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I have a three story house. The first floor is an apartment that is completely covered by the second floor. It has it's own panel which is inside the apartment and mostly uses GFCI breakers. There is a circuit that just provides power to a row of ceiling lights on the first floor, and it often trips during heavy rains (and can be immediately reset).

I plan on replacing the GFCI breaker, I already had one that was getting flakey and was tripping whereas it's replacement (also GFCI) would not.

But I can't figure out where the connection to the rain comes in. The first floor is recent construction and there's nothing on that circuit that leads outside or powers anything outside. The two floors above are dry, so unless there is a stream of rain sneaking down two floors along the inside of the walls and getting into the ceiling circuit, all without causing any visible damage, I can't quite see how the rain is the cause.

Is it possible that the breaker is getting flakey and it's just the general humidity that's causing it to trip?

Any other possibilities that I am overlooking?

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    Does any of the wiring on that circuit go through the crawlspace below the floor? It can be worth opening up the boxes along the circuit and see if any of them have water inside -- note that water can sometimes travel to an indoor box by wicking through the cable itself due to capillary action, if the other end of it is exposed to water. – Nate Strickland Apr 16 at 17:46
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    @NateStrickland Yes, that "wicking" is why NM cable is banned outdoors. Inappropriate use of NM where UF should be used could be responsible for this. – Harper Apr 16 at 19:02
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People who have GFCI trips are way too quick to blame the detector rather than consider it may be doing its job.

Rain is very consistent with a genuine ground fault. Since NM cable jacket is at least modestly effective at excluding water, you should check each of your junction boxes on the circuit for water ingress. Not because of a two-bit electrical problem, but because the water is rotting out your house.

The "ground fault" part can be tested.

Regardless of load

Ground fault current will definitely be observable if you take the hot and neutral wires and tape them together (so the two wires are right next to each other) and put the clamp meter around that. The amount of load on the circuit won't matter, since magnetic fields from normal current will cancel out (hot current and neutral current should be equal and opposite).

If the ground fault current is returning via a ground wire, you will see it with a clamp meter on the ground wire. The amount of load on the circuit won't matter.

Another good ground-fault test is to plug in a device via a 2-prong "cheater" (takes 3-prong, has 2-prong and a grounding tab). The tab must not touch ground and the device must not touch anything or be wired/connected to anything. If it stops tripping the GFCI in that mode, that means the device has a ground fault.

If all normal loads are removed

If all intended loads are off, you could also clamp the hot or the neutral.

If it's a hot-ground fault, you can also spot it by putting a neon 2-wire voltage tester or other extremely small load in series with the hot or neutral on a circuit with no loads attached. (I don't know if the fault is on hot or neutral). The current flow will make it light.

Dimmers are loads. Did they bootleg ground?

Keep in mind dimmers, motion sensors, lighted switches, non-hand-crank timers, smart switches etc. are loads themselves. Most of them need neutral, and often, when a neutral wire is not present in the box, the installer will bootleg neutral off of ground. This is a ground fault and will trip a GFCI. And such loads are typically so small that they are barely at the threshold of detection for GFCIs. As such, they don't reliably and instantly trip.

Anyway, if you had any of these ground faults, then all you did by changing GFCI is get a duller GFCI (higher threshold of detection). You should still check for, and fix, any ground faults.

  • I should add and clarify a few things: First, I've had to swap two of my identical GFCI breakers already, because one of them was tripping in the bedroom anytime we plugged in a newer, hi quality UPS. Swapping the identical GFCI breakers fixed that. I suppose it's possible there's a ground fault issue with that UPS, and it was just on the boundary of the two GFCI breakers, but I'm skeptical of that and starting think that some of the breakers are lower quality and reaching end-of-life – David Ljung Madison Apr 17 at 21:02
  • Secondly, I haven't ripped the walls out, to see where the wiring goes, but it seems really hard to believe that water is making it's way down two floors and then into the wiring (which was done to code about 20 years ago) and then across the ceiling or into the circuit somehow. All of this without any water damage showing on the ceiling or ending up in the crawlspace underneath the first floor which is dry? I'll check the dimmers and see how they are wired, and I'll look at the rest of the tests, but I did want to somewhat justify why I'm more skeptical of the GFCIs than the average bear. – David Ljung Madison Apr 17 at 21:04
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    @DavidLjungMadison You're not more skeptical than the average bear. Everyone is skeptical because ground fault problems feel alien and difficult to fix, so people try to deny them. But they're not difficult to detect; and if you haven't tried, how can you say it's not a ground fault? – Harper Apr 17 at 22:28
  • I don't find them alien - I just don't trust the breakers since replacing one solved a problem on a different circuit. I suppose that could have been a threshold change, but there was really nothing on that circuit that should have caused any leakage... – David Ljung Madison Apr 19 at 3:14
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    @DavidLjungMadison Yeah, the names really don't help make it clear AFCI and GFCI are totally different things. Yes if these are AFCI breakers than my advice is invalid. – Harper Apr 29 at 19:26

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