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I'm planning to replace a number of 3-way switches with smart switches and as a necessary first step have been trying to map out the wiring in each of the relevant boxes. This is what I've come up with (it's Powerpoint, try not to make fun ;)):

Wiring Map

This shows 3 boxes on two floors. A through I are all box entry points. The colors represented are what is actually used (except gray has been substituted for white). A simple gray box in the diagram is just a connector. The dotted lines are what I believe is going on in the walls, but that's where I would love to get a second opinion.

The tricky one for me is the leftmost box (2nd Floor Middle Stairway Bottom) because it contains wires from two different circuits, as I confirmed using a voltage tester (even when the breaker for "Circuit" was off, the purple and peach wires in that box were hot - which I guess isn't illegal but I was none too pleased to learn).

I assume the orange wire coming out of S1B's common terminal splits at or near light 1, in the 2nd floor ceiling, providing the load for light 1 and also ultimately for light 2, on the 3rd floor ceiling, as the orange wire from D-E is unbroken. I couldn't find this particular configuration as an example of multi-light 3 way wiring, though. I guess the other possibility is that the orange wire goes C-D-E unbroken, and at light 2 splits and goes all the way back down the stairs to light 1 through another conduit in the ceiling, but at the end of the day I'm not sure it matters.

What I'm less sure about is the neutrals. At first I thought the white at C connected to the white at D, but then it occurred to me that that would mean that the neutrals from 2 different circuits were all connected (if we assume the white at A and B are on the same circuit as the purples and peach). That would be a big no no, right? So on second thought, it seems like the white at D must be the return from light 1, and the whites in that 2nd floor box, like the purple and peach, have nothing to do with the circuit I'm interested in. Rather everything on the circuit I care about ultimately starts and returns through F, which I believe connects to an outlet on the other side of the wall.

So, does this seem right or could there be other possibilities I haven't considered? And how could I be more certain?

The reason this matters is that it determines where the smart switches have to go, and what kind I have to use. And also I just want to be sure that when I play with the white wires in the 3rd Floor Stairway Top box that I'm not going to get in trouble with that mystery circuit that shares the 2nd floor box.

Thanks for any insights!

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  • (Assuming USA.) One of the wiring techniques no longer allowed is to use a single neutral for two hot circuits (12/3 w/G), thus saving copper. The hot circuits must be from different incoming legs of the main supply, i.e. they are 240VAC hot-to-hot and both are 120VAC hot-to-neutral. Since the hots are 180 degrees out-of-phase the neutral current can't exceed what a single hot can supply. It also means that if you need to move a breaker it must stay on the same leg. Pull the cover of the breaker box to confirm the arrangement: typically two breakers feeding black and red with a common white. – HABO Feb 1 at 22:12
  • FWIW, my fallback for wire tracing it to disconnect the devices and non-ground connections in all of the boxes and then ring out the wiring with a long wire, alligator clips and a DMM. Run the wire from one box to another and have at it, checking all combinations. Always check for voltage first, then continuity only if the voltage is zero. Continuity from white to ground indicates the bonding at the breaker box, i.e. you've identified a source wire. Label everything as you go and then diagram the result. – HABO Feb 1 at 22:18
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    @HABO You're talking about a multi-wire branch circuit and those are perfectly fine. You have to follow more rules than you used to (handle ties; pigtailed neutral) and they don't play very well with common AFCI breakers, but yeah... legit. – Harper - Reinstate Monica Feb 1 at 23:37
  • Oh interesting, so you're both saying it's possible that the whites at C and D actually do connect and are not part of two totally isolated circuits. So I guess I need to figure out what circuit those purple and peach wires in the leftmox box are on and then look at the breaker box to see if they have a shared neutral. Either way I don't think it should affect my smart switch config but it would be nice to know the whole picture of what's going on in there. Thanks both. – Peter Moore Feb 1 at 23:58
  • What make and model are the smart switches in question, and can you get us photos of the insides of the boxes involved please? – ThreePhaseEel Feb 2 at 2:06
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I assume the orange wire coming out of S1B's common terminal splits at or near light 1, in the 2nd floor ceiling, providing the load for light 1 and also ultimately for light 2, on the 3rd floor ceiling, as the orange wire from D-E is unbroken. I couldn't find this particular configuration as an example of multi-light 3 way wiring, though.

Right. Most novices get intrigued by those "color-by-numbers" drawings of 3-way circuits all over the web, which are made for use with /3 cable. Your installer took full advantage of being in conduit and having a variety of wire colors... by doing something totally bespoke that is correct for your installation.

In other words the original installer took a procedural approach to a 3-way circuit:

  • Connect both 3-way switches with 2 travelers (lavender here)
  • Connect always-hot (pink here) to the common of one 3-way switch.
  • Connect switched-hot to the other 3-way common and all lamps.
  • Connect neutral to all lamps.

Treat each wire individually and you can see what they did makes sense.

I believe you will find conduits C and D both go to the lamp box of lamp 1. The (lavender) travelers pass right through the box.

I believe that lamp box also has the purple, peach and that other neutral passing through it. These wires have nothing to do with your 3-way circuit; leave them alone. No, you can't poach neutral from that. Neutrals must stay with their partner hots.

The good news is, this being conduit, you can have anything you want anywhere you want - you just have to pull it. So you can pull neutral down from wherever C goes, onward to D if necessary. However since you will then have 2 neutrals in the box, you must clearly mark which is which, e.g. by wrapping associated wires with a couple loops of tape.

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  • Thanks! That all makes sense, and so it sounds like you think the whites at A B and C DON'T actually connect to the white at D, and that the white at D is just lamp 1's return, am I right? Anyway at this point I'm planning to just put the smart switches in that 3rd floor stairway top and use the neutral (and hot obviously) coming out of F for both. The Zooz Zen27 works with analog 3way switches and doesn't require neutral at the other switch location, so I can leave that leftmost box alone. It's very crowded (1 gang) and may already be out of code, but def doesn't need more wires! – Peter Moore Feb 2 at 13:45

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