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I’m replacing the original switch with a smart switch. When I opened it up, it seems whoever installed it previously used a regular light switch.

The red wire went straight into the old light switch (where a black wire would usually go).

One black wire was screwed in. The other black wire went straight in.

Below are pictures of the original switch and my wiring of the smart switch. I would just cap the red wire?

What could the red wire be used for in the future? It is not hot as tested with a contact tester.

https://ibb.co/hKLNrXB https://ibb.co/C24ZDYW

Thank you
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2 Answers2

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Why would you think a wire connected to the old non-smart switch would not be connected to the new smart switch? If anything, a smart switch often requires more wires, commonly (but not always) needing a white neutral wire added.

In any case, colors don't matter in US wiring. Or rather, they only matter a little:

  • Neutral is white. White is not always neutral (some 3-way, 2-wire 240V, etc.)
  • Green, bare, yellow/green - ground.
  • Black, red, blue, yellow, etc. - hot, switched hot, traveler, etc.

So the "usually black" is simply because of what you have seen. In a modern 3-wire switch loop, it will usually be red. In conduit - black, red, blue, yellow, etc.

In your case, the most likely setup is:

  • The two blacks (one on screw, one backstab) are effectively daisy-chained. If your new switch can legitimately take two wires under a screw (screw-to-clamp), great. If not, use a pigtail to get them together with one short black wire that then goes under the screw. Or if the switch comes with pigtails, connect it to the two black wires.
  • The red wire is the "switched hot". Connect it to "switched hot" or "load" on the new switch.
manassehkatz-Moving 2 Codidact
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It looks like you have a three conductor cable from the switch box to the fixture (Black, Red, White, plus uninsulated ground). If you had two switches in the box, this would allow separate control of the light and the fan, if the light/fan fixture allowed this. But since you have only one switch the light and fan were operated in unison.

One way to do this would be to connect three wires to the switch. The line hot (a black wire) would be connected to one side of the switch. The switched hots (a red and a black) would be connected to the other side of the switch. In the fixture one switched hot would connect to the light and the other switched hot would connect to the fan. This seems to be what you had originally.

Another way to do this would be to connect only one of the switched hots (B or R) to the switched side of the switch and not use the other switchable wire at all. In the fixture housing you would use a short wire "a jumper" connecting the light and the fan terminals.

What functionality do you want to achieve with these smart switches?

Jim Stewart
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