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I have a box with a switch and a dimmer side by side. Last year I replaced them with smart switch/dimmer and had no problem until the dimmer stopped working. I am replacing the dimmer, but foolishly and overconfidently didn’t document the wiring and now I have confused myself.

The bundle for the switch has a black and a white wire. The bundle for the dimmer has black, red, and white. (There are also grounds for both.) Both the smart switch and dimmer need ground, neutral, load, and live.

The leaves one short on the switch (the bundle has white and black). Originally, there was a wire bridging the dimmer and the switch. I think the load went from the switch, to the dimmer, and then to the red in the second bundle, but I am not certain enough to test.

Using a tester, the only hot wire I detect is the black from the two-wire bundle. None of the wires in the three-wire bundle read as hot.

How should I wire this? (Keeping in mind that this was working fine until I decided to mess with it this morning.)

And obviously, never forget to document!

Top photo is the current situation. Bottom is the original (pre-smart).

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  • What lights does this power? Are the two sets of lights together?
    – Harper - Reinstate Monica
    Commented Dec 21, 2019 at 16:09
  • The lights are separate.
    – iayork
    Commented Dec 21, 2019 at 18:23

2 Answers 2

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The black wire from the /2 cable needs to be pigtailed to the LINE terminals on both devices

We know, from your testing, that the black wire from the /2 cable is the incoming always-hot wire; this means that the black and red wires from the /3 cable are the outgoing switched-hots for the two sets of lights this box controls. Therefore, we wire the box as follows:

  • Black from the /2 gets nutted to a black jumper wire, or pigtail (12AWG THHN always works) that goes to the LINE screw on the smart switch and also to the LINE wire (or pigtailed to the LINE screw) from the dimmer
  • Black from the /3 goes to the LOAD screw on the smart switch
  • Red from the /3 gets nutted to the LOAD wire (or connected to the LOAD screw) on the dimmer
  • All the white wires get nutted to each other and to a white pigtail that heads off to the silver neutral screw on the smart-switch (if the dimmer has a silver neutral screw, it needs to be pigtailed to the neutral-junction as well)
  • And all the ground wires (green and bare) get nutted to each other and to a green or bare pigtail that heads off to the green ground screw on the smart-switch, and to the green ground screw on the smart-dimmer if it has a screw for ground, too

Then you can button the box up, turn the power back on, and enjoy your new smart switch!

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  • 1
    This worked, and as I connected things I realized it’s exactly what I had done before. Should have documented ...
    – iayork
    Commented Dec 21, 2019 at 16:29
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If any of your smart devices don't already have pigtails, add them for Line (black), Neutral (white) and Safety Ground (green, yellow/green or bare).

Let's mark wires with colored tape

You must use bare, green or yellow/green for safety ground. All these always join.
You must use white or gray for neutral.
Hots can be anything else. For always-hot black is great.
For switched-hot I like red, with blue as an alternate if there are two.

In the black-white-red cable, white is neutral. Black and red are switched-hot to lamps. Red is already the right color, but the black wire gets blue tape.

On the smart dimmers, LOAD is the one that is "switched" hot to the lamp. On one smart dimmer, mark LOAD red. On the other, mark LOAD blue.

Match up all wires of same colors.

So all 4 white neutrals come together.

For grounds, you just have to know green, yellow/green and bare are the same thing, and connect them all. Connecting all grounds of those colors is SOP in every box. This is not true of Neutrals (but it happens to be in this box).

I expect 4 grounds, 4 neutrals, 3 blacks and 2 each blue and red.

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  • This is good advice in general, but the problem here is that there are seven wires coming in to the box (two ground, two neutral, two black, one red) and we need eight going in to the two switches (two each ground, neutral, line, load), so we can’t match up the same colors. My question was how to split the seven into eight, and ThreePhaseEel’s answer solved that.
    – iayork
    Commented Dec 21, 2019 at 21:23
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    @iayork I think you're expecting a 1:1 match of wire:wire. Not at all. I'm saying (after remarking) match up all wires of a given color. So 4 neutrals come together, 4 grounds come together, 3 blacks come together, and 2 each of red and black. My whole point is, after you identify by function, you don't need to count wires anymore. If it's blue, it goes with blues. Further, this marking will be useful 5 years later when you revisit this box; you won't have to headscratch it all over again.
    – Harper - Reinstate Monica
    Commented Dec 21, 2019 at 21:31

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