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?