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I'm in the process of finishing my basement, and am almost done with the electrical rough-in. The whole thing is open studs at this time, so any sort of reconfiguration is easily possible.

I have a 20amp AFCI circuit breaker supplying the bathroom's GFCI receptacle, all with 12 gauge wires. This receptacle then supplies power to the light and fan via a double gang box.

Intermittently and randomly, the GFCI trips itself, but only when either the fan or light switches are toggled. It can go several dozen toggles with no issue, and then suddenly trip. This can happen either with the fan or light switch; I haven't had any luck identifying exactly why this happens.

The GFCI outlet is around 7 years old (came with the house; I repurposed it in the bathroom). Load and line are set correctly, and the switches are also wired correctly.

Is there anything I need to specifically look for, to help identify why this happens, and to avoid it in the future?

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I suspect the electronic circuitry of the AFCI is drawing some quite small current from hot to ground, which looks like a ground fault to the GFCI. This current is probably not enough to trip the GFCI normally, but when switching an additional load on the protected side of the GFCI, there could be some transient currents from hot to ground that take the GFCI over the trip threshold sometimes Remember, the GFCI is supposed to trip with as little as 5mA of ground fault current.

Try connecting the light and fan to the line (unprotected) side of the GFCI. I don't think code requires the fan and light to be protected by the GFCI in any case, just the receptacle.

  • Even if the Arc fault were to be drawing current, it should be equally on both hot and neutral conductor. FYI, all bath exhaust fans within the tub or shower "zone" are REQUIRED to be GFCI protected. – Kris Oct 19 '16 at 11:07

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