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I had two black wires, one red wire and two white wires coming from the ceiling (I suspect there used to be a fan with a light). In the switch box, there was a black and a white wire (with a green ground wire).

I wanted to install a light fixture so I tied the red wire (which I discovered was the constant feed) and one of the black wires (which I figured extended to the switch) together in the ceiling with a nut, the two white wires and the blue wire from the light fixture together and the remaining black wire with the brown wire from the fixture.
With a regular switch, everything works fine: lights turn on and off. With a fried dimmer (the switch part works but not the dimming part), everything works fine (except the dimming of course): lights turn on and off With a new dimmer, nothing works: lights don't turn on. I know the dimmer works because it works in another location just fine.

How come? Does a fried dimmer act as a regular switch?
Can I even use a dimmer with that configuration? Did I wire it wrong?


Thanks for taking an interest in my question. Here are pics of the ceiling box and of the switch box. I did some testing and:
- The red and black wires that are in the same cable as a white one have voltage (left side of the pic)
- The remaining white in that cable and the white and black wires in the other cable (right side of the pic) do not have voltage
- All wires in the switch box have voltage (including the green one)

If I attach my light fixture to the black and white wires which are in the same cable as the red one (leaving the red one with a nut by itself), the light works with a normal switch. It does not with a dimmer.
Any other configuration of white and black wires tied to the fixture won't work.
Initially I had tied the red and one of the black wires together but that does not seem necessary to turn on the lights with a normal switch. However, the problem remains the same.

I don't understand why a normal switch would turn on the light and not a dimmer...

Ceiling box switch box

  • 2
    Can you post photos of the insides of all boxes involved? – ThreePhaseEel Oct 29 '19 at 4:18
  • It sounds like you guessed wrong on the wire for hot , neutral, feed ect. The old dimmer may have an internal switch at some point that bypasses the dimmer circuit, if the new dimmer is 100% electronic and you do not have the line on the correct wire it can’t turn on. I would double check with a meter to verify where the hot is. – Ed Beal Oct 29 '19 at 13:54
  • What make and model is the new dimmer? – ThreePhaseEel Oct 30 '19 at 0:45
  • @ThreePhaseEel It's a Leviton Rocker Dimmer DSL06-1TW – Nicolas Oct 30 '19 at 1:49
  • I take it both black and red in the /3 cable are de-energized when the breaker for the circuit is turned off? – ThreePhaseEel Oct 30 '19 at 2:56
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Try swapping LINE and LOAD on the dimmer

Older analog-triac dimmers were like single pole switches in that they didn't care which way around in the circuit they were connected. However, you have a newer dimmer that is more sophisticated (and has to be, in order to dim LEDs properly without a neutral reference), and thus cares about the distinction between its LINE side and its LOAD side; in your case, the black screw is LINE, and the red screw that isn't covered by the label on the dimmer is LOAD.

As to the wiring in the ceiling box, the white wire in the /3 cable is neutral going to the fixture (the fixture's blue wire), while the red and black wires are always-hots, along with the black wire in the /2 cable to the switch box. The white wire in that /2 cable, then becomes the switched-hot returning from the switch in an old-style switch loop configuration, going to the brown wire on the fixture, and needs to be flagged with a wrap of electrical tape at each end as a result. Of course, all the grounds need to be connected together as well.

With the ceiling box wired as above, we then move to the switch box, where the black wire goes to the black screw and the taped white wire goes to the red screw that isn't covered by the label. The green screw, of course, gets connected to the grounds in the box.

  • Thanks for your answer. The switch was already wired the way you describe so I must have gotten the ceiling box/flxture wrong. So to make sure I understand correctly: I should tie the black and red wires from the /3 cable and the black wire from the 2/ cable together with the brown wire from the fixture, and the white wire from the /3 cable with the blue wire from the fixture, and leave the white wire from the /2 cable by itself (with a nut of course)? – Nicolas Nov 1 '19 at 1:04
  • @Nicolas no, that'd turn the fixture on all the time, rendering the switch irrelevant. The brown wire from the fixture needs to go to the switched-hot coming back from the switch, instead of the bundle of always-hots. – ThreePhaseEel Nov 1 '19 at 1:24
  • I did that and it didn't work. The black wire in the /2 cable is not always hot as you described so that might be where we're doing it wrong. Basically, attaching the fixture to the black and white wires in the /3 cable (black to brown and blue to white) works (lights turn on and off) with a normal switch but not with a dimmer. It's too bad I can't use a dimmer but I can live with that. I appreciate all the help, thanks a lot for taking the time to answer me. – Nicolas Nov 1 '19 at 3:02
  • Hm, it sounds like the there's another junction box in this mess somewhere...can you find it? – ThreePhaseEel Nov 1 '19 at 3:38
  • Yes, the /2 cable wires probably go somewhere else but I can't find where. It's fine, it works with a regular switch and that's good enough for me. Thanks for taking the time to answer my question! – Nicolas Nov 2 '19 at 0:11

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