Wiring A Three Way Switch

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Like a single-pole switch, a three-way switch controls the flow of power in an electrical circuit, but from two different locations instead of just one.

This type of switch is useful, for example, when you want to be able to turn on a stairway light from either the top or bottom of the stairway, or a detached garage light from either the house or the garage.

Such switching requires special three-conductor or three-way switch cable with ground.

This type of cable is usually round, rather than flat like conventional nonmetallic (NM) cable, and it contains an additional, insulated conductor—a red wire.

Three-way switches also differ from single-pole switches in that they have three screw terminals instead of two: a COM terminal (dark screw), and two traveler screws to connect wires that run between switches.

The switch also has a grounding screw.

The switch does not have either an ON or an OFF marked position because the COM terminal alternates the connection between two different switch locations, allowing either position to potentially close the circuit.

You must consider three different cables when wiring a three way switch: the feeder cable, the fixture cable, and the three-wire cable.

The typical wiring method is to tun the two-wire hot feeder cable into the first switch box, and then the three-way switch cable between the first and the second switch box.

You can then run a second two-wire fixture cable between the second switch box and the fixture box.

An alternative m ethod is to run the hot feeder into one switch box; then run the three-way switch cable from the first switch box to the light fixture and then to the second switch box.

Either method initially requires that you run the hot feeder to a switch box.

It's also possible to run power first to the light fixture, but this method is not preferred because it's more difficult to troubleshoot if there's a problem in the circuit.

When wiring a three way switch the switch has one operable contact and two fixed contacts.

In the first position, the switch is open; in the second position, the switch is closed and the circuit is completed through switch box A; in the third position, the switch is also closed but the circuit is completed through switch box B.

A THREE-WAY SWITCH has three terminal screws and no ON/OFF positions.

The dark colored screw terminal screw is the COM, or common, terminal.

The two light screw terminals are switch leads, known as "travelers."

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