I see NEC 300.3(b) coming up quite a lot in discussions about neutral wires and smart switches. I'm looking at a 3-way configuration I have now with analog switches and how I can properly modify it to support smart switches, but I'm not sure the current configuration even complies with 300.3b to begin with. Note none of the other 3-way switches in my house are like this:

enter image description here

P = power source.

Question 1 Doesn't the fact that the C-D conduit carries lamp 5's power source (plus travelers) but not its neutral violate 300.3b? Or if I'm wrong, how am I misunderstanding that section?

Question 2 If I were to change the configuration to the following, would that cure the code issue, assuming there is one? Or is this improperly "stealing" a neutral from the "other devices" to power my smart switches? Does the fact that Lamp 5's return doesn't go through any conduit carrying its switched power source just screw me?

enter image description here

(FYI these are GE Embrighten switches - S5 would be the main switch, S5AO would be the add on). Everything is on the same circuit.

  • What color is the dashed white wire intended to represent? Mar 21 at 13:01
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    White (but used as traveler currently). Just was distinguishing from the actual neutral. Mar 21 at 13:04
  • Meant to say white with black tape, sorry. Mar 21 at 13:23

No, you're actually fine on that front here

The C-D conduit meets 300.3(B) despite not having a neutral in it for the same reason old-style switch loops were compliant with that rule: all the power is coming back via the same wiring path it went to the switch on. In other words, current in on the hot = current out on one of the two travelers, which are both in the same conduit as the hot wire, thus there is no issue with a stray loop.

However, there's no need for the gymnastics with the neutral to the add-on switch

However, your plan does something rather needless by running neutral all the way from the main switch's neutral terminal to the add-on switch. Instead, you can simply tap the neutral connection for the add-on switch off at the light fixture box instead by nutting the white wire from C in with the rest of the neutrals there.

This also is a chance to fix the Code issue that does exist with the current wiring

There is one problem with the existing wiring though, and it has nothing to do with improper wiring loops, but with color. You see, the reason you're permitted to remark a white wire in a cable as a hot is because you can't get cables with arbitrary-colored wires in them without going to great length and expense to have them custom-made for you. However, that's not an issue in conduit jobs, where the electrician gets to choose which wires and colors to run when they pull the wire through.

As a result, that white traveler wire violates NEC 200.7(C) point 1 in two ways:

(C) Circuits of 50 Volts or More. The use of insulation that is white or gray or that has three continuous white or gray stripes for other than a grounded conductor for circuits of 50 volts or more shall be permitted only as in (1) and (2).

(1) If part of a cable assembly that has the insulation permanently reidentified to indicate its use as an ungrounded conductor by marking tape, painting, or other effective means at its termination and at each location where the conductor is visible and accessible. Identification shall encircle the insulation and shall be a color other than white, gray, or green. If used for single-pole, 3-way or 4-way switch loops, the reidentified conductor with white or gray insulation or three continuous white or gray stripes shall be used only for the supply to the switch, but not as a return conductor from the switch to the outlet.

Since you're turning the white wire in C-D into a neutral, and already have a neutral in the A-B conduit, you can simply remove the extra white wire from the conduit (slowly and carefully!), or abandon it in place by capping it off at both ends if you'd rather not fuss with it at this point.

  • "all the power is coming back via the same wiring path it went to the switch on" - That is the best explanation of 300.3(b) i have heard, and it finally makes sense. Thank you! Also great point re: rewiring at the fixture box rather than repurposing the entire traveler line. Finally, my apologies, the traveler white does have black tape to mark it, but always a good reminder there. Mar 21 at 13:19
  • Just one follow up though - for the smart switches, is it ok that the switches' neutrals would be upstream of the light's? Literally every wiring diagram for smart switches that require neutrals show the switch downstream of the light being switched. That's the only part of this that still bothers me. Mar 21 at 13:23
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    @PeterMoore -- yeah, the black tape is fine if you're working in a cable wiring method such as NM, but a) you're required to use it for a supply to the switch (which is fine on A-B by my reading of the Code text but breaks down on C-D since the travelers are the return path there), and more importantly b) isn't permitted at all in conduit work for the reasons I mentioned above (it's literally unnecessary -- they simply could have pulled another red instead of another white) Mar 21 at 13:23
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    @PeterMoore -- on that one last followup, it's a distinction without a difference :) it'll work fine either way 'round since the loads (lamp and smart-switch-electronics) are in parallel anyway, just like two light fixtures on the switch would be Mar 21 at 13:24
  • "isn't permitted at all in conduit work" ohh wow ok. Yeah the CD line is really weird. It's small and almost looks like it's just a romex cable shoved through on both ends. I couldn't run anything else through that line even if I wanted to. Like I said, not like anything else in the house and definitely has provided the biggest challenge so far. Thanks again for the advice, much appreciated! Mar 21 at 13:30

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