While working a bathroom remodel I encountered an odd issue. The lights are on a 3-way setup with one switch at each end of a walk-through bathroom. Housing the switch at one end is single-gang box also being used as a junction for two other circuits --one feeding the GFCI receptacles in the bathroom, and a completely separate pass-through circuit to the hall. Needless to say, the box is overcrowded. The 3-way circuit and the GFCI circuit are powered using a single 4-wire cable that enters from the breaker panel, with one hot for the lights and one hot for the GFCI. Both circuits share the remaining single neutral and ground wires. All the neutral and ground wires in the home are connected to the same bus bar. This 4-wire cable enters with a second 4-wire cable that powers the hall circuit. Because the entire box shares the ground wire of the first cable, this second cable has two wires not being used. After much frustration, I eventually figured the voltage found in these unused wires was a phenomenon of magnetic induction (which I assume everybody agrees).

The phantom voltage on these two wires is as follows:

Wire A to ground is 68v

Wire B to ground is 47v

My question is regarding the phantom voltage of 115 between wires A and B, which matches the mains, except no current. [EDIT: I should have pointed out 68+47=115] Is this coincidence, and what useful information can this tell me? My gut is saying it has to do with opposing phases of the 220v mains but pffft, I don't know. Please pardon my wayward terminology, I'm not a formally trained electrician but I've read through this forum and it appears some of you will know what's happening here.

Thank you in advance, Handyman in Oregon who hates when he can't figure stuff out.

  • 1
    "Two wires not being used" is very bad news. One of the most basic rules in electrical wiring is "currents must be equal" or "current must go back the way it came". If you put a clamp ammeter around any cable, it should read zero. It's possible that some yutz saw what you did, that ground and neutral land on the same bus in the main panel, and decided they were interchangeable elsewhere too. They are not. Ground should never flow current. Now about that 4-wire (/3+ground) cable -- it has two hots, black and red. Do they go to different breakers in the main panel? – Harper - Reinstate Monica Feb 1 '18 at 23:29
  • @Harper I think the house was wired using surplus or recycled wire, as nobody would run 10/3 to power an outlet or light, it's not economical. I agree it is bad form to have separate breaker circuits share a neutral wire but since the gauge of the wire was sufficient to carry the load of both circuits, I left it as I found it. I did not find any ground wires being used to carry current. All three hots entering the box have their own 15A breaker. The bathroom doesn't need 30A but it did before I removed a space heater installed in the ceiling. I replaced switches, lights. Didn't change wiring. – Nathan Feb 2 '18 at 0:01
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    "bad form to have separate breaker circuits share the same neutral wire" What makes this bad form? We call that a three-wire circuit. Each hot has to be on different phases of the panel bus bar and the when doing this the AC wave is 180 degree out of phase and thus they cancel on the neutral. If both legs were carrying the exact current it completely cancels on the neutral. If you screw it up and put both hots on the same bus then the waveforms add together and you can overload the neutral. – C Knight Feb 2 '18 at 2:31
  • @Nathan Anytime you learn about electrical from Google, you get swiss cheese knowledge because you simply don't know which questions to ask. (Books, read through, solve this problem.) If you want to google "multi-wire branch circuit" it will fill in a little more of the swiss cheese. There is nothing wrong with them if they are competently installed. If not, they're a death trap. I am asking about yours so we can render it safe. – Harper - Reinstate Monica Feb 2 '18 at 4:24
  • Which isn't too hard, it may involve a $9 breaker or a $3 handle tie. – Harper - Reinstate Monica Feb 2 '18 at 4:33

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