Electricians - GFCI breaker problem

Factual Questions
1

I’m pretty sure I know what the problem is but I wanted to run it by those with some actual expertise…

As part of replacing the ancient lights in a bathroom I swapped out the old circuit breaker with a GFCI breaker (old post-WWII construction, so it’s early-type romex and no ground, wet location). It works fine, however several times the breaker would just trip after the lights were on for a while.

Finally tracked down the triggering event - if we turn on the lights in the kitchen or TV room that makes the GFCI breaker for the bathroom trip.

ETA - when the GFCI trips it goes to the middle position, not the complete “OFF” position. As long as the TV room lights are on the GFCI will not reset, turn off those lights and we can then reset the GFCI no problem.

The kitchen/TV room are on a different circuit with their own breaker. This was never a problem before.

My suspicion - when I pulled the old bathroom breaker out and installed the new GFCI breaker, I connected the GFCI pigtail to the panel neutral bus (as specified), unhooked the hot wire from the old breaker and connected it to the hot terminal on the new breaker and then located the neutral for that same circuit and moved it from the panel neutral bus to the neutral terminal on the GFCI breaker (again, following the installation instructions). I have a feeling that I may have accidentally pulled the neutral from the TV room circuit and plugged that into the GFCI neutral - I tried to be careful about locating the right wire but there are several cables in that spot.

The fact that the TV room circuit breaker is located directly below the bathroom breaker would make it easier to pick out the wrong neutral.

Would hooking the wrong neutral into the GFCI cause this behavior (GFCI trips when the circuit that actually uses that neutral gets a load - e.g. turn on a light switch)?

Note - while I am not an electrician I have done a substantial amount of wiring on my home as part of remodelling including installing a subpanel, running multiple circuits and so forth, I’m quite comfortable with this stuff. I have a feeling I just made a simple mistake which I’ll be able to identify when I get home today (shut off the power, open up the panel and recheck those wires).

2

Connecting the wrong neutral to a GFCI will certainly trip the GFCI if any current flows through that neutral since the currents in the hot and neutral from the GFCI’s point of view will be different. Any current through the hot would also trip it though since that would be coming back through a different neutral.

Is the bathroom light on the GFCI circuit or just the bathroom outlet(s)?

Based on your post, it seems like you think the GFCI is connected between the hot going to the bathroom and the neutral of another circuit, but the GFCI trips only occasionally. That doesn’t make much sense to me. The GFCI should trip continuously whenever there would be current through either the hot or the neutral if they are both from different circuits.

3

The bathroom light and the bathroom outlet (only one) are both on the GFCI breaker. There is an adjacent bedroom which is on the same circuit as well (it’s my stepson’s bedroom, and to be safe I unplugged/turned off everything plugged into the outlets in his room to check whether that was causing a problem, it’s not).

When I say that the GFCI breaker trips occasionally I mean that it happens immediately when the TV room lights are turned on - hit that switch (or another switch on that same circuit, it also feeds a kitchen light) and you can hear the click as the GFCI breaker goes off.

4

If all you did was grab the wrong neutral, then the GFCI would trip every time you turned on something on the other circuit.

However, it would also trip every time you turned on the bathroom light. If I understand your posts correctly, that’s not happening.

You’ve got something else going on here.

5

if the neutral to the bathroom was joined to another circuit after the breaker box then it might cause what you are seeing.

6

Correct, the bathroom light stays on just fine as long as I don’t turn on something that’s on the TV room circuit.

Some other things that came to mind:

  1. Like I said there is a lot of ancient wiring in the house and perhaps the GFCI is hooked up 100% correctly and it’s just revealing the presence of a pre-existing problem. Exactly what I dunno.

  2. Electrician who wired the main panel messed something up - what might have been done that only is an issue now that there’s a GFCI breaker will take some diagnosis.

  3. Problem with the wiring of the TV room circuit - again, this would require that the problem be such that it’s only an issue when the bathroom breaker is GFCI.

  4. I messed something up when I installed the new bathroom light. That was my first thought but it’s pretty simple stuff and it doesn’t touch anything on the TV room circuit. If I did something boneheaded in the bathroom I would think that the new light would either work or not work, it wouldn’t trip when the TV room light is switched on.

7

Hm, I can see that…what would be a good way to test for this condition? For example:

Turn off power to panel.
Unplug b/r neutral from GFCI (leave GFCI pigtail alone).
Put continuity tester from b/r neutral to neutral bus.

If I see a light, that indicates that the bathroom neutral is connected to another neutral in the house which is not great.

Can confirm it’s specifically connected to the TV room neutral by unplugging the tv neutral from the neutral bus, then check for continuity between b/r and tv room neutrals.

The problem is then locating and fixing that link. Hopefully it’ll be in a box (switch or outlet) somewhere along the way as opposed to sealed up in the wall…

8

Yep. Google “shared neutrals ground fault” (without the quotes).

9

And just to complete the circuit (snort) it was indeed a shared neutral.

Of course now that I knew what to look for it was obvious - the two breakers in question are fed from a 3-wire cable. Thank you all for your advice!

Next time a remodel project rolls around that crap is getting fixed. I’m sure doing it that way saved somebody a little time decades ago :frowning:

10

Can it be “fixed” by getting a double pole GFCI breaker for the two circuits sharing the neutral? Is that then safe and to code (assuming no large motors on the other circuit)?

11

Quite possibly however I don’t need a GFCI for the other room and those double-pole breakers are rather pricey (over $100). I’d rather isolate the circuits.

On the other side of the wall from the breaker box is the old fuse box (electrician re-used it to contain some splices when he installed the breaker box); I opened that up and can see where a couple of neutrals are pigtailed together so I’m pretty sure that I can open that up, remove a hunk of drywall (screwed into place and it covers the cables coming out of the breaker box) and then I’ll have ready access to everything.

Assuming that the two circuits are readily accessible there I can fix that easily - I have a feeling the electrician just ran a single 12/3 cable into the breaker box from the splices in the old fuse box. All I should need to do is turn off the power, unsplice the necessary wires, remove that hunk of 12/3 and replace with two lengths of 12/2, one for each of those two circuits.

Of course many projects have started with “This will be pretty simple, I just have to remove/replace failed part X” and ended with me explaining to my wife why it’s going to cost $500, require six trips to the hardware store, one new power tool and replacing the ceiling :smiley: