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I have three way switches at the top and bottom of my stairs. There is a light at the top of the stairs controlled by the switches and I wish to add a light at the bottom.

A 14/2 runs first to the light at the top (hot and neutral from the breaker), then a 14/2 runs to the switch at the top (hot supply and switched hot back to the light), and a 14/3 runs between the switches (travelers and switched return).

It would be very difficult to run a new 14/2 (switched hot and neutral) from the light upstairs to the light downstairs, but it is easy to run a 14/2 from the downstairs light to the switch box downstairs.

I want to know if it is acceptable to connect the hot switched from the downstairs 3 way to the light and run a separate neutral to the breaker box, or another nearby circuit's neutral.

skill level: electrical engineer / homeowner, but not an electrician. (know how to not burn the house down, but don't know the electrical code)

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How difficult would it be to replace the 14/3 between the switches with two 14/2 wires? –  Niall C. Oct 7 '10 at 12:26
    
+1 for a very well-asked question! Clever solution too but I have no idea what code says about this. –  Mike Powell Oct 7 '10 at 12:40
    
can the nearby location be the same neutral line? (on the same circuit?) –  mohlsen Oct 7 '10 at 13:24
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3 Answers

up vote 2 down vote accepted

You're describing a shared neutral, and some local codes allow it, some don't. However, it's usually described with closely related circuits, like a single 12/3 wire feeding alternate receptacles in a kitchen's two 20A circuits.

In general, I think they're a bad idea because if circuits A and B share a neutral, you could potentially still have current in A's neutral with A turned off at the service panel. In addition, if A and B are on different phases and there's a problem with the neutral, you could have 240V across all your devices. Using a ganged breaker mitigates those risks somewhat.

How difficult would it be to replace the 14/3 between the switches with two 14/2 wires? That would get neutral to the downstairs switch and wouldn't raise any eyebrows come inspection time. If that would also be difficult, next step is to talk to an electrician that knows what's allowed in your area.

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I don't think it would be up to code if you used two 14/2 wires in this situation. most code states that the hot and neutral must be run in the same conduit. I don't see how you could achieve that with two separate wires. –  Tester101 Oct 7 '10 at 12:57
    
@tester: switched live and neutral on one, the travelers on another. My wiring book shows that arrangement for a circuit with multiple light fixtures between two 3-way switches. –  Niall C. Oct 7 '10 at 13:30
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I understand that, but how are you connecting the 'new' neutral wire to the circuits neutral in the upstairs light? You would have to run an extra wire from the light down to the switch box, so in essence your re-running the entire circuit (which James does not want to do). –  Tester101 Oct 7 '10 at 16:00
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@tester: good point, I missed that. That'll teach me to answer questions when the dog wakes me at 4am! :) A possibility is to run power to the lower light from the service panel, then 14/3 to the switch to carry neutral, and remove or cap off the existing line to the top light. –  Niall C. Oct 7 '10 at 16:12
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@James: If your going to run more wire between the switches, just add a 14/2 line (leaving the 14/3 alone) and while your at it carry that line right up to the upstairs light. That way you can just run the new 14/2 right to the new light, and you'll be done. –  Tester101 Oct 7 '10 at 16:36
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You'll want to run a new 14/2 line from the upstairs light to the downstairs light, that way everything is up to code and is done right.

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This makes more sense than accepted answer. If you're running a new wire anyways, run it to the light (assuming it's accessible, anyways), or to the upstairs switch. –  gregmac Oct 7 '10 at 21:39
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@gregmac: This answer makes more sense as what to do, but the other answer says why my idea was bad, which was what I asked. –  James Oct 8 '10 at 6:42
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Tester is absolutely right. Don't over complicate the job. Put the light fixtures, as many as you want, in a simple parallel circuit from the existing one with 14/2wg/nm and you will be fine and confirm to current code. simple, huh? BTW, adding an extra neutral on a single phase 120vac leg cannot introduce 240vac. impossible.

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Good catch on the neutral not introducing 240vac, I overlooked that part and focused on the wiring aspect. –  Tester101 Oct 9 '10 at 17:40
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