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Is this an acceptable way to add a light to a 3-way circuit?

My whole-house fan is currently on a dedicated circuit. I want to convert the single-pole switch to a 3-way. My plan is take the existing switch leg to fan and make it the traveler; the hot remains in the existing box. On the other end I can add a 3-way switch and run a new switch leg back to fan. Since the existing switch is downstairs, I cannot get a neutral from the fan circuit. Can I use a neutral from another circuit as-log-as it comes from the same phase side of the circuit panel?

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Can you give us a sketch of what's existing and the location of the new switch? We can help, but need to know the geography of the circuit. –  Chris Cudmore May 7 '12 at 20:18
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marked as duplicate by Tester101, BMitch, ChrisF May 17 '12 at 21:03

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No you can not share neutrals. There are several reasons for this:

  • If you share a neutral with more than one circuit than the return currents are additive, IE: two twenty amp circuits with a shared neutral will return as much as 40 Amps through a 12 AWG wire that is designed for only 20Amps. This will cause the wire to overheat and possibly catch on fire.
  • GFCI and AFCI breakers will not work properly.
  • It is against the NEC code.

Probably other reasons that I am not thinking of right now.

I'm not sure how you figure to wire this. You mention not having the neutral in the switch box, but you don't use a neutral in the switch box. The neutral doesn't get switched only the hot leg and traveler. If you sketch a wire diagram we can help you plan it.

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+1 for a good "no, but" answer –  Nate May 7 '12 at 20:19
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No, you should not do this. Think of what would happen if someone wanted to work on that other circuit - they would flip the breaker and assume its off, yet there is still current on the neutral coming from another circuit! Zap! Even worse, someone disconnects the neutral without knowledge that it used elsewhere and now you have an open neutral!

As well, it's worth mentioning that the neutral has no phase - they are all tied together (and tied to ground at the main panel).

The correct way to do this is to run a new cable.

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