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I'm replacing (yet again) my bathroom dimmer switch. The old switch had 3 wires: 2 black and one green. The two black wires were attached to the wires coming out of the wall and the green wire was attached to nothing. The new switch has 4 wires: black, red, red/white striped, and green. Can I attach the new switch somehow? If not, what would you recommend when I only have 2 wires (circa 1930) coming out of the wall (besides rewiring my house)?

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    Did you read the installation instructions?
    – Tester101
    Sep 10, 2014 at 23:11
  • What is the make and model of the dimmer?
    – Tester101
    Sep 10, 2014 at 23:11
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    Does this dimmer go bad often ? Is it controlling just lights or is it also hooked to a fan or a heater and how many watts do the lights add up to?
    – user24125
    Sep 11, 2014 at 0:22

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Not sure what make and model dimmer you're trying to install, so I'll describe a common installation. Wire colors could vary, depending on manufacturer.

  • Black is the common terminal, and is attached to the ungrounded (hot) conductor feeding the switch.
  • Red is the switched lead (or a traveler in a 3-way installation), and is attached to the switched hot going to the light.
  • Red with white is a traveler, used for 3-way installations. In your situation, you'll just cap this one off.
  • Green is ground, and is attached to the grounding conductor.

Single pole installation
From Lutron Luméa® Installation Guide

If you don't have grounding conductors in your home, you could install a GFCI breaker and you'll be code compliant.

Lutron devices typically have a red and a red/white stripped wire, whereas Leviton has two red wires.

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  • This answer might also be helpful.
    – Tester101
    Sep 10, 2014 at 23:34
  • even though you qualified by stating "ungrounded (hot)", I would be careful using the term "common" to describe a hot conductor. The neutral conductor is commonly referred to as the "common" conductor in the US. Jeez, was that too many "commons"?
    – Jimmy Fix-it
    Sep 11, 2014 at 1:32
  • I've never heard the "neutral" referred to as the "common". Can you provide any sources (aside from forums and/or yahoo answers), where the grounded (neutral) conductor is referred to as the "common"?
    – Tester101
    Sep 11, 2014 at 2:09
  • I sold residential and commercial electrical supplies for 20+ years in SoCal, many electricians here refer to the neutral conductor as the "common". I s#*t you not. Maybe because they all lead to a "common" bus at the panel? Not sure of origin but absolutely true. I do not "yahoo" for my answers or comments, if I do not know by experience I do not post, when I am wrong I admit, apologize, and learn.
    – Jimmy Fix-it
    Sep 11, 2014 at 2:22
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    Because a certain area uses the wrong term for something does not make it correct. "Common" used as a term for a neutral is NOT a correct term. Common definitely IS a correct term for one of the connections to a 3-way switch.
    – Speedy Petey
    Sep 11, 2014 at 2:28
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Dimmer Switch for Alexa from Amazon. This is how I wired it to the existing 3 wires. The ground and neutral from the switch, green and white, had to be connected to the plain ground wire from the wall. The black wire from the switch, power in, had to be connected to the white from the wall. The white one from the wall could have been a black one when you have two black ones, in which case it wo u8 led be the one coming from the fuse box carrying 120v. The red one from the switch transfers electrical power to the lamp. enter image description here

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