I have 2 light switches, and want to replace the switch on the right with a dimmer switch that requires 3 wires. Link to the dimmer switch. The switch on the left doesn’t do anything, so I planned to cap the unused wires and use a plate that covers the left side.

The left switch has a red wire going straight into the back, and a black wire going straight into the back below the red one. This switch has terminal screws but none of them have any wires connected.

The right switch has 3 terminal screws: 1 green which is not connected to anything, and 2 that have been painted over and are both connected to black wires.

I am confused about this configuration, and this is my first time doing anything electrical so not able to find this online. There’s a bunch of insulation behind the switches so hard to tell what wires are available.

Can someone help me with which wires should connect where on the new dimmer switch?

This is how the 2 switches are wired This is what is behind the switches

  • Hi Alex, this question is too vague. Try editing the question. Add information about "a dimmer switch". Even better, if you can add photos of the new switch, the old switches and the "squashed copper wire" then you are more likely to get an answer here. Commented Jun 19, 2023 at 5:00
  • @robertchapin thank you for the advice; I have added pictures and a link to the dimmer switch. On closer look I’m not sure I saw copper wire after all so I removed that line.
    – Alex
    Commented Jun 19, 2023 at 5:12

1 Answer 1


You might be able to DIY, depending on some extra preparation.

What's hard to see in the photos is maybe a metal 2-gang box (?) that has been stuffed full of fiberglass.

The first step is to turn off the circuit breaker(s) for both switches. Then verify there is no voltage on any of the terminal screws. I prefer a multi-tester when checking for voltage. If the paint is interfering with the multi-tester probes then leave the switch in the on position so you can verify the lights and circuits are off.

The second step is to remove all of the insulation. It doesn't belong there and could interfere with the heat sink on the new dimmer. I suggest wearing gloves for this task.

If you don't find a 2-gang box, or if you find a plastic box with no copper ground wires, then STOP.

Now the difference between the old switch and the new switch is that this new model has polarized terminals, which is why one screw is red. Due to the amount of paint on the old switch, you will likely need to remove the two black wires for testing. With those wires separated and capped or otherwise safely away from other people and metal parts, turn on the circuit breaker, then use a multi-tester to determine which black wire has the line voltage (hot). Label the hot wire, then turn off the circuit breaker.

The hot wire will attach to the black terminal. The other black wire is the light and will attach to the red terminal. Remember, no paint is allowed on the stripped wires. Clean it by scraping gently, or cut and re-strip.

"Switch on the left doesn’t do anything" is not a good reason to cap wires. It could be set up for an old ceiling fan or a wall receptacle. It's also possible the backstab connection has failed and the switch simply isn't working anymore.


Based on the comments, there is a substantial fire hazard in the existing work. Foam insulation is a flammable material and should not be present inside a junction box. The failed backstab connections are likely to arc, providing an ignition source to that insulation. Removing the broken switch and the foam should be a higher priority than the dimmer upgrade.

  • I’m working on removing the insulation, will report back once I am done. I think it’s the spray-on insulation and it’s gotten hard and crunchy so it’s slow progress. Thanks for the advice.
    – Alex
    Commented Jun 19, 2023 at 6:04
  • 2
    Oh that's foam? Gross. Foam should never be inside a junction box. Maybe start around the front edges to verify a box is in there and not just a metal bracket. Commented Jun 19, 2023 at 6:07
  • How can I tell if it’s the entire box? I scraped on the inner edge and found it extends at least 1 inch into the wall, but I’m not sure how deep the metal bracket would be. I am working my way through this house to try to fix everything that was done wrong, so I’m not surprised this is also wrong (the insulation)
    – Alex
    Commented Jun 19, 2023 at 6:22
  • Most mounting brackets are quite thin. It sounds like the box just needs to be cleaned up so you can see the rest of it. Ideally a blunt tool like a flat head screwdriver would help, but if the foam is so tough that it needs a knife then that's a tricky situation. Maybe even worthy of a separate question. Commented Jun 19, 2023 at 6:33
  • 1
    Yeah that backstab was a fire waiting to happen. You can cap those 2 wires (individually) for now. The damaged switch should be removed or replaced. Commented Jun 19, 2023 at 6:46

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