Wednesday, March 24, 2010

how to wire a three way switch


How to wire a three way switch

I don't wire enough three way switches to keep the logic straight in memory, so I made a logic diagram of several ways they can be wired and a PDF version of this diagram to print out.

A three-way switch has four screw terminals

  • green ground screw for the green or bare ground wire which is used to ground the switch to the main electrical panel.
  • dark common screw for the hot wire which either comes from the main electrical panel or goes out to the fixture, depending on where the switch is in the circuit.
  • Two brass traveler screws for the hot wires which run from one switch to the other.

Switches interrupt only the hot wire to the light fixture

  • Switches never interrupt the neutral wire or the ground wire.
    • The ground wire is connected to the switch to ground its metal parts for safety.
    • The neutral wire is never connected to a switch.

CAUTION - the white wire may be hot

You'll note in the diagram that in some wiring schemes the white wire is used as a hot feed. This is contrary to the general rule that the white wire is grounded at the main electrical panel. So, as always in working with electric wiring, never assume a wire is dead. Turn the circuit breaker off. Test it. Test it again. Be careful.

For a PDF version of this diagram to print out download this file:

Three Way Switch Wiring Diagram - PDF version
If you want more information The Home Improvement Web has a good article with details about how to install a three way switch.

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