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Apologies if this question is answered elsewhere. I couldn't find it specifically.

I'm planning on installing two multi-gang switch boxes, where all of the switched lighting will be on the same branch circuit. Both of the boxes will have a 3 way switch, which in combination will control one set of lights. All other switches in these boxes are single pole switches.

I'm trying to determine the best way to provide power to each of the switch boxes. Considering I need to run travelers between the two boxes, is it permissible to run a 4 conductor cable (2 travelers, 1 hot, and 1 neutral, 1 ground) between the boxes? This allows me to bring power into one box and through to the next box for additional switched wiring.

Thanks, once again, for your help in advance!

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Yes, that's fine. From the box with the supply coming in, You can run a /4 cable between the boxes --

  • always-hot (e.g. Black)
  • neutral (must be white) - this works because all loads are on the same circuit
  • two travelers (red/blue, or red/white-red depending on if it's 12/4 or 12/2/2 cable).

However in this configuration, the onward power to the lamp must come from the other box.

If you want supply and lamp coming into the same box, you would need /5, and at that point, for availability reasons, you are better off using /3 and /2. The /3 would be entirely dedicated to the spur 3-way, and its white wire would not be neutral. Onward hot and neutral would be in the /2 cable.

As always, you are better off using colored tape to mark the wires by their purpose instead of being stuck with the default colors of cable, which are to distinguish wires. The two travelers don't even need to be distinguished, I mark both with yellow tape.

In the /4 or /2/2 configuration, you cannot substitute two /2 cables for the /4. You will get eddy currents and vibration between the cables if you do: fatigued wires, arcing, trouble. The wires in a cable are bound tight together for a reason.

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    One other option for the /5 situation would be to connect the two boxes in question with a length of 1/2" ENT/smurf and run THHNs in it – ThreePhaseEel Nov 19 '18 at 1:41
  • Thanks for the response. Just added a diagram to clarify how the other loads would be set up. I'm not quite following "the onward power to the lamp must come from the other box." As indicated in the diagram, the 3-way would supply power to their switched lamps via the travelers that enter the second box. Other loads are supplied by power from either box. If I do use /3 and /2, why would the white of the /3 not be neutral? I would use two travelers (red and black) for the 3-way switch, and the white would be connected to the white/neutral of the lamps being switched by the 3-way. – Uncle Woody Nov 19 '18 at 1:50
  • @ThreePhaseEel I actually suggested that in my answer "or run 5 separate wires in a conduit (colored/marked appropriately)" – manassehkatz-Reinstate Monica Nov 19 '18 at 1:58
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    @UncleWoody What Harper is referring to is that if you had power -> light -> 3-way -> travelers -> 3-way, then the "travelers" section would be 3 wires - 2 travelers plus switched hot. Using his preferred color scheme, you would label black & red with yellow for travelers and white with red for switched hot. It would be like hot + neutral in terms of balanced currents within that /3 cable, but would NOT be an actual neutral conductor. The white would actually be connected to the hot side of the lamps with the white/neutral of the lamps connected to the panel white. – manassehkatz-Reinstate Monica Nov 19 '18 at 2:00
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    You could run it with /2 and /3, but to keep currents equal in all cables, you'd need to have 2 neutrals as mana' describes, and the 3-way lamp's neutral would have to come back in the /3 cable all the way to the rightside box, totally firewalled not to interact with the other neutral in any way. Talk about confusing! Try a real electrical supply (not Home Depot etc.) for the /4 cable. Really for everything lol, just walk in there and tell them you've heard Home Depot's prices aren't in line. That'll set you as price conscious. – Harper - Reinstate Monica Nov 19 '18 at 2:44
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The cable between the two 3-way switches is already normally a /3 cable - 2 switched-hot travelers (i.e., only one carrying power) and the neutral (not connected to the switches but connects to the switched light and must parallel the travelers).

If you want to add another set of loads, you would actually need /5 - 2 travelers, 2 neutrals (one for 3-way lights, one for the other load), hot for the other load. Which really means just use a /2 and a /3 run together or run 5 separate wires in a conduit (colored/marked appropriately).

Thanks to Harper for more details: This can be done with a /4 using a single neutral, provided the lights are connected to the 2nd of the 3-way switches:

panel/power --> switch 1 --> travelers/hot/neutral --> switch 2 --> lights and other load

This way the neutral current matches the combined hot (other load) and travelers (3-way lights). However, this could be a little confusing for anyone working on this in the future. Plus, if you ever wanted to split the loads to different circuits you would not be able to do so - though with LED lighting that is far less of an issue than it used to be.

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  • Thanks @manassehkatz. I hadn't thought of the shared neutral. Is this perhaps a question of balanced loads within a cable? I had assumed sharing the neutral would be compliant, but even if it is, I don't want my design to be confusing or far from standard. I'll wait for clarification on the neutral, but probably will do as you say and run the /2 and /3 if that is standard practice. – Uncle Woody Nov 18 '18 at 23:14
  • From a purely logical/technical design, one neutral in that cable (obviously based on everything being on one circuit breaker) is perfectly fine because everything will be, by definition, balanced. But there might be other code issues based on that (I know almost nothing of code, compared to the pros) and I can see it being confusing to others in the future. It is perfectly normal to have one neutral (and hot, of course) come into a box and split to multiple switches, other boxes, etc. But here part of the hot is switched already (the travelers), so I just don't know. – manassehkatz-Reinstate Monica Nov 18 '18 at 23:17
  • You don't need 2 neutrals if they're the same circuit. – Harper - Reinstate Monica Nov 18 '18 at 23:33
  • @Harper As I said, I wasn't sure. Thanks for giving a definitive answer. – manassehkatz-Reinstate Monica Nov 19 '18 at 0:21

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