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I am trying to figure out my options for installing a smart switch in my configuration without running a new wire with a neutral. This is quite similar to other questions I'll reference later, so apologies in advance if I am failing to take the right lessons from them.

Here's my configuration:

  • Two circuits on adjacent breakers, one serving outdoor lights and the other serving outdoor outlets.
  • A 2-gang box has two switches, a smart switch controlling the lights and a basic single-pole switch controlling the outlets. One of those switches is already a smart switch (controlling the lights). The other switch is a basic single pole switch.
  • A junction box sits between the electrical panel and the switches
    • A Black/red/white/ground cable goes to the panel, with black being hot for one circuit, red hot for the second circuit, and white a shared neutral.
    • Two cables go from the the junction box to the box with the switches. A black/red/white/ground cable carries the hot/load/neutral for the smart switch and a black/white/ground cable carries the hot/load for the single pole switch.
    • Two other cables (black/white/ground I believe) leave the junction box, one for the lights and another for the outlets.

Can I replace the single-pole switch with a smart switch and use the same neutral wire that the existing smart switch is using, given that the two circuits are sharing a neutral at the junction box? My impression from a few answers is that it should be a code problem more than an electrical one. My read of the answer to this question is that even if it would be electrically okay it's a code violation because the neutral for the smart switch isn't in the same cable as the hot between the junction box and the switch. On the other hand, nobody raised an objection to this question where it was deemed to take the neutral from another switch in the box as long as it was the right neutral and presumably they were not in the same cable, so maybe there's something okay here?

If I can replace the switch the easy way I will do that, but if I can't, another option would be to move the lights and outlets to the same circuit. The lights are now LEDs and the outlets will only be used to power LED lights, so they do not require much current. Is this viable/better?

Edit: Adding some pictures...

  • The boxes The boxes (the panel is a foot or two to the left)
  • The cables leaving the junction box for the switch box In the junction box, the cables leaving for the switch box
  • the incoming cables in the switch box The incoming cables in the switch box
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  • As long as you have full sized handle tied breakers it is fine, we had a question a while back that hat a tandem breaker and that would overload the neutral, but not a problem with 2 pole or 2 full sized handle tied breakers.
    – Ed Beal
    Commented Jun 8, 2021 at 22:00
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    It would be electrically OK if it was DC power. Since it's AC, it's not electrically OK. It's not about "which circuit", it's about wire grouping. ThreePhaseEel's answer describes how to rearrange the /3 and /2 cables to handle neutral properly. You'll be happier anyway with that, since it brings a whole leg of the circuit here, instead of it being two switch loops. (by the way /2 switch loops were banned in NEC 2011).
    – Harper - Reinstate Monica
    Commented Jun 9, 2021 at 18:34

3 Answers 3

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What you have right now is a MultiWire Branch Circuit - MWBC. That is wonderful because it lets you get twice as much power over a 3-wire cable as over a 2-wire cable. But the way things are wired right now, it doesn't do what you need with the existing wires. However, rather than changing to a single switch, which would of course limit you to just 1/2 the circuit, you should be able to change to two smart switches using 1/2 the circuit. Here goes:

Panel

  • Remove the red wire from the breaker. You can use this breaker for a new circuit if you want to.

Junction Box

  • Incoming power - cap the red. Connect black and white to the 2-wire cable going to the switch box.
  • Outgoing (to lights) cable - connect black to black of 3-wire cable, connect white to white of 3-wire cable.
  • Outgoing (to receptacles) cable - connect black to red of 3-wire-cable, connect white to white of 3-wire cable.

Note that unlike a typical switch box, the whites here are not all together. There are two sets of neutral/white wires - one for the line side (from panel, to switch box) and one for the load side (from switch box, to lights & receptacles) and never the twain shall meet.

Switch Box

  • Connect black of 2-wire cable to "hot" of both switches (the old smart switch and the old dumb/new smart switch)
  • Connect white of 2-wire cable to "neutral" of both switches and to white of 3-wire cable.
  • Connect black of 3-wire cable to "load" of old smart switch.
  • Connect red of 3-wire cable to "load" of old dumb/new smart switch.

The end result of the switch box is that it will look like a typical light/fan setup with power in on one cable and two switch loads out on the other cable.

The key is that matching hot/load (switched hot) and neutral are always together:

  • Panel - 2 of 3 wires in the cable (3rd wire no longer used)
  • Junction to Switch - 2 of 2 wires in the cable
  • Switch to Junction - 3 of 3 wires in the cable
  • Junction to Lights, Junction to Receptacles - 2 of 2 wires in each cable
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  • What? A mwbc that is on 2 full-size handle tied breakers the neutrals are 180 out of phase so they will not be over loaded. With single phase an overload will trip the breaker 100% of the time the neutral is not overloaded. The 2 separate circuits can go different directions and be controlled by whatever switch.
    – Ed Beal
    Commented Jun 8, 2021 at 21:46
  • @EdBeal Absolutely not a problem of neutral overload. My concern (and I think my wiring changes account for this appropriately) is that if you have one switch on /3 = hot-A/load/neutral and the other switch on /2 = hot-B/load then if switch 2 itself uses neutral (borrowed from the /3) then there won't be a balance/matched current in the /3 cable. Switch A is off and drawing 1 watt in standby. Switch B is functioning and drawing 10 watts (broadcasting WiFi, whatever...) - the /3 will have 1 watt on hot, 10 watt on neutral = unbalanced = bad (eddy currents?)
    – manassehkatz-Moving 2 Codidact
    Commented Jun 8, 2021 at 21:57
  • @EdBeal If you had two GFCIs - one for each branch - in that main junction box, the simple "grab the neutral from the other switch" would result in both GFCIs tripping.
    – manassehkatz-Moving 2 Codidact
    Commented Jun 8, 2021 at 21:59
  • try again gfci’s will nuisance trip without a switch that’s why pros like me don’t use them much any more a multi wire branch circuit is just that all the return goes back on the neutral.
    – Ed Beal
    Commented Jun 8, 2021 at 22:11
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    Thought experiment: Future owner decides they want a convenience outlet in this switch box. Grabs hot and neutral that are going to switch B. Plugs in a portable heater 1,500W. Now that /3 cable has 1 watt (standby current of switch A when "off') on the hot, 0 watt on switched hot *and 1,500 W on neutral. That /2 cable now has 1,501 watt on hot (1 watt standby for switch B when "off" + 1,500 W for the heater) and 0 W on switched hot. So each cable is essentially a single wire at ~ 1,500W instead of a pair (or triple) of balanced wires.
    – manassehkatz-Moving 2 Codidact
    Commented Jun 8, 2021 at 22:24
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You'll have to either rearrange things or redo the wiring run

Your problem is that the original installer lashed this together from the 12/3 and 12/2 NM they had on their truck instead of stopping to think about this for a moment. Right now, it's not a Code issue, but it becomes one as soon as you add the second smart switch, whereas it wouldn't have been had the installer put a conduit between the two boxes instead of using cables for this run.

Why? It has nothing to do with the multi-wire branch circuit (a red herring in this case) and everything to do with the need to keep currents balanced in cables to avoid the generation of errant inductive loops in your walls. This stems from NEC 300.3(B) and 310.10(H), as those inductive loops can induce eddy currents in ferrous objects (such as fasteners and box-parts) inside them, which in turn heats them up with the attendant consequences of having hot nails in your wall.

Fortunately, it is possible to rearrange things to make it all work on one circuit. You'll have to use the /2 cable to bring always-hot and neutral from the lighting circuit to the switch box, then retask the /3 to bring both switched-hots and the load-side neutral back to the junction box, and not connect the two neutral bundles (line-side and load-side) together in the switch box. If that's not an option for you, then you'll need to take the two cables between the two boxes (junction and switch) out and replace them with a conduit run; a ½" ENT ("smurf tube") with individual THHN wires in it is inexpensive and will do the trick provided the cable runs themselves are within the walls.

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  • Briefly, four wires need to be run close. Since that type of cable does not exist, some conduit should be used. Or 3 cables.
    – user263983
    Commented Jun 9, 2021 at 1:07
  • @user263983 -- /4 NM is a thing, albeit not /5
    – ThreePhaseEel
    Commented Jun 9, 2021 at 1:20
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    @manassehkatz-Moving2Codidact -- thanks for the explanation, edited
    – ThreePhaseEel
    Commented Jun 9, 2021 at 1:23
  • Pls provide a link, I never seen that.
    – user263983
    Commented Jun 9, 2021 at 11:28
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    Almost. If it is really 6" (or even a few feet) between the boxes, then the conduit (smurf tube) solution makes a lot of sense. But what you do then is not put the cables through it, but rather get some individual wires (THHN - can't just shuck the cables to get wire for this purpose) to match all the necessary connections. If you do that, you can actually get just black & white & green, because you can use black in place of black & red - but label all the black pieces using colored electrical tape to make it clear what goes where in case you need to do any future repairs/changes.
    – manassehkatz-Moving 2 Codidact
    Commented Jun 9, 2021 at 16:26
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It should be fine to use the neutral in this situation because its already shared between the two circuits. This is a "multi-wire branch circuit" (MWBC) and the breakers that the red and black wires go back to should have their handles tied so that they trip at the same time. In the first question you linked, the objection was using a neutral from a unrelated circuit and the possibility of overloading stemming from that.

In a MWBC, the tied handles on the breakers protect the neutral from an overload on either the red or black circuit. Also, the neutral that goes to the smart switch is only used to power the switch electronics and not the load being switched. The current is going to be in the low milli-amp range and would not pose a hazard.

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6
  • I don't think that is legitimate. Take an extreme case - new smart switch uses 1,500 W (OK, that would be a bit extreme...smart switch heater) - all 1,500 W will be on the white of the /3 for neutral, but the hot would be in the /2.
    – manassehkatz-Moving 2 Codidact
    Commented Jun 8, 2021 at 20:50
  • The way I read it, the MWBC breaks out into the /2 and the /3 and goes to the switch box, so everything is already running on the same neutral back to the main panel.
    – JPhi1618
    Commented Jun 8, 2021 at 20:57
  • This looks correct the mwbc branches off as they usually do and either branch can be controlled or used as needed. @manassehkatz maybe I missed it but the op did not provide the wire or breaker size but if the device was 1800w on a 15 amp circuit that is 100% loaded but there is a safety factor no electronic switch is going to draw enough to make a difference even at that level.
    – Ed Beal
    Commented Jun 8, 2021 at 21:55
  • @EdBeal I actually agree that practically speaking there shouldn't be a problem - a typical smart switch draws very little current. But in principle, it would end up unbalanced in the /3 cable.
    – manassehkatz-Moving 2 Codidact
    Commented Jun 8, 2021 at 21:58
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    It will never be unbalanced at the breaker. The unbalanced part is in the /3 cable. That cable has one of the hots, the neutral already split, so it should match only that one hot, and the matching switched hot. If you grab that neutral for the other switch, that segment of neutral (junction to switch) will no longer match the hot in that same segment.
    – manassehkatz-Moving 2 Codidact
    Commented Jun 8, 2021 at 22:21

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