I'd like to add a neutral wire to an old switch box so that I can install a smart switch that needs a neutral wire.

I have a mud room and porch that each have a light fixture. The lights are independently controlled by a pair of stacked switches. Both fixtures are on the same breaker.

The wiring is old, and there isn't a neutral wire in the switch box.

I need to verify this, but I'm pretty sure 3-wire cable brings hot, neutral, and ground up to the junction box for the mud room light, and from there the same to the junction box for the porch light.

I think that neutral is then run in each junction box to each light fixture. The hot is then wired with 3-wire (including ground) cable to the switch box, which sends switched-hot back via the same 3-wire cable, to connect to the hot side of the light fixture.

I'd like to replace the stacked switches with a single smart switch that controls both lights (so that they would turn on/off together rather than being independently controlled).

My idea is to run hot/neutral to the switch box via the cable that currently is being used to switch fixture 1. The cable that is currently being used to switch fixture 2 would continue to supply the switched hot that is necessary for fixture 2. The cable that is being used to take neutral and hot from fixture 1 to fixture 2 would then be repurposed to take the switched hot from fixture 2 to fixture 1, and to take neutral from fixture 1 to fixture 2.

Does this sound code compliant, and aside from code compliance, does it sound like a reasonable idea?

  • It looks like you've lost the original account and have a different account you're using to edit. You should flag this so a mod can merge the two accounts for you
    – Machavity
    Commented Feb 4, 2019 at 21:17
  • diy.stackexchange.com/help/merging-accounts
    – isherwood
    Commented Feb 4, 2019 at 21:33
  • Not quite. Current mustn't go in a big circle. That's a bad idea with AC power. The neutral must tag along with the switched-hot along the whole circuitous route, and cannot shortcut to the lamps, even though it is right there in lamp 1. Commented Feb 4, 2019 at 21:33
  • A simple drawing of what you have and what you want to do would help immensely here.
    – JimmyJames
    Commented Feb 4, 2019 at 21:55

1 Answer 1


What you have -- ?

Note that colors signify function, even though the actual cable color is whatever standard cable colors are. Here, red is switched-hot, white is neutral.

enter image description here

You can't cross the streams - er, send current going in a loop. This is AC. It'll do bad things.

A basic rule is that currents must be equal in each cable or conduit. Think about an army of ants exploring a tree. Anywhere on the tree, if you count the number of ants that went up a branch, the same number will come back down. Currents are equal in tree topology - but branches can't touch. If an ant goes up one branch and comes down another, currents are not equal. Hence, circuits must be a rigid "tree" topology and loops must be prevented.

You see in the first drawing where there's a "Great Wall" separating the two switches.

So we do the following.

enter image description here

See how neutral goes a bit farther, but this avoids sending current in one direction around the triangle, and it is a tree (vine, anyway) not a loop.

**Notice how you will have two separate neutral splices in the left box. Don't combine them and don't mix them up!

  • 1
    I keep trying to visualize what the OP is asking -- if you have the time, adding his proposed (but highly discouraged!) diagram would be educational!
    – rrauenza
    Commented Feb 4, 2019 at 23:21
  • 1
    @rrauenza in my second diagram, OP wants neutral to leap the "great wall" and avoid neutral looping down to the switch and back. Commented Feb 4, 2019 at 23:26
  • @Harper - thanks so much, that's very helpful! Your diagram is what I eventually realized (after your comment to my question) might be possible. My follow-up was going to be what I should do with the two neutrals in the first box - but you've answered that, too, so thank you again! But my question now is whether I should mark the two neutrals somehow in the box to make it clear they shouldn't be joined. If so, is there a convention I should use for that?
    – Michael K
    Commented Feb 5, 2019 at 0:14
  • @MichaelK the best option is gray tape or shrink tube. White would be legal but would not be seen. You can't use any other color because in cable, marking white a hot color makes it a hot. Commented Feb 5, 2019 at 0:36
  • One last question for @Harper: What about for true grounds (green, bare copper)? Is it ok to join them without regard to a Great Wall? That is, to combine all grounds in every box?
    – Michael K
    Commented Feb 5, 2019 at 17:28

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