GFCI (panel) breaker trips on neutral connection

Question

I have a circuit previously with a regular 15A breaker, powering some lights, a closet outlet, and two (tile) floor outlets. This latter is now going to power a large aquarium. Because these outlets are recessed into the floor (and wet mopping could conceivably get into them), I think they probably should have had GFCI protection all along, but I definitely want it with the Aquarium.

The house dates from 2005, by the way, so the wiring is not particularly old, and was (hopefully) done to a recent code.

Rather than guessing what was upstream of what, I opted for a panel breaker. It's Cutler Hammer style, and I added the breaker, hooked the white from the load to the white terminal, the black to the black, and the pigtail back to the neutral bar where the original neutral was (and indeed it was there).

Note the circuit works fine with the regular break, but trips immediately on reset. I've unplugged everything but the lights and it still trips. Perhaps more interestingly, it will trip with the hot disconnected from the breaker, but just the neutral connected.

The breaker does not trip with no neutral and no load black connected (but with the pigtail connected).

Here's my guess -- somewhere in the house, no idea where, the installing electrician connected this neutral to some other circuit's neutral. Both may be still connected to the panel, but due to distance the extra neutral is causing a problem.

Or guess #2 - somewhere in the house, on this branch circuit, the neutral is hitting the ground (though frankly I don't think it would cause this symptom of tripping with the neutral-only connected to the breaker).

Does that sound correct? If indeed this is the problem, it is pretty hopeless trying to find this, some wiring is in the slab, some in the attic (much of which is pretty inaccessible).

Or is there something else I can check?

The obvious thing is start disconnecting circuits, but because of the light/outlet mixture that's easier said than done.

Lacking a better idea I'm going to put a GFCI on the first outlet in the floor -- it won't really protect that outlet (or more precisely the wire leading to it), but at least it will protect the aquarium equipment itself.

Any ideas what it could be?

Answer 1

A GFCI uses a current transformer (CT) to detect differences in current between the ungrounded (hot), and the grounded (neutral) conductors. Since the GFCI is tripping with only the grounded (neutral) conductor connected, that means that there's current on the grounded (neutral) conductor. This would lead me to believe, that this neutral is tied to the neutral of another circuit somewhere.

You'll want to make sure this isn't part of a multi-wire branch circuit. If the neutral in question enters a cable with two hot conductors, it's likely a multi-wire branch circuit.

If not, you'll want to look for places where this circuit might be in close proximity to other circuits. A multi-gang box with two or more devices, would be a prime suspect. If you find such a location, you'll want to open it up and make sure the grounded (neutral) conductors are separated properly. It's common; especially with DIYers, to simply connect all the grounded (neutral) conductors in a box together. However, this can lead to the exact problem you're describing.