In my research on how to install a 3 way switch, i've seen different ways on how to install the wires but i haven't found the schema that would help me in my situation.

Here's a screenshot of my installation.

  • The background in green is my current installation
  • The background in yellow is what i want to add
  • i have access to the wires of my current installation. I can change if i need to

enter image description here

Is there a way to install a 3 way switch like in the schema above by leaving the power source where it is? If yes, a detailed explanation would be welcome!

  • Just as an interesting observation. I had to look several times at your dialog to see where the colors change (I'm slightly color blind). In the future, you might want to demarcate areas like this by a line as well as by color, or perhaps by color in one area and white in the other. – Michael Kohne Apr 30 '12 at 1:03
  • @Michael, sorry about that! I've made this image very quickly so i didnt took the time to polish it :) – Alexandre Jobin Apr 30 '12 at 2:15
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    This Answer contains diagrams of a bunch of different configurations, the first one is the one you need. – Tester101 Apr 30 '12 at 11:41

Yes, it is possible. But you may need to replace the existing switch. The key to making it work is that you need a connection behind the existing switch between two wires.

You need to do this:

  • The existing switch needs to be a change-over switch, switching active.
  • The two poles of the existing switch are cabled to the new switch, as well as a single return wire. (That's three wires between the switch plates.)
  • The new switch is also a change-over switch, switching the return wire between the other two.
  • Behind the existing switch, you connect the return wire to the light socket's active wire. This is the connection which doesn't connect to the existing switch.

Normally you would use the loop terminal for the last point, but that might already be in use for the neutral connection. In that case, you will need an in-line connector.


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    I think i've found the schema for your post. Could it be the second one from this website ? familyhandyman.com/DIY-Projects/Electrical/Switch/… – Alexandre Jobin Apr 30 '12 at 1:54
  • Yes, Diagram B would be what I had in mind. – staticsan Apr 30 '12 at 7:33
  • You WILL need to replace the existing switch. You will need a length of 14/3 wire and two three-way switches. Normally in the packages for the switches (may be on the inside of the box itself) there are directions for what you want. Basically, there will be a hot, neutral, common, and ground on both switches. Hot, neutral, and ground are self explanatory, the common is the wire that "tells" the other switch if it is on or off. – ShoeMaker Apr 30 '12 at 20:19
  • This advice is almost correct -- you'll need to use 14-4 instead of 14-3 if this is in a bathroom or habitable room, in order to provide a neutral at the newly added switch. (Means you won't have to faff with retagging the white wire, either.) – ThreePhaseEel Mar 17 '15 at 0:43

I like the diagram here - (On this page go down to the part labeled '3-way Switch Variations' and look at the first one). Basically, you run an extra wire out to the new switch to carry the hot out to the end of the chain, then hook things up per normal 3-way. You can instead run the lamp's hot wire out to the other switch, it really doesn't matter.

What matters is that you'll need 4 wires from the existing switch box out to the new switch. You'll have the usual white/black/green + a red wire as the traveler.

You'll need to replace the existing switch with a 3-way switch, and of course the new switch will need to be a 3 way as well.

When done, DO make sure you test all 4 possible switch configurations (up/up, down/down, down/up, up/down)! It's possible to screw this up such that you get the two switches chained instead of operating as a 3-way.

  • You only need three wires to the new switch, not four. – staticsan Apr 30 '12 at 1:15
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    @staticsan: 3 wires plus the ground wire = 4. – Brock Adams Apr 30 '12 at 1:25
  • Ah. Australian switch mechs usually do not require a grounding wire (and it isn't technically required for the circuit, anyway). – staticsan Apr 30 '12 at 7:34
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    When you say "you need 4 wires", like @staticsan I read that as 4 + ground. Ground has to go everywhere, so it is generally not mentioned. This is a standard convention- For instance, if you go buy your standard NMD/Romex "14/2" wire, you'll get 2 14AWG conductors (white/black) plus an unshielded ground. Likewise, 14/3 (which is probably what will be used in this situation) refers to 3 conductors (probably black/white/red) plus a ground (unshielded or green). – gregmac Apr 30 '12 at 17:50
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    Should also note: when you're using a white wire as hot in this way, you must mark it as hot: this is done using red or black marker or electrical tape. By code white means neutral. – gregmac Apr 30 '12 at 17:52

The way I would do it:

  1. Run a 3-conductor (+ground) Romex wire out to the other switch location. (if it's already there, great).
  2. Run a 2-conductor (+ground) Romex wire to the light (again, if already there, great).
  3. On the remote switch (the one without the power OR light run), put the red and black wires on the two terminals other than the "common".
  4. The white wire is, by code, normally the neutral "return path". In this case, however, it must carry the "hot" potential from the combination of the two switches. So, take some black or red (it MUST be one of those two colors; black is easier) electrical tape and wrap a length around the end of the insulation of the white wire. This marks the wire as being "hot" despite the actual insulation color. Now, connect this wire to the "common" terminal.
  5. Back at the box that has the panel, light and traveller conduits, connect the panel's hot wire (should be black) to the common terminal of the switch, and the black and red wires of the traveller to the non-common terminals. Take the white wire from the traveller conduit, and mark it with black tape (remember it's really a "hot" wire). Join this marked wire to the black wire of the lighting conduit, then join the white neutral wire of the light to the neutral from the panel.

The other way I would do it, which would be my preferred method if all the wires weren't already there, is to invest in these Lutron Maestro Dimmers:

enter image description here

They make handheld remotes and wall switches. Basically, one master switch actually hooks into the house wiring and controls the light, and that switch also responds to other switches that send IR or RF commands to turn the light on, off or dim. This actually has some advantages over a traditional hard-wired 3-way switch:

  • Any dimmer control synced with the master control can be used to dim the lights. In a traditional 3-way setup you can put a dimmer into the circuit, but the lights can only be dimmed from that one location and not from the other switch.

  • These systems can provide 3-way, 4-way, 5-way, etc switching with relatively simple wiring.

  • The available handheld remote allows you to dim the lights from your easy chair.

The only trouble is they're expensive ($25-35 per device, roughly double Lutron's non-Maestro decorator dimmers) and they either require batteries or to hook into existing house wiring for power (though it doesn't have to be the circuit they control). But, they can do some pretty cool tricks; you can use these to coordinate all the lights in your home, add timers and motion sensors that run programs for an entire room's or house's lighting, etc etc.

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