I am replacing an existing outlet with a GFCI outlet. The existing outlet functioned fine, but I want GCFI protection on the circuit. It appears to be your standard daisy-chained outlet box - 2 black hots, 2 white neutrals, 2 bare copper grounds. Both grounds are bonded to the metal junction box.
I'm using my multimeter to find which wires are the line to the breaker box, and which wires continue downstream to feed the rest of the circuit. I disconnected the old outlet and I'm measuring the wires looking for 120V. Hot 1 to Neutral 1 measures 120V. Okay, I found my line wires. So I connect Hot 1 and Neutral 1 to the GFCI outlet line terminals, and Hot 2 and Neutral 2 to the load terminals. I turn the breaker on and the GFCI trips right away. Okay, time to double check my work!
I remove the outlet from the the circuit. I double checked and there's still 120V between Hot 1 and Neutral 1. But...there's also 120V between Hot 1 and Neutral 2. No volts between Hot 2 and any neutral.
Why is there voltage via Neutral 2 when it should be disconnected from the circuit with the outlet removed? There should be no path via Neutral 2 back to the breaker box, so I shouldn't see 120V, correct? I think I have a ground-neutral short somewhere, or perhaps the neutrals from 2 circuits joined together somewhere. Is there another scenario I'm not thinking of?