I'd appreciate help on an odd problem. I'm pretty good with electricity but stumped on this.

I installed a GFCI in the middle of a 120V outlet circuit. It works correctly, protecting itself and all the downstream load outlets.

But, on a completely separate circuit (separate breaker), I also have an outside floodlight set on a pair of three-way switches.

When I switch on the outside lights on this circuit, the GFCI I installed on the other outlet circuit trips.

Two different breakers. I understand how (say) an open ground or grounded neutral could pop the GFCI -- but how is the possible on a separate circuit?

The electrics were installed new in 1995. The outside lights were probably installed a couple of years later, before I bought the place.

Any ideas?

Thanks--

Terry

The grounded neutral is most likely the cause, which when then happens, with the neutral being a current carrying conductor, can send current back into the grounding system, or vice versa, and find the GFI, since all grounds and neutrals are connected together at the main disconnect.

Since the floodlight is what causes the GFI to trip, this circuit should be completely examined , looking for miswired splices, splices that came apart or shorted in a junction box, a staple that is too tight and pinching the cable, things of that nature.

I would also be checking the main panel and service grounding to be sure that it is done properly and in good condition.

Don't rule out incompetence, either. If someone pulled a shortcut and wired the neutral of the floodlight back into the GFCI protected circuit (but still used a hot wire from a different circuit), that would cause it to trip. Follow tkrussel's advice, though, and thoroughly examine the floodlight circuit.

(And yes, this did happen in my house. Neutral from one circuit used with hot from another. Don't ask)

FWIW, I had something similar. 12-3 to a far end of the house. 115V outlets on each side. Put two GFCI's on each side, and the remaining outlets on either GFCI leg. False trips all the time. I reran two 12-2s and everything worked fine.

Go figure.

Quote:

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Originally Posted by tbentley

I'd appreciate help on an odd problem. I'm pretty good with electricity but stumped on this.

I installed a GFCI in the middle of a 120V outlet circuit. It works correctly, protecting itself and all the downstream load outlets.

But, on a completely separate circuit (separate breaker), I also have an outside floodlight set on a pair of three-way switches.

When I switch on the outside lights on this circuit, the GFCI I installed on the other outlet circuit trips.

Two different breakers. I understand how (say) an open ground or grounded neutral could pop the GFCI -- but how is the possible on a separate circuit?

The electrics were installed new in 1995. The outside lights were probably installed a couple of years later, before I bought the place.

Any ideas?

Thanks--

Terry

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Some where in your house you have ground and common connected... ground and commond are only to be connected in your main box... no where else in the wireing