How to wire a three way switch
A three-way switch has four screw terminals
- A green ground screw for the green or bare ground wire which is used to ground the switch to the main electrical panel.
- A 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 white neutral wire or the green or bare ground wire in the circuit.
The green or bare ground wire is connected to the switch to ground its metal parts for safety.
The white neutral wire is never connected to a switch. Be aware that in a three way switch circuit the white wire is used as a hot feed when a black feed wire is not available - so don't assume that the white wire is never hot. Be careful.
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.
NEVER assume a wire is dead.
Turn the circuit breaker off. Test it. Test it again.
For a PDF version of this diagram to print out download this file:
If you want more information The Home Improvement Web has a good article with details about how to install a three way switch.