I recently installed some AFCI/GFCI circuit breakers. One of them kept tripping immediately, so I tracked down the problem to where a neutral and ground were joined. (There was just the one neutral in that box.) To confirm that this was the problem, I disconnected the neutral from the ground and ran a neutral wire to another nearby box in the same circuit and connected it to neutral there. Problem solved!
Almost.
S---------L
|
S3------S3
|
|
O--------O
It looks like the above. The outlet (O) on the left is fed into from earlier in the circuit. It connects to the right, which connects up to a 3-way switch (S3), which connects left to the other 3-way switch, which then feeds a regular switch (S) to a light (L) (independent of the 3-ways). S3 on the right has an isolated neutral from O that is capped off. S3 on the left therefore has no neutral to contribute to the S and L part, so they just tied the neutral there to the ground, which was my problem.
To test this, I ran a neutral between the left 3-way switch and outlet, joining their neutrals. That's what worked. But when I ran the 12-2, I also joined their grounds, and that caused the AFCI/GFCI to trip immediately when I turned it on. (Actually, with a half-second delay — it tripped immediately if there was any load on the circuit.) And if I disconnect those grounds it works again.
So there's my question: why would having an extra ground there be the difference between AFCI/GFCI tripping or not tripping?
The bottom receptacles are all on that circuit, too, but always hot (not switched).
It's all working, so this isn't urgent, but I'm baffled and would like to know what's going on!
Thanks.
Best Answer
The existing wiring is wrong
Obviously someone added the last/top switch and lamp to the end of the 3-way. It was just a switch loop. So rather than run proper cable, they misused ground as neutral, to free up a wire for always-hot. Your last switch circuit grabs onto the bootleg neutral and always-hot.
This wiring is illegal and wrong, and is inherently incompatible with GFCI.
The choke point is the /3 (4-wire) cable between the two 3-ways. You really need:
Unfortunately you are considering this wiring "normal" on the logic that "it works", and figuring there is nothing wrong with it, so it ought to work with GFCI. And expending a lot of brain power trying to figure why it isn't. This has also poisoned your mind from having good discipline about separating neutral and ground. They should never be connected (except at exactly one place.)
No brain power needed; there's a neutral-ground fault at the first (right, upstream) 3-way. That's because neutral and ground are wire-nutted together.
How to make this fiasco work
Run additional cable. Between the 3-ways replace the 12/3 with 12/4, allowing hot, neutral, 2 messengers and ground. Or install a parallal 12/2 to provide hot neutral and ground to the non-3way switch.
How to make this fiasco work without running additional cable
Smart switches. You need a multi-way smart switch. Whichever light is controlled by the 3-ways, its wires come into one of the 3-way locations. That is where you put the "master" smart switch. The "remote" goes in the other 3-way location. The "choke-point" of /3 cable becomes:
This is a relatively easy solution for a smart switch. We aren't a shop-for-me site but the right switches should be readily available.