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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.

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  • Can you replace/re-run any of this wiring? Also, can you provide photos of the wiring in the switch boxes?
    – ThreePhaseEel
    Aug 5, 2017 at 4:47
  • This is a case of a GFCI working as designed and protecting you from someone else's screw-up. Neutral is not Ground - they are not interchangeable.
    – brhans
    Aug 5, 2017 at 13:15
  • I can't really replace any of it without tearing the wall open. But I may have an idea of the problem: could a ground loop cause the AFCI to trip? I made one when I attached the new ground; it created a rectangle about 3ft x 10ft. I'll have to figure out how to get pictures here, so I'm going to look into the ground loop issue first. Thanks!
    – cstarr
    Aug 5, 2017 at 13:19
  • brhans -- Right; I fixed that problem. My question is about a second ground-to-ground connection causing a trip. Thanks!
    – cstarr
    Aug 5, 2017 at 13:21

1 Answer 1

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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:

  • always-hot
  • messenger
  • messenger
  • neutral
  • ground

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:

  • Always-hot
  • Neutral
  • Smart switch control/communication
  • Ground

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.

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  • Thanks for the reply. Just for the record, I'm not considering it normal -- I know it's wrong, and I know that's incompatible with GFCI. What I ended up doing was running 12-2 from the outlet below the switch on the left. (The ground/neutral connection was in that box, not the one on the right.) I joined that to the switch loop for the light and completely separated the 3-way switch from that. The thing I'm trying to figure out is why joining the grounds from those two loops caused a problem. I think it's because it creates a ground loop -- can you confirm or deny? Thanks again.
    – cstarr
    Aug 5, 2017 at 19:08
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    No, the issue is that you still have a neutral tangled up with a ground somewhere. Since absolutely no current whatsoever flows through ground under normal conditions, a loop would be irrelevant. It's also possible you have an unrelated, bona-fide ground fault in one of the switches or lamps.
    – Harper - Reinstate Monica
    Aug 5, 2017 at 19:17
  • I think I traced the whole circuit and only found the one neutral/ground connection, and I've fixed that as noted. Everything works fine now with the AFCI/GFCI breaker. The relevance of the loop (potentially) is that it allows for an induced current to flow in the ground. It wouldn't be much, but it seems that it was enough to trip the breaker. The only difference between what I have now and what tripped the breaker is closing that ground loop (no neutrals involved). Still a puzzle! Many thanks for your help, though. BTW: I've never heard of a smart switch. Now I want to go read about them!
    – cstarr
    Aug 5, 2017 at 19:30
  • 1
    A GFCI device will be completely oblivious to any current flow on the ground wire, since it is not connected to the ground wire in any way. Check it out, it's not!
    – Harper - Reinstate Monica
    Aug 5, 2017 at 20:18

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