I'm trying to replace this regular light switch with a dimmer. After I open the cover, there are 2 groups of wires from the wall. I don't know why the white wires are not used, but this is the original wiring.

Left Group

  • 2 white (not used)
  • 1 copper for ground (connected to right group ground)
  • 2 black
    • letter B in the pic, plugged in to the bottom of switch
    • letter C in the pic, screwed to the bottom of the switch Right Group
  • 1 white (not used)
  • 1 copper for ground (connected to left group ground)
  • 1 black, letter A in the pic, plugged in to the top of switch

enter image description here

Now my question is how do I connect to the dimmer below? I know the green wire goes to the ground copper, and the red with a sticker should not be used since I'm not doing a 3-way. Which one should I connect the black and red with?

My educated guess is:

  • connect A with dimmer red wire
  • connect B and C with dimmer black wire
  • connect ground copper with dimmer green wire

Does that sound right?

enter image description here

  • 3
    The whites are absolutely being used. They just don't connect to the switch.
    – Ecnerwal
    Dec 4, 2021 at 2:10

1 Answer 1


Your original switch was a 50 cent cheapie. It had both screw terminals and backstabs. In your photo, the "B" backstab is internally connected to the yellow screw. So the installer is using the dual connections to splice together "B" and "C", as well as connect them to that leg of the switch.

It's very common to bring power (always-hot+ground) to a switch, and then daisy-chain off of it to power another switch or outlet. Most likely, that accounts for hots B and C.

So "A" must be the switched-hot to the light. I prefer to mark wires with colored electrical tape to identify them by function, and the preferred color for switched-hot is red. I would mark "A" red.

The preferred color for always-hot is black. So B and C stay black.

Your smart switch also uses those same preferred colors. Join em. Done.

"That was easy"

  • My new switch is a LED dimmer but it's not a smart switch. So I connected A to dimmer's Red, B and C to dimmer's Black, and copper ground to dimmer's Green. But, sometimes the light doesn't turn on or have a 2 to 3 second delay if the dimmer was on a low position. No delay if position is on high. I heard dimmer can introduce a delay, but isn't that too long?
    – Ray Cheng
    Dec 4, 2021 at 6:02
  • The dimming on the switch is adjustable, so I adjusted it, but now the dimming range is less, but it seems to get rid of the random delay.
    – Ray Cheng
    Dec 4, 2021 at 6:43
  • That's all too utterly normal for LED dimmers and LED lights.
    – Ecnerwal
    Dec 4, 2021 at 14:35
  • @RayCheng Yeah, look into the tech of how triac dimmers work. It will become obvious that the design was optimized for "cheap" and optimized exclusively for how incandescent bulbs behave. It was very imprecise to begin with. It doesn't work on LEDs. "Dimmable" LEDs are specifically designed to endure it, analyze the waveform, and give the brightness level the triac dimmer seems to be asking for. It's a black art. There are other technologies designed specifically for LED dimming, but they require different wiring schemes. Dec 4, 2021 at 21:20

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