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I have a light switch that controls 2 lights in the hall. The wire to the switch carries power to the switch with the black wire and the white wire carries the power to the light. I have access to the switch box and can run a three wire (black, red, white) to the location for the other switch. Can I wire this so I have three way switches?

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Yes you can. You need to replace the current switch with a three way.

Run the new three wire cable between the old switch box and the new switch box.

The wire that carries current to the old switch is attached to the common terminal of the replacement three way.

The wire that brings current to the light in the existing switch box is connected to one of the wires to the second remote three way switch. In the new switch box, this will be connected to the new switch's common terminal.

The other two wires are the travelers between the replacement switch and the new second switch.

NOTE: Be sure to mark the white wire in the new cable with black tape or marker on both ends. That wire will be hot.


Old Wiring
Old Wiring

New Wiring
New Wiring

  • NOTE: As of NEC 2011, a "neutral" is required at all switch locations. So technically while this will work, it will not meet current codes. – Tester101 Jan 22 '15 at 12:28
  • Hope you don't mind that I added a couple images. – Tester101 Jan 22 '15 at 12:30
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    @Tester101 Thanks much for the illustration and the note. My old school ways keeps blanking out the neutral-at-switch rule, even though you keep reminding us (thankfully). – bib Jan 22 '15 at 12:31
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    Interesting code point: is it necessary to bring this whole thing up to code in this case (as it would require pulling a new wire between light and switch and/or new power feed to switch), or is that a judgement call the AHJ makes? – gregmac Jan 22 '15 at 22:35

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