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We called out the contractor who did our crawlspace encapsulation because the GFCI receptacle that serves the crawlspace loads (dehumidifier, sump pump, and two humidistat controlled fans) trips occasionally - I think this happens during the rainy season when humidity is high during the rainy season, but since the battery backed pump sounds an alarm when it loses power we disconnected one of the fans which stopped the tripping.

The contractor is putting forth the theory that since it is a 15 A circuit (14 AWG NM cable and a 15 A breaker) there may be an overload if all of the devices startup at about the same time and this may cause the GFCI to trip. That makes no sense to me because I don't think GFCIs work that way - but he is adamant that he has seen this happen.

I'm thinking that we must be seeing some leakage from one of the fans or the humidistat - but I don't want to be going off half-cocked, my years as an electrician are long past and I never did residential work.

Could startup loads from the motors down there be the cause of the GFCI tripping?

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Overloads don't trip a GFCI. Period. If there was an overload, it would trip the actual circuit breaker. Since this is a GFCI/receptacle (as opposed to a GFCI/breaker) and the GFCI trips, that is a GFCI problem - 100% - and not an overcurrent situation.

14 AWG raises the question of the size of the breaker. If it is 15A then everything is good. If it is 20A then the breaker is oversized for the wires. If that is the case, the breaker should be changed to 15A. That is a general (fire) safety issue, unrelated to the GFCI issue. But even if it is 14 AWG on a 20A breaker with 18A flowing through (therefore over the allowed safe current for the wires but not enough to trip the breaker), it would cause overheating of the wire and potentially a fire - but would probably not trip the GFCI.

Now back to the real problem. Something is tripping the GFCI. Most likely that is some current leaking to ground. That is what GFCI is designed to protect. That also perfectly matches when humidity is high during the rainy season. When water gets in places in electrical devices that it shouldn't, electricity can move in unwanted pathways. It takes a lot of current to trip the regular breaker. It only takes a little to trip a GFCI or to injure or kill someone.

The good news is that it sounds like you already found the problem: "we disconnected one of the fans which stopped the tripping." If that's a consistent/reliable fix then the problem is either in that fan or the wiring going to it.

As far as startup loads: anything is possible. GFCI and AFCI both have sophisticated electronics. A GFCI should be relatively simple and immune to anything except actual current leakage, unlike AFCI which is trying to detect some relatively hard-to-detect problems and could be "confused" more easily by motors 'n things. However, newer GFCIs with automatic testing have more sophisticated electronics - and therefore are more susceptible to problems from other causes.

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    The issue could also be that there's a small amount of ground current leaking through more than one device, with no single, or two, devices being sufficient to trip the GFCI on its/their own. While it's likely a single device/wire run, the conditions of "humidity is high during the rainy season" make ground current leakage more likely everywhere. Disconnecting one device stopping the tripping shows that some of the leakage current is flowing through that device. While using instruments to test each would be best, it's also possible to connect only one device to see if the tripping occurs. – Makyen Aug 6 at 16:19
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    Based on my own understanding and your backup I pushed back pretty hard on the "tripping on overload" theory and low and behold they found that one of the extension cords they had "installed" to bring power to the suspect fan was showing evidence of having been sitting in water - the "waterline" showed immersion of both the ground and the hot conductor. I think we know the culprit. I'm not sure how they had managed to leave the plug end of the cord sitting on the ground - or how I missed it, but that is another question… – dlu Aug 6 at 19:19

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