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I was testing the GFCI in my garage and the "test" button would not work. The receptacle had power but pushing the test button would not cut power. I turned off the breaker and took the receptacle out of the wall.

The box in the wall has 4 wires coming into from one piece of conduit (no ground wire as conduit has been used throughout the house as a ground). A red and a light gray (almost white) wire were on the "line" of the GFCI and a black and white wire were on the "load" of the GFCI.

To see if the line and load were backwards, I capped all 4 wires, turned the breaker back on and used a non-contact volt tester. To my surprise both the red and the black wires were identified as carrying voltage. Since all wires are in conduit, I thought perhaps the voltage tester was reading phantom voltage.

I bought a new GFCI, turned the breaker off and hooked up the red and light gray wires in the "line" of the GFCI and kept the black/white wires capped. My multitester read 123 volts and the receptacle worked fine. All tests of the GFCI were fine. I then capped the red/light gray wires and hooked up the black/white wires in the "line" of the GFCI. The receptacle worked fine and my multitester again reads 123 volts.

Moreover, in both of these configurations, I am not able to identify a downstream plug or light that isn't working properly. Thus, I am convinced that I have two hot wires coming into the same box and i have no idea why this would be the case.

I'm inclined to keep the red/light gray hooked into the "line" of the GFCI and keep the black/white wires capped in the box. Any thoughts?

  • Can you trace the conduit to the junction box that feeds it? It sounds like there's a wiring error here... – ThreePhaseEel Feb 14 at 23:30
  • If the box had originally a split receptacle with a downstream connection, then wouldn't there exist 5 wires in the box; White, Red and Black bringing power in from the panel, and Black and White continuing downstream? Did you use your digital multimeter on both Red and Black conductors? Or try Red to box (Ground) and then Black to box (Ground). – Chris Taylor Feb 15 at 3:22
  • I haven't been able to trace the conduit to an upstream junction box. – Dean Feb 15 at 4:58
  • I did use the digital multimeter on both red and black and both were 123 volts. I also haven't been able to identify any downstream devices. Harper's theory that the duplex receptacle had one circuit (red/gray) for one sock of the duplex receptacle and another circuit (black/white) for the other socket seems to make sense. I know the prior owner (who was the original owner of the house) did some wiring in the garage. For example, he had wired lights in the ceiling and made connections outside of a box. – Dean Feb 15 at 5:06
  • You really need an actual voltage meter to be testing things test it with one of those and let me know what the actual volts are – Paul Feb 15 at 17:47
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I think I know what happened.

First, having red and gray for a second circuit is perfectly reasonable, I do it all the time in conduit.

Anyway. I suspect the original circuit wiring had a plain duplex receptacle with both tabs broken off. One socket was served by red and gray. The other socket was served by black and white. Why? Because he had 2 large loads he wanted to power -- table saw and dust collector, for instance. But for whatever reason he wanted to git-r-dun on one duplex receptacle instead of two.

Mind you, in such a case, Code requires that the two circuits be on common maintenance shutoff, because both circuits are on the same yoke (i.e. Receptacle body). It is not required for them to have common trip, or be on opposite poles, but you'll get both as side-effects.

When he was selling the house, the home inspector did a walk-through and red flagged the lack of GFCI in that location. He went to Home Depot, bought a $16 GFCI+receptacle combo device, noted the 4 screws on the receptacle, noted the 4 screws on the GFCI, and did the obvious. Didn't work, he couldn't figure out what was wrong, and he just wanted to sell the house... so he gave up. Inspector walked through, saw GFCI, approved.

Now if you don't care about GFCI, go back to the original plan.

If you don't care about the original plan, cap off the black & white and use the red & gray (or vice versa), attaching only to the LINE terminals and leaving the warning tape on the LOAD terminals.

If you do want the full use of both circuits, then:

  • if surface mounted, swap that box for a 2-gang deep (2-1/8") 4" or better a 4-11/16" square large box with appropriate 2-gang cover plate.
  • if flush mounted, get a Surface Conduit Starter Kit, and go off wherever you please (it can be 1 inch) and fit a surface box. Extend the white/black to the surface box.
  • Alternately, use a plain 1-gang extension, a 1" or so nipple to a 4" or 4-11/16" square box.

Then you mount two GFCIs, using only the LINE terminals. Red/gray to one, black/white to the other.

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