Electrical – How to add a light fixture to an existing 3-way circuit

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I'm trying to figure out how to wire a utility light to the existing light circuit in my basement. I've attached a diagram of the switch that I'm hoping to tap into (Switch 1). The switch is at the top of the stairs and there is another switch (Switch 2) on the circuit at the basement door. Coming into Switch 1 is a 3-wire cable wired to the switch as shown in the diagram.

When Switch 2 is on, the red wire at Switch 1 is hot and when Switch 2 is off, the white wire at Switch 1 is hot. When Switch 1 is turned on, both the red (or white) wire AND the black wire are hot. (This is probably all obvious to an electrician…)

Any help would be appreciated.

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Best Answer

You can't just pick any random point and fork a lamp off that. They do not work that way.

The simple way

3-way circuits have several tricks and traps, and you can't get anywhere with them unless you really understand how the circuit functions, how switches are designed, and I really, really recommend wires be marked by function with colored tape.

So the simple way to do an end-run around all that complexity is supply the second lamp from the first lamp's location. You are simply grabbing the 2 wires (and ground) supplying the first lamp, and paralleling it to a second lamp. Badabing, done. You never have to think about 3-ways.

The hard way

You can stop and train up on wiring and 3-way circuits. I'm all for education. But in this case, this will only make you realize you can't get there from here. You cannot feed a second lamp from this location.

The smart way

This is a different kind of smart, where we toss traditional 3-way wiring into the dumpster of history, and go with "Smart Switches". There is a huge variety of smart switches, and we're not a "Shop For Me" site... but there's at least one out there that'll Git-r-Done for you.

What's unique about smart switches is we toss out the goofy schemes and oddball wire functions found in traditional 3-way wiring. We reduce the wires to 3-4 functions:

  • Always-hot goes to every switch location, and use Black.
  • Neutral goes to every switch location and every lamp, and use White.
  • Switched-hot goes from the "master" smart switch to the lamps, using whichever wire color is left.
  • Some smart switches require a datacomm signal between switches, that'd most likely be red. Red can't do 2 things at once.

Ground is a safety shield and can't be used for any of these.

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