All ground wires within each box are connected together and then connected to the outlet itself (see the wiring diagram, below). - HANDY

One way to wire a 3-way switch system