I have some (seemingly common, but specifically unanswered) concerns about providing a neutral wire to smart switches that are in a single multi-gang box (4-6 switches on different circuits). I am wondering if it is acceptable to use a single neutral line run directly from the electrical panel to serve all the switches, even though they are on separate circuits with their own neutrals at the fixtures.

See diagram of what I am thinking below. Four "smart" switches requiring neutral on separate circuits with extra neutral run from panel

This question relates to this and this question (to which the answers were "not ok"), but the difference being that I am not "stealing" a neutral from another nearby outlet or switch, but running an extra neutral directly from the panel.

To better answer @ThreePhaseEel , the setup is a bit out of the ordinary, and the current (inherited) physical setup is like this:enter image description here

  • How is this new neutral wire being routed?! Jun 28 '17 at 3:54
  • All through rigid emt conduit, connected to an exposed steel 4x4 raceway. The neutral wire will be in the same conduit as the hot wires on their way to the switch. All other circuits are branches off of the 4x4 raceway. *actually, two of the switches will come from one raceway on one side of the room, and two will come from the other, so there are two sticks of rigid conduit entering the box, with a raceway on each side of the room.
    – Ryan
    Jun 28 '17 at 3:57
  • By "raceway" do you mean "junction box"? Or are you talking about an actual wireway here? Jun 28 '17 at 4:02
  • Actual wireway, like this link
    – Ryan
    Jun 28 '17 at 4:04
  • Which subpanel is feeding which switches/lights in this setup? Jun 28 '17 at 4:15

Keep 'em separated

Since you have two subpanels feeding this setup (one for each "side"/half), you'll need to make sure that the switches for each side have a neutral return back to the corresponding subpanel, and only the corresponding subpanel. In other words, the two left-side switches have a neutral wire going back to the left-side subpanel, and the two right-side switches have a neutral wire going back to the right-side subpanel.

Cross-coupling the two smart-switch-neutrals in your setup violates 300.3(B)/310.10(H) and creates a neutral current loop that causes inductive heating of metal parts (like your raceways and boxes) as well as magnetic field EMI due to the loop area of the current (old-school picture tubes were susceptible to this in particular).

Neither of these neutrals needs to join with the light fixture neutrals on its side until the subpanel neutral bar, though. The smart-switches are loads of their own (to power the "brains"), and running an extra neutral homerun in the same cable, conduit, or raceway is OK by code, as per 300.3(B) (otherwise, /2/2 NM wouldn't be Code legal, for that matter):

(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding con- ductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

Load isn't a concern here, but Code is

The primary concern for multiplexed neutrals is having multiple hots on the same phase return via the same neutral, which causes the currents on that neutral to be the sum of the currents on the various hots. In this situation, though, that isn't a concern as smart switches don't draw much power at all.

However, technically speaking, unless you punch the circuits down as such to have currents cancel (i.e. a MWBC -- such things do exist in the three phase wye world, whether they are two-phase/three-wire or three-phase/four-wire), your proposal violates 200.4(A):

(A) Installation. Neutral conductors shall not be used for more than one branch circuit, for more than one multiwire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in this Code.

  • I can run a neutral line from each sub. To confirm that I understand: By cross coupling, you mean running all neutrals of the smart switches off of one subpanel? It is alright to have two switches on one subpanel side share the same neutral even though they are on different "hot" lines of the same subpanel (and the have independant neutrals at the fixture that return to the same subpanel). This would not be a case of cross coupling, and would not cause a load imbalance between wires? (i.e. let's imagine in the first drawing there is one subpanel instead of two...is it acceptable then?)
    – Ryan
    Jun 28 '17 at 4:40
  • In short, is this correct
    – Ryan
    Jun 28 '17 at 4:55
  • @Ryan -- that's what I'm envisioning, yeah Jun 28 '17 at 11:34
  • One caveat though -- I'm assuming this is all protected by regular breakers, not ground or arc fault interrupters, correct? Jun 28 '17 at 11:37
  • Another caveat is that each switch is on a separate breaker, regardless of which subpanel, right? If the hot wires feeding the switches are on separate poles, the shared neutral will carry the difference of the loads since the power from each pole of the transformer is 180 deg out of phase with the other. If they're on the same pole, the shared neutral will carry the sum of all the loads. The saving grace here is that the smart switches themselves are small loads and don't add up to more amperage than the wire can safely carry. But code may call for a neutral per switch, depending. Jun 28 '17 at 12:41

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