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I found my question difficult to phrase, so I mocked the wiring diagram up in Photoshop. I have a 3-way switch setup like so: 3-way switch diagram

I am going to upgrade this to a Z-Wave enabled pair of 3-way switches with dimming, but both switches would require a neutral to function. Easy for switch #1, but since switch #2 is on a switch loop, it doesn't have a neutral available from the 12-3 I ran to it.

However, it is in a receptacle which has power for a different 3-way switch (to a different light) that is further along on the same circuit. What I want to know is, is it ok for me to draw power for the neutral from the line that feeds switch #3 (proposed wires illustrated in purple below)? 3-way diagram with neutrals added

It SEEMS to me like that may be ok as it is just connecting to the same conductor at different places, but I would rather be safe than sorry and ask more knowledgeable folk before buying the product and just wiring it up.

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  • Are your boxes metal or plastic?
    – ThreePhaseEel
    Commented Nov 24, 2015 at 3:23
  • Now I'm curious. Why does Metal or Plastic matter?
    – Billy left SE for Codidact
    Commented Jan 6, 2016 at 5:57

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Practically speaking, this arrangement should work without an issue, as everything is on the same circuit and a smart-switch isn't going to draw enough current to significantly heat a metal box by induction. However, there are some very sticky Code-compliance issues involved with funky neutral routing -- the 300.3/300.20 mess is not at all clear, and if you have metal boxes, what you describe is technically a Code violation unless you modify the box to put a slot between the switch-loop cable and the incoming-power cable.

A plastic box, though, is immune to these sorts of issues...but the neutral not being in the same cable as the hot could still be seen as a 300.3 type of issue by the AHJ. So, you're on the right track, but double-check with your local electrical inspectors to make sure that they won't ding you on this!

Of course, running a 12/4 instead of the 12/3 for the switch loop lets you run a neutral with the loop -- simply connect the switch loop's neutral to the smart switch, and then do not connect the incoming-power neutral to the switch loop's neutral in the loop-end box, or else you'll definitely be violating 300.3(B)/310.10(H) due to the inadvertently looped/paralleled neutral!

And while you're at it with the walls open -- run a 12/4 for the other switch loop -- the NEC has a requirement for the neutral to be present now, even if you aren't putting in a "smart switch" straight away. (Just wirenut it off at the unused end if you want to keep the dumb switch there)

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  • Thanks for the quick response. I have plastic boxes thankfully. I actually have the wall opened up right now anyway. I was originally going to put the standard dumb switches in, and ran the 3 conductor wire. Now that I'm thinking about upgrading to smart switches, would the safer way to do this be to just rip out the 3 conductor cable and replace it with a 4 conductor cable? Then use the 4th wire to run the neutral.
    – Colton McCormack
    Commented Nov 24, 2015 at 3:50
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    @ColtMcCormack -- that's how it's normally done, yeah -- see my edits.
    – ThreePhaseEel
    Commented Nov 24, 2015 at 4:22
  • Sounds good. However, your last paragraph got me thinking. The wall that the second pair of switches run through is not currently open. To add insult to injury, while the 4th switch is in a receptacle with another switch that has power, that other switch is on a completely different circuit. After a little research, it looks like you can buy these z-wave dimmers with wireless 3-way functionality. So, you just use 2-conductor wire everywhere and "link" switches for 3-way functionality via wireless programming. I might just convert both pairs to use that. Your help has been much appreciated!
    – Colton McCormack
    Commented Nov 24, 2015 at 4:47
  • @ColtMcCormack -- if the wall there's closed, there's no need to re-run the switch-loop right away. Just keep it in mind for future smart-switchery or whenever you open it up...
    – ThreePhaseEel
    Commented Nov 24, 2015 at 4:49
  • One of the principal advantages of home automation devices is that you do not need to physically wire 3-way circuits (with the split hot paths.) I would recommend you install the same switch modules in each of the four places. Treat the 12/4 as if it were 12/3, and use it to supply power to the second modules on each circuit, and wire nut their load terminals. Each switch will think its controlling its own light, and two switches will be doing nothing in reality. Then simply program the do-nothing switches to control their appropriate partner switch, and vice versa if they have indicators
    – Billy left SE for Codidact
    Commented Jan 6, 2016 at 5:59

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