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Moved into a new house recently. Half of it has ungrounded outlets so I installed a few strategic GFCI outlets to protect those. After doing this, I noticed that a light switch in the basement will cause a GFCI in the upstairs living room to trip. This light switch controls two lights in the basement. This is what it looks like after opening it up

metal box showing switch partially pulled out of wall and wiring behind
Click any image to embiggen

Wider view of switch above showing the whole switch

Image of the other side of the switch showing no wiring

Image of box and wiring with switch removed

It's a little hard to see but both blacks are connected to the outlet. The white wire is actually the hot. If I connect the white to either black I get ~80v. If I connect both blacks together and connect the result to the white I get ~95v. If I wire up the outlet with just one black and the hot white, one of the lights turns on and the GFCI does not trip. What can I do so that both lights will turn on without tripping the GFCI?

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  • Some clarification would be helpful. Your pictures show the light switch wiring, but it's very puzzling. There are two cables entering the box, left and right. Each has a black wire and a white wire. It appears that both black connected to the top screw on the switch, and the white wire from the right cable is connected to the bottom screw. Is that correct? Is the white wire in the right cable connected to anything?
    – Mark
    Sep 29, 2022 at 5:11
  • Also, how are you connecting white and black wires, and where are you measuring the voltages? Are the lights on the same circuit as the new GFCI in the upstairs living room?
    – Mark
    Sep 29, 2022 at 5:14
  • There is black tape on the white wire in the first image, but it's missing in the last image. I'm 99.99% certain that was there to mark the white as the always-hot wire in a switch loop. This means that power comes to the light, then a loop drops to the switch to switch the circuit on and off. If you're treating this as a neutral, that'll definitely trip the GFCI and/or the breaker.
    – FreeMan
    Sep 29, 2022 at 12:32
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    Notice the burn marks on the white wires. Something is overloading or leaking. I suspect there's a lot more connected to this power than just two light bulbs. I would trace the wires and take them out of the mix.
    – GenShira
    Sep 29, 2022 at 22:57

1 Answer 1

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Most likely causes:

#1 You have a light fixture faulted to ground. This is a common problem because light fixtures sometimes have uninsulated screws that can easily contact a ground wire inside a junction box. On a switched light, you might not notice the problem because the light appears to work normally, and there is nothing wrong with the switch.

Also, it looks like one of the white wires isn't connected anywhere in any of the photos. This suggests one or both of the fixtures might be operating without a neutral conductor.

To resolve this, turn off the circuit breaker and check all connections for the switched fixtures to make sure they are well separated. Cover exposed screws with electrical tape if it helps to maintain a safe installation.

#2 Damaged insulation on that white wire. It's really hard to tell from that photo, but you might have a white wire contacting the grounded metal box. I would check this while you've got the switch out.

p.s. Don't test voltage from hot-to-load. Instead, hold one of the probes on the ground wire and then reference the other wires from there. Voltage is measured in parallel, current is measured in series. Resistance and continuity are a different technique as well.

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