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I’m trying to install a set of light switches to control my fan and lights. The left switch has two black cables running to it and the right has a red and black.

I installed both switches and the left doesn’t power on when I flip the breaker. I’ve confirmed it isn’t a manufacturing issue with one of the switches by uninstalling/reinstalling them in the reverse order. In both cases, the left side is the one not receiving power.

Below are the original and new configurations, respectively:

Original Configuration

New Configuration (Left side not getting power)

The new red wire on the left connects to the nest of black wires similarly to the original bottom black wire. I’m afraid what may happen if I reverse the wires going to the red and black on the left switch, though I don’t know whether that will do any actual harm.

Any clear issues with how I rewired these?

  • The guide also mentions the switches are only compatible with a one-way, single-pole circuit. I had to do some reading, but I don’t believe I’m looking at a three-way circuit, am I? – Justian Meyer Aug 10 at 22:27
  • Difficult to be sure, but it looks like you have LINE & LOAD reversed on the left switch. – brhans Aug 10 at 22:43
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You need to reverse the line and load on the left switch

  • Is there any risk to reversing them to see? If that’s the case I’m guessing it didn’t matter what side line/load were configured on the original switches so left and right were configured differently? – Justian Meyer Aug 10 at 22:45
  • That did the trick. Thanks for the help! – Justian Meyer Aug 10 at 22:48
  • They do not designate line and load on a single toggle like these. – JACK Aug 10 at 22:52
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These new smart switches are active devices, and power themselves between supply (their black) and neutral (their white obviously). Therefore they care about the difference between supply (their black) and switched/lamp power (their red). Your old switches didn't care about that.

You need the smart-switch black (supply) to go to the bundle of black wires. The smart-switch red (switched) must go to the lamp/fan, which are the red and black that go into the same cable.

Remember, do not dim a motor. Motors respond very badly to triac style dimming intended for lights, they need a different type of thing called a fan speed control.

  • That’s great advice. Fortunately these are switches and not dimmers, but you stopped me from trading them in for a dimmer set instead, so thank you. – Justian Meyer Aug 11 at 0:32

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