Scenario: In my guest bathroom there is a light fixture box, outlet, switch for the fart fan, and switch for the light, each in their own box - aside from the two switches, which are together in one box. The circuit is fed by a 12/3 coming from the main panel that enters the light fixture box. One hot is being used to feed a different circuit on a different breaker, the remaining hot is used for the bathroom. There is another 12/3 between the light fixture box and the outlet box, then another 12/3 between the outlet box and the switches box. One leg of these is being used to carry the hot into each box, and the other is being used as a switch leg to get back to the light fixture box from the switches boxProblem: I recently replaced the normal outlet with a GFCI, which I have wired correctly. When I flip the switch for the light, it trips the GFCI. Switching on the fart fan and plugging in a load to the outlet doesn't cause a trip. After much metering and labeling of wires, I've discovered that when the hot wire in that first 12/3 run is connected to the feeder from the main panel, the switch leg lights up a hot stick and meters at 5v, which I assume is what is tripping the GFCIAny clues?
the operation of a GFCI is as such: a toroid current transformer encapsulates the grounded and ungrounded conductor, and the electronics in a GFCI will cause the GFCI to trip when there is a current of 4-6ma or higher being detected by the current transformer. the idea being that if there is a difference in current between the grounded and ungrounded conductor, current must be taking another path back to source, ie a ground fault. the reason you are having issues with a multiwire branch circuit is because there are two ungrounded conductors and one grounded conductor. the grounded conductor is carrying the difference in load between both ungrounded conductors, which is being detected by the CT, causing the GFCI to trip. the way to alleviate this is to either run a new branch circuit that only feeds your bathroom, or install a 2 pole GFCI breaker in the panel the multiwire branch circuit originates from. the GFCI breaker will be simpler, but they are a lot more expensive than normal breakers.
>>2714688>one hot is being used to feed a different circuit on a different breaker??? Are you talking about a MWBC? Those breaker handles need to be tied together, it’s better to think of it as a single circuit. Better to get one 2-pole GFCI/AFCI breaker and be done with it. Wiring MWBC with gfci receptacle can be tricky.
>>2714688If you are feeding the light switch from the load side of the GFCI, the neutral for the lamp also has to be tied to the load side neutral of the GFCI. Basically, unless you ran 14/4 wire from the drop to the outlet, you can't.
>>2714688>One hot is being used to feed a different circuit on a different breaker, the remaining hot is used for the bathroom.
>>2714694I replaced all my breakers with arc/ground fault breakers because I care about my family and don't want them to burn or die.
>>2716178this is bait