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How can I determine which neutral in a sub-panel is for a particular (tripped) breaker? We have a sub-panel in the basement for circuits that are supplied by an automatic generator. There are eight breakers in that box, and seven neutral wires on the bar, none of them labeled. (I think one slightly larger wire is for a ganged pair of breakers.)

I've tried disconnecting several of the wires, one at a time, but when I try to reset the breaker, it trips again. Could it be shorting to the bar in general, rather than to its associated neutral? I think there may be a hot-to-neutral short somewhere in the circuit, but I need to isolate the correct neutral before further testing. Something tells me it could involve shutting off all eight breakers in that box, and maybe disconnecting the generator so it doesn't fire up (or at least not supply any juice) when I open the main breaker.

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You should disconnect all power to your sub-panel (turn off your main breaker and your generator). Be absolutely sure the power is off to avoid serious injury or worse. I'd suggest using a voltage sensor pen to verify the panel is cold.

You will need a ohm/continuity meter for this test.

Pop the breaker in question out of the panel, leaving the hot wire connected. Turn on the breaker and test continuity between the back of the breaker and the neutral bus. Since the breaker is tripping, you should see continuity/low resistance. If you do not, it's possible the short only exhibits under higher voltage but more likely, you have a bad breaker.

Assuming you do see continuity, remove neutral wires one at as time from the bus and recheck continuity. When you lose continuity, you just removed the neutral you're looking for.

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First disconnect all power to the panel (including the generator, like you mentioned). If you're sure it's a hot-to-neutral short, you could use the following the find the breaker:

  1. Remove all the breakers from the panel (leave the hot wires connected to the breakers).
  2. Switch the breakers on (still removed).
  3. Do a continuity check between the hot on the breaker and the neutral bar.

If it is indeed a hot-to-neutral short, you should get a "beep" on the breaker that has the short (but not on the others). From here, you can find the neutral by disconnecting neutral wires from the bus until the continuity stops.

Note that this answer only applies when you know there's a hot-to-neutral short on a (only one) circuit.

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    This will work only as long as all loads are switched off on all circuits. Any circuit with loads switched on, e. g. a light fixture, will show continuity. Since the OP knows which breaker is tripping, he's better off only removing the one breaker and disconnecting neutrals until the continuity goes away. Also, he must be sure that all power is removed before starting or else he will destroy his meter and probably injure himself. – DoxyLover Apr 14 '15 at 4:45
  • @DoxyLover, I like your answer better. You want to add it as an answer? If not, I'll modify my answer to reflect what you said. – watkipet Apr 14 '15 at 14:29
  • I do have my "idiot stick" always available as the final check when I'm "sure" I shut everything off. I found out yesterday that shutting off the main breaker at the home's original panel starts the generator. I don't know yet if pulling the lever on the transfer switch will reliably isolate the generator from the breakers. (I don't want it backfeeding into the grid, either.) I've got my multimeters all ready to go, once I'm sure everything's OFF and I've found the correct neutral to test for continuity where there shouldn't be any/much. Thanks again. – Track_Time Apr 14 '15 at 21:20
  • @DoxyLover: Will the "bad" breaker show continuity to the bar in general? IOW, if I label the neutrals 1-7 top-to-bottom and then disconnect them one at a time, the meter will show an open as soon as the correct neutral for that circuit is disconnected from the bar? – Track_Time Apr 14 '15 at 21:40
  • @Track_Time - If you've disconnected the breaker from the panel, then yes. The only electrical path from the breaker to the bar will be through the correct neutral unless you have a short between circuits, in which case, you have a bad situation that really needs an electrician. However, since no other breaker is tripping, I doubt you have this situation. – DoxyLover Apr 14 '15 at 21:58
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First, never disconnect the neutrals -- basic safety rule.

All neutrals are connected together, and also to ground, either at your home entrance and at the utility company, so if you have already disconnect the neutral the short circuit, if any, should be from the phase line to another phase or to the metallic boxes or conduits.

Consider also that some thermal/magnetic circuit breakers can go bad, and trip off even with low amps.

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  • That's the first time (in a lot of researching of this) I've seen anything about not removing neutrals. – Track_Time Apr 17 '15 at 15:31
  • Dang five-minute limit for editing. Re the breaker possibly being the cause, I did order a new breaker (same make/model) "just because." Last night, while doing the tests that DoxyLover and Watkipet suggested (with the panel "dead cold off"), I noticed the breaker had gone into the tripped position. I reset it and then tried moving the lever around to see if it could manually be put into the tripped state, but it wouldn't stay in the middle. – Track_Time Apr 17 '15 at 15:42
  • Glad you found the problem & solved it. – jcatech Apr 19 '15 at 10:25
  • Regarding disconnecting the neutral, that's the latest thing you should do, and only if gonna remove that circuit branch, or you find that the neutral wire is broken or wrongly connected. The neutral is your safety guard. Always disconnect the phases or the return wire (hot) for troubleshooting. – jcatech Apr 19 '15 at 10:54
  • Actually, I'm stumped. I might just close it out, and start anew. I never found which neutral is for the "bad" breaker. While the panel was cold, I did swap the hots (line out) between #6 (problem circuit) and #8. I reset the main and tried again, but while #6 (on 8's line) stayed on, #8 tripped, so I think I've ruled out a bad breaker. Moved 'em back, and decided to check amps on circuit 6, and it was 119-127 amps before it tripped. I have some new resistance readings on the GFI in the bathroom, but I don't know how relevant they are if neutral and ground are still bonded at the panel. – Track_Time Apr 19 '15 at 19:53

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