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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?

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  • Just because you have the option to use LOAD terminals, does not mean you are required to. Try putting the warning tape back on LOAD and pigtailing to LINE. Is there a light switch in this box?
    – Harper - Reinstate Monica
    Commented Aug 28, 2018 at 6:22
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    Did the old receptacle have any of its tabs broken off? These are found between the screws on each side.
    – Harper - Reinstate Monica
    Commented Aug 28, 2018 at 6:23

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You are correct. There is a pathological path from Neutral 2 back to the neutral bar in your service panel. Neutral 2 should not be connected to anything but the cold sides of the loads on Hot 2. You should track down and eliminate this path, even if you do not want to provide GFCI protection to the downstream loads. It is a persistent hazard to anyone working on the wiring, and may carry an unbalanced current that can lead to inductive heating.

The most likely culprit is a bootleg ground somewhere further along the daisy-chain. Other possibilities are an accidental contact between a bare ground wire and a neutral terminal screw, an unnecessarily borrowed neutral, or failed insulation in one of the loads.

You could start by unplugging everything from the downstream outlets. Keep checking the voltage between Hot 2 and Neutral 2. Remove the outlet cover plates and pull the outlet devices out of their boxes, without disconnecting any wires. You will probably eventually either disturb the accidental contact, or encounter a white wire that doesn't make sense.

As always, be careful around live wires. Wear gloves and try to handle devices with one hand only.

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