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Is it possible for a black wire to be hot, but to cause reverse polarity in a GFCI when it's connected to the brass screw?

A no-contact voltage tester shows that the black wire is hot. The white wire and ground wires do not trigger the voltage tester at all.

Yet, when I connect the black wire to the brass screw on a new GFCI receptacle, and the white wire to the silver screw, and the ground wire to green, the polarity is reversed. The no-contact tester shows that the large slot on the receptacle is hot, while the small slot does not trigger the tester.

I've tested with multiple GFCIs, so it's not a case of a bad receptacle.

Likewise, putting a different, LED contact tester across the white wire and ground wire will cause the tester to light up. Putting it across the black wire and ground wire does nothing.

How is this possible?

  • Hot and neutral are reversed upstream of this GFCI. Believe your LED tester (with actual contacts) over the "non-contact" one in this case. – Ecnerwal Nov 17 '14 at 3:54
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As I've said elsewhere on this site, I find no-contact voltage testers to be unreliable. I've seen both false positives and false negatives with mine, especially in the presence of multiple conductors, some of which are hot. The only thing I would use a no-contact voltage tester for is to confirm a condition I already know, and even then I would only use it to test if an entire circuit is live or not, not individual conductors.

If you are getting confusing readings, the first thing I would do is use a proper multimeter to get reliable voltages, then you can go from there.

But to answer your question: no, the GFCI should not be reversing the polarity of the circuit. Are you sure you're wiring up the GFCI correctly? Most (all?) GFCI outlets have different "LINE" and "LOAD" terminals, which are you using?

  • Yes, I'm connecting the wires to the LINE terminals. Black to brass screw (explicitly marked HOT and is on the side of the short slot), and white to silver screw. I'm tempted to agree that something is wrong with the tester. But the no-contact tester shows the expected results on all other wires I test in other receptacles in the house. Likewise, the contact tester (two pins that I touch to wires or push into the slots) works as expected in other receptacles. It's lighting up when touched to the white wire and the ground -- yet the no-contact tester is clearly saying the black wire is hot. – confusedGFCI Nov 17 '14 at 3:50
  • @confusedGFCI: If you ignore the no-contact tester and just used your wired tester, do you still show voltage on the neutral side of the outlet? Have you tried this with more than one GFCI outlet? – Hank Nov 17 '14 at 3:54
  • Yes, if I install the GFCI with black to brass and white to silver, then the wired tester will light up if I touch one pin to neutral and one to ground. I've tried multiple GFCIs in this box and the same thing happens. – confusedGFCI Nov 17 '14 at 5:13

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