Pull 14/3 between two switches to bring neutral to one of them

Question

I've read a bunch of similar questions, but none actually detail the wiring for what I'm trying to accomplish. This is in the US, and in my state NEC 2020 applies.

I have two switches on the same circuit (controlling different lights) where one has a neutral, the other one doesn't. They're only a few inches apart so I'd like to pull Romex 12/3 from one to the other so both have a neutral. It's not possible to pull a neutral from the lights since the room is wallpapered and I can't open up the walls there.

I know I'm not allowed to pull just a neutral wire because the currents have to be balanced. From the new 12/3 cable in the first switch, do I connect black to the permanent hot (black) and bundle the neutrals? How are the wires attached in the second switch?

Answer 1

If they are on the same circuit (which you say is the case) and the old-style switch loop is wired correctly (which you have to check) then:

Disconnect white wire at the switch with a white wire connected to it. Find the join in the associated light fixture between black and white and disconnect it, capping the always-hot black feed there, it will be unused. Also disconnect the neutral associated with the incoming always-hot from the fixture, as it will also be unused. Maintain any onward connections that may feed other fixtures.

Run 14/2 (/3 is not useful) between the two switches, connected to always-hot and neutral (probably black to black and white to white and ignore red, but not a guarantee, just a convention to use red for switched hot.)

At the switch formerly without neutral, join your white neutral wire to the white, and your black always-hot to the switch. Be nice to your (future) self and put some red tape on the black wire going to the light connected to the other terminal on the switch.

At the associated light, connect the black coming from the switch to the fixture (which it already should be) and the white coming from the switch to the fixture white/neutral. DO NOT connect the white from the switch to the other neutral(s) in the light box. If the switch black is not connected to the light, you need to identify it and remove it from an improper switch loop connection to always-hot.

This is required to keep hot and neutral in the same cable. You can't just bring over neutral without also bringing that neutral to the associated light in the same cable as it gets hot. And you can't make loops unless you are on British-derived rules.

Answer 2

That's a 3-way switch. There may be neutral there, but there's no guarantee that always-hot is there. You must pull always-hot and neutral from the same place.

So you need to map that 3-way system and see if always-hot does indeed come here - that is, it would have to be the first switch on the chain with power at the switch.

I realize all you care about is powering your smart switch, but I think you already understand you can't just poach neutral from anywhere. At that point why not just bootleg ground? LOL I'm kidding.

Here, your goal would be to tap always-hot and neutral from elsewhere, bring them to the switch, then "flip" what is presently a switch loop, so it is now powered from the switch. This would mean supply hot and neutral at the light would be discontinued and the wires carefully capped off. I'm fine with leaving the wires energized, as long as it's securely capped off. Definitely don't cut the wires! That would be much worse.

It's not enough to be on the same circuit. The currents in all the wires in any given cable must be equal and opposite, so the magnetic fields balance out and we aren't creating an accidental "transformer" by sending AC power in a big loop. Thus, all related wires must travel in the same cable. The cables must be a "tree" topology with no criss-crosses creating a loop. Obviously, when all non-ground wires in a cable are disconnected and capped off, that doesn't count as a loop.