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Adding a 3-Way Switch to an Existing Lighting Circuit

February 2, 2007

Q: Can I add a swich to an existing light circuit so the light can be controlled from two switches? I have a light controlled by a single switch, can I add a second switch?

A: Yes you can. Most home owners call this 2-way switching, but in fact, it is called 3-way switching.

To get started you need a materials list. You will find all of these items at your local home improvement center, hardware store or electrical supply house. For this installation your will need:

  1. Either 12/3 or 14/3 NM cable (match existing size)(length to be determined by distance from existing switch to new switch – i.e. 5′ down from existing switch + 25′ across floor joists + 5′ up to new switch = 35′)
  2. 1 – 1-gang old work (remodel) box
  3. 2 – 3-way switches
  4. 1 – 1-gang switch cover plate

Now that you have your materials, let’s get started. Go to your breaker box and turn off the breaker that protects this circuit. If it is not labeled, turn on the light and have someone stand near the light and you flip breakers until you find the correct one. Once you have found the correct breaker, label it so you will not have this problem again. Now go to your switch, remove the cover and verify that the power is off with a volt-meter.

Next you need to remove the switch and install a 3 conductor, with ground cable between your existing switch and the new switch location. You need to match the existing wire size that is already in place for your lighting circuit. This is either going to be 14/3 or 12/3.

Installing the cable from your existing switch to the new location is difficult for me to explain without more information. You will either need to fish down to the basement or crawl space, or fish up to the attic and over the the new location. 

Now “cut-in” your new switch box matching the height of the existing light switches in the house. To do this place the box’s front side against the wall, level it and trace around the outside edges. Now use a drywall saw and cut along the line to remove the drywall.

Fish your new 3 conductor cable into this location and push it into the electrical box. Insert your electrical box and fasten it to the wall. Next strip the outer sheath off of the cable. Strip each wire to remove approximately 3/4″ of insulation. Curl a loop in the black, red and white wire. When wrapping the wire around the screw, you want the loop to close on the final turn of the screw. 

The bare copper wire terminates to the green ground screw on the 3-way switch. Now terminate the black wire to one brass colored screw and the red wire to the other brass colored screw. The white wire is a hot wire and not a neutral so, you need to mark this wire black. This is easiest done by wrapping electrical tape around the wire or using a permanent marker. After the wire is marked, terminate it to the black screw. Install the switch and cover it.

Now go to your existing switch. Strip off the outer sheathing of the 3 conductor cable and terminate the bare copper wire with the rest of the grounds and leave a “pig-tail” to connect to the ground screw on your new 3-way switch. Connect the black wire to one brass colored screw and the red wire to the other brass colored screw.

There were 2 wires that were connected to your old light switch. You need to determine which is the power supply and which one goes to the light. The one that goes to the light, terminates to the black colored screw on the 3-way switch. The power supply connects to the white wire (marked black) with a wire nut and tucked back into the box. Install your switch and cover.

Go to your breaker box and turn on the circuit breaker and go test the new 3-way switching. The light should turn on or off from either switch in any position.

If this explanation does not work for your scenario, please submit more information in the comment section.     

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