I'm replacing a dimmer switch with a regular switch, but the only configuration that works involves wires from my another switch. What's going on?

Question

EDIT: Per the excellent answer below,this was (to my surprise) a three way switch. Thankfully I had a few extra three ways lying around! In addition, the other switch on the three way was wired incorrectly, I guess by the previous owner, which was making things even more confusing. They had apparently flipped one of the travelers and the line. Good times! All fixed now at least

I'm swapping out a dimmer switch for a smart switch, or was attempting to. I have tried a regular switch instead with still no luck.

Kitchen lights: A and B were wired up to the dimmer switch. The dimmer switch's ground was the left pink mark. The dimmer switch's neutral was connected to the C terminal for the second (pendant lights) switch.

Pendant lights: C and D were wired up to a regular switch. The ground was the right pink mark. As mentioned, the C terminal also supported the neutral for the first (dimmer) switch.

I swapped the dimmer switch out for a smart switch. I connected the ground to the same ground wire and the neutral to the bundle of white wires in the box. This was my first indicator that something was weird, because the smart switch did not have power. Moving the smart switch's neutral wire to the C terminal of the second switch provided power to the smart switch, but not to switch the lights on.

Here I did try the second neutral bundle in the box, but that also provided no power to the smart switch. Only the terminal on the second switch provided power.

Thinking something could be wrong with the smart switch, I used an extra switch I had lying around, a regular switch. This also did not work.

Next, I removed the switch and tried just touching the wires together, with power on. No luck. I tried the dimmer switch again in the original configuration, and it worked. I removed the dimmer switch again and attached wire A to the C terminal of the working second switch and wire B to the D terminal of the same second switch. Interestingly, this worked, but now I had one switch controlling both the pendant lights and the kitchen lights, which isn't exactly what I want lol.

I read online that in some cases, dimmer lights specifically need a dimmer switch. I swapped out one of the lights for a regular lightbulb, but this made no difference in any testing.

Removing both switches, I started testing different wires together.

• A+B - nothing
• A+C - kitchen lights on
• A+D - nothing
• B+C - nothing
• B+D - nothing
• C+D - pendant lights on

So, uh, any idea what's going on here??

Answer 1

Sounds like your C is your "hot" a.k.a. "line" (or possibly switched hot from a 3-way switch configuration with the one you call "Dimmer"). Dumb switches usually wouldn't have "neutral" connected.

A appears to be the "active" traveler if this is a 3-way setup as I suspect, or possibly switched hot to the kitchen lights. That is why A + C turns on kitchen lights.

B is probably the other traveler. You can verify this by looking for the other switch that usually controls the kitchen light and flipping it, then connect C + B which should turn on the kitchen light if I have the correct interpretation.

D is the switched-hot to the pendant lights. It needs a "hot" or "line" to connect to the other side of the switch so that when the switch is "on" the light is on.

Hopefully you've learned to photograph and label your wiring before disconnecting things; which doesn't do you much good now, but next time you need to do some rewiring, you can take some "before" pictures to help figure out how to put things back properly and help identify each wire's purpose.

What should you do now?

First, make sure you're allowed to work on electrical in your area. If you're a renter, changing out light switches for smart switches is typically a no-no. Ditto if you're a landlord. If you own your own home, you may still be in a city that requires permits or licensed electricians only.

Assuming you're able to do your own electrical:

1. Get a voltmeter. Learn how to use it.
2. Use the voltmeter to figure out if you have actual neutrals in the box on the same circuit as these switches and lights. You can't trust the wire color, it's just an indication.
3. If you don't have true neutrals here, you need new smart switches that don't require neutral (they usually run on batteries).
4. If you do have neutrals, you probably need a 3-way smart switch for the one you call "dimmer" and a regular smart switch for "pendant lights" switch.

Assuming my interpretations above are correct (test and make sure!) Wire up A and B to the travelers (brass terminals) of the 3-way smart (dimmer?) switch. C needs a pigtail to the common (black terminal) of the 3-way switch, then you connect the neutrals to the switch's neutral and ground to switch's ground.

For the pendant switch, connect "line" to C and "switched" to D, and then the neutral and ground as normal.