GFCI wiring problem

8 years ago

I've been slowly wiring the receptacles in the unfinished basement of my vacation home, each future room to have its own breaker and GFCI receptacle. No problem with the first three, but the fourth fails in the following way: the first receptacle is a GFCI and all the rest are "slaves". Before closing the wall, I checked all receptacles with a 3-light circuit tester (to check for missing grounds and/or reversed wires) and all passed. After putting up wood paneling as a finish wall surface, I plugged a shop vac with a 2-wire cord into one of the downstream receptacles and the GFCI tripped. The same thing happened with the other downstream receptacles, but not with the GFCI itself. I opened up all the boxes and did a visual inspection and cannot find anything wrong. Short of unwiring everything, any suggestions as to what to look for? The ground wires on the connecting cables are connected to the metal boxes and the ground connection on the receptacles themselves.

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Comments (3)

Ron Natalie
8 years ago
If the downstream receptacles trip only when a load is connected, I can almost guarantee that you didn't connect BOTH the hot and neutral to the "load" side of the GFCI. If you connect he neutral to a way that bypasses the GFCI (i.e., to the incoming feed or possibly to some other grounded point), it will exhibit this problem.

Of course, there's a slight possibility that your shop vac has a ground fault.

If the GFCI would trip even without a load, I'd have guessed you had a neutral-to-gorund fault somewhere in your wiring (the neutral wire or the terminal on the receptacle is touching the metal box or the uninsulated ground wire).
mtvhike thanked Ron Natalie
mtvhike
Original Author
8 years ago
Thanks, Ron. Remember, the shop vac is only 2-wire, so it cannot have a ground fault. Actually, since I posted this, I discovered the problem: the neutral wire on the load side had pulled out of the receptacle and shorted to the metal box. Since the GFCI receptacles were working, I didn't look there yesterday. After reconnecting this wire, everything is working correctly. I still don't know why my 3-light tester didn't pick this up - maybe it cannot see neutral-to-ground shorts.
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Ron Natalie
8 years ago
last modified: 8 years ago
I said it is unlikely, but it is not IMPOSSIBLE. If one side of the vac line is connected to exposed metal, there may be enough leakage to trip the GFCI. In fact, this is EXACTLY why GFCIs exist to begin with.

3 light testers are stupid. All they do is look for roughy at least 90V between each of the pins. The HOT to GROUND and HOT to NEUTRAL should light. The GROUND to NEUTRAL should not. It has no way of differentiating ground and neutral (let alone tell if they are connected).