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Voltage at a GFCI pug in the bathroom goes down 20 volts when I plug in a 1800 watt hair dryer from 120 to around 100. At the same time when I measure the voltage at a plug in the kitchen it goes up 20 volts from 120 to 140.

The bathroom GFCI triggers randomly with nothing plugged into it. The GFCI doesn't seem to trigger by loading or voltage fluctuations.

After checking the breakers and grounds an electrician told me it must be a Edison transformer issue and to call Edison which I did. Edison found nothing wrong yet.

Help

  • I have you tried replacing the GFCI unit yet? – auujay Jun 16 '14 at 19:35
  • Age of wiring, location, would help. – Bryce Jun 16 '14 at 20:55
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    This sounds like a bad or loose neutral. Check all the devices on the circuit, looking for bad/loose connections. – Tester101 Jun 17 '14 at 12:49
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A faulty shared neutral can do exactly that.

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I had the same thing in my house, the upper floor of which was wired long after the house was constructed, and was an obvious case of "homeowner wiring". The upper floor was wired with an ungrounded "Edison circuit": that is, two 120V hots and a shared neutral.

This is a really bad idea for a lot of reasons, but here are just two:

  • Though the voltage on the shared neutral is always zero, the current through a system is constant. Which means that in theory you could end up with a situation where there was 15 amps on each hot and 30 on the neutral. The circuit breaker is on the hots, not the neutral, so it will not trip for an overcurrent situation, even though the neutral might be getting dangerously warm.

If the circuit is wired in this way then it is wired incorrectly; in a correctly wired Edison circuit, the currents subtract rather than add. Every Edison circuit I've seen so far in a real house has been wired incorrectly. If you work out the sequence of events which led to this situation, often you find that the circuit was wired correctly when it was installed, and then a later modification caused it to become incorrect. Use caution when modifying existing circuits if you do not have a complete understanding of the whole system.

  • A GFCI measures the difference in current between the hot and the neutral. I discovered that my upstairs was an Edison circuit when I installed a GFCI -- figuring that if I didn't have a safety ground at least I could have ground fault protection -- and it triggered at random. Clearly the neutral carrying the current for both hots means that it is often going to be unequal to the current on one of the hots.

Commenter Tester101 below notes that this only happens if the circuit is miswired; again, in my house, the circuit was miswired. It might be in yours too.

Fortunately for me the Edison circuit in question was wired up after construction of the house and the conduit it was in was easily accessed. I replaced it with a proper pair of circuits, each of which had a dedicated neutral.

Good luck! These can be a real pain to deal with.

If you're going to be rewiring stuff in the kitchen and bathroom anyways I encourage you to bring the whole thing up to code: dedicated GFCI circuits in the bathroom and kitchens, at least two in the kitchen, and dedicated circuits for high-initial-current inductive loads like dishwasher, refrigerator, microwave, disposal, and so on.

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  • If you're sharing a neutral (aka multiwire branch circuit), the neutral will only carry the unbalanced current. Meaning if you have 15 amperes on each hot, the neutral will have 0 amperes. If the hots are 10 and 12 amperes, the neutral will have 2 amperes. – Tester101 Jun 17 '14 at 12:48
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    @Tester101: Correct me if I'm wrong, but that's assuming that the circuit was set up with the two hots "split phase". Given the idiocy of the homeowner wiring I've had to deal with thus far, I wouldn't be surprised if the two hots came from the same phase. – Eric Lippert Jun 17 '14 at 13:37
  • Maybe, but then the hots would come from separate single pole breakers, instead of a single double pole breaker. Breaker panels are designed so that alternating slots are on opposite legs. So if the breakers are next to each other, they're on different legs. A multiwire branch circuit (or shared neutral circuit) is required to come from separate legs, so without seeing the wiring that's how I would assume it's wired. – Tester101 Jun 17 '14 at 13:53
  • @Tester101: Right, that's how it should be done, assuming you're going to do it at all, which I think is a bad idea. In taking apart my hundred-year-old house I have found many places where the previous owners made let us say questionable choices regarding the wiring. In my particular case the Edison circuit was originally run to a fuse box, not breakers. I don't know how you correctly wire a fuse box with an Edison circuit, but whomever upgraded it to a breaker box did not do so correctly either. Best to assume it is wrong until proven correct! – Eric Lippert Jun 17 '14 at 15:17
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    If the circuit was wired the way you describe, the OP wouldn't see what they are seeing. For the voltage to swing link that, the hots must be on separate legs. If they were on the same leg, you might see the voltage drop at both locations, but you wouldn't see them seesaw. – Tester101 Jun 17 '14 at 17:17

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