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I have a junction box in my garage with three light switches. They are all on separate circuits and controlled by separate circuit breakers. I replaced one of them with a switch/outlet combo.

All the neutral wires that come into the box are twisted together with a wire nut capping all of them together. Do I have to pigtail a single wire from the capped group of neutrals (none of them is connected to any of the other switches) or can I just select the neutral wire that is associated with the black hot wire that I have connected to the new multi switch/outlet?

Location: North Carolina, USA

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  • Are you sure the switches are all on separate circuits? If so, the neutrals shouldn't all be tied together...
    – mmathis
    Dec 4, 2017 at 16:08
  • If the different lights are indeed on different circuits then the original installation is flawed. The neutrals from different circuits should never inter-mix.
    – Paul Logan
    Dec 4, 2017 at 16:16

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You must separate the neutrals completely. Neutrals must be monogamous only with their individual hot. That is a gravely defective installation. If any neutral wire breaks, the hots together will overload it.

No need to untangle the Gordian knot right now, there's a simple workaround. I once had an installation where I had 5 hots on 2 neutrals. (4 is feasible if two MWBCs are exactly correct, but this was not "exactly correct"!) As a simple fix, I moved all the hots to one breaker. This removed the overload issue, and for me since I was in conduit, was a compete solution. For you, you'll still have neutral current returning on a different cable than hot current, and that needs fixing. But it'll buy you time.

If you have one cable with two hots and one neutral, wired like a 120/240V circuit except it splits into multiple branches, that is called a ”multi-wire branch circuit" or MWBC. If you have an existing one, learn all its tricks and traps. Please don't create any new ones, as they are obsolete. (they don't play well with GFCI/AFCI)

GFCI breakers are very good "detectives" for this, they will trip if there is any neutral hanky-panky. Being in a garage, they would need GFCIs if they were new work. Fixing wiring defects is not new work.

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    Also, other than not being electrically acceptable, it plays hell if anyone comes on and tries to troubleshoot a problem. Count at least doubling your labor.
    – Retired Master Electrician
    Dec 4, 2017 at 17:43

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