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)
Best Answer
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.