An existing circuit in a bath (in TX) has the following diagram.

Can the 3-way light switch be replaced by a dimmer switch (without changing the wiring)?

Does the other 3-way switch need replacement at the same time?

R is for Red, B for Black, W for White, G for Ground.

Edit: I have incorrectly labeled 3 as Line. It should be 2.

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Achievement Unlocked: Wire a box with 6 different wire functions without using any phase tape

Sorry, no points for that "achievement", just a lot of brain pain. Let's try this.

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Ah, suddenly I can hear myself think

Most of this looks pretty straight forward. The top /3 cable is the supply side of a 3-way circuit. The bottom /3 cable supplies light and fan in the usual manner.

But something rather squirreley is going on with cables 2 and 3 here. You've labeled cable #3 as "Line" but then, feeding all the switches off the black wire of cable 2 doesn't make any sense.

So this raises more questions than answers.

What might make more sense is if cable 2 was actually line, and comprised a multi-wire branch circuit, with one pole serving all loads here, and the other pole heading off to parts unknown in cable 3. Does the breaker for this circuit have 2 handles and the handles are tied? (on Square D "QO" this might look like a double-wide breaker with a single handle).

  • As it was the only 2-wire and not having a red I assumed it would be the line. But I could easily be wrong. The breaker box has a few cross marks for this room (indicating previous owners had an issue too) and I found too late that more than one handle controls the flow in that area. I have to check with a multimeter to be sure what is the line. There are no physically "tied" handles in the breaker. Just single handles.
    – Maesumi
    Jan 4 '21 at 2:48
  • @Maesumi OK, try out the hypothesis that cable 2 is actually supply, and red and black wires are supplied by different breakers. Does that make actions jibe with measurements? Jan 4 '21 at 4:52
  • Yes #2 is the line. My mistake.
    – Maesumi
    Jan 5 '21 at 15:31

Since it appears that this 3-way is where power is supplied (line end), yes, you can normally change this one out for a dimmer without changing the other one.

For the 3-way dimmers I have personally used, the "non-power-supply-end" (load end) does not work. Others may vary.

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