I'm trying to install a 3-way switch for a specific light but I'm not sure how to do the wiring.

The existing switch box has quite wires inside:

3x 12-2 1x 12-3

The hots (black) are all tied together, the neutrals are all tied together, and the grounds are all tied together.

However, the existing switch has one hot (black) wire connected, and a RED wire from the 12-3 wire.

The light fixture itself only has 12-2 connected (black, white, and copper/ground).

How would I install a second (three-way) switch to this setup?

  • 1
    You will need to pull new 12/3 cable from the switch you have described to the new 3-way switch you are adding. Then of course you'll have to swap your existing switch for a 3-way switch. When you ask "how," it's very vague-- there are quite a few steps to pulling wire, installing box(es), and wiring traveler wires. How familiar/comfortable are you with electrical work?
    – mjohns
    Jul 4, 2015 at 22:14
  • Thanks for the information! I'm very familiar with pulling cables (mostly cat 5/6, coax, etc) but only know the basics of electrical wiring but I understand the safety. However, what wires do I connect from the new 12-3 to my existing switch? That's the part I'm unsure about.
    – SofaKng
    Jul 5, 2015 at 0:24

1 Answer 1


Assuming this is the setup you have described:

enter image description here

You will need to pull a new 12/3 cable to your new 3-way switch. You will also have to replace your existing switch with a 3-way switch. Your wiring setup will look like this:

enter image description here

Because your lighting circuit's power is supplied by a conductor in a 12/3 cable, the figures above were modified as follows:

POWER SOURCE / 2-Wire Romex with Ground (i.e. 12-2) - I modified these to show your 12/3 wire with your RED supplying the power to your lighting circuit (since you described the BLACK in your 12/3 as being spliced into a bunch of other black conductors). From here, everything else should be one-to-one.

  • Thanks! The part I'm confused about is that I already have a 12-3 wire at the current switch. The current switch is connected to a RED and BLACK wire instead of BLACK and WHITE. (and the switch is definitely not connected in 3-way to any other switches)
    – SofaKng
    Jul 5, 2015 at 1:44
  • A three-way circuit is not the only reason to use 12/3. Another more common use is a switched outlet or a fan/light combination ceiling box.
    – longneck
    Jul 5, 2015 at 2:13
  • Thanks... Yeah, I have seen it used for fan/light combinations as well. However, how would I hook-up the wires given my current wiring setup?
    – SofaKng
    Jul 5, 2015 at 2:18
  • @SofaKng, I think you are getting caught up on wire colors. All that matters in this setup is (1) What is currently supplying power to your lighting circuit, and (2) Keeping the colors consistent in your traveler wires between your two new 3-way switches. Look at the revised figures in my edited answer.
    – mjohns
    Jul 5, 2015 at 4:30
  • why do you show red, black and white plus yellow ground, but state the power is 12-2 romex?? (on 3-way between lights). Seems the red should be black and the black wire spiced together and going nowhere should not be shown. I'm confused. May 1, 2017 at 19:58

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