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I am in the process of wiring a new bathroom circuit that includes normal lighting in addition to a fan/light combo. There is a three-way switch that will control the light in the bath fan (as well as the other lights.)

Another switch controls the fan. The three-way switch (lighting) is serviced by a 12/3 run, and the outlets and fan switch are serviced by a 12/2 run, both on the same circuit (notice the single "source power".)

The light part of the fan/light combo is served by the hot on the 3-way switch. The fan is served by the hot from the "outlet" run switched by the fan switch. The neutral for both is from the "lighting" run. Notice the neutral from the "3-way run" connected to the "light" and on to the light/fan.

Boiled down: is it allowed to use a single neutral on the 12/3 for two "hots" that are served from separate runs of the same circuit? Possibly similar to this answer.

Picture of the circuit:

Circuit diagram

  • I hope that the little neutral jumpered off the ground at your lower Outlet box is a drawing error ... – brhans Apr 4 '17 at 19:18
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While the neutral to the fan is on the same circuit as the other devices, the original drawing suggested that it is not in the same raceway or cable as the hot wires to the fan. This is not allowed under the code:

NEC 300.3 (B) - All conductors of the same circuit ... shall be contained within the same raceway ... [or] cable ...

UPDATE: The revised drawing does indicate that the neutral wire is in the same cable as the hot wire(s). Consequently, the neutral is on the same circuit and in the same cable, as allowed under the code. While there are two hot wires, they are actually the same circuit and merely one hot, using two wires and switched separately for fan and light, a very common pattern.

  • It is in the same cable, a 12-3 which runs from the box, to the first light, then finally up to the fan/light. The "3-way light switch, outlet, fan switch" are in a 3-gang box. Another drawing error! That neutral going to the fan/light is the same coming from (outlet/fan switch) run. I'll update the drawing. – Peter J Apr 4 '17 at 19:30
  • I appreciate the answer. Definitely makes sense. The inspection is coming up in a few days, and I didn't want any surprises. – Peter J Apr 4 '17 at 22:39
  • Does Harper's answer raise any concerns, namely any imbalance of the switched-hot for the fan traveling from the 12-2 outlet run to the neutral on the 12-3 run? – Peter J Apr 5 '17 at 1:51
  • I don't see the imbalance he suggests. There is both a hot and neutral on the same circuit in that cable. – bib Apr 5 '17 at 19:52
  • It's a hot and neutral from the 12-3 run, and an additional hot from the 12-2 run. Same cable. – Peter J Apr 5 '17 at 20:41
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Another way of putting it is that the currents must be equal (cancel each other out) in each cable.

As you have drawn the picture above, I'm a little confused by what's going on with the 3-way switches, but you will have unequal currents in the rightside 12/2 run.

You could still do it by adding some more 12/3 and islanding the 3-ways, i.e. Have both 3-ways be dead-end spurs off the "junction", then bringing neutral and switched-hot for the light up on /3 via the fan switch. If that makes sense.

  • I think that makes sense to me, but I want all lights (including the light in the fan) to be switched-hot on the 3-way. – Peter J Apr 5 '17 at 1:36
  • What can I clarify on the 3-way? I'm sure it's drawn iffy. – Peter J Apr 5 '17 at 1:37
  • Basically for the fan, the current is travelling from the 12-2 run hot and back the 12-3 run neutral. Since it's the same source power circuit, I thought it would still be safe. – Peter J Apr 5 '17 at 1:43
  • @PeterJ yeah, that last is what you shouldn't do. If there is any metal between those cables ( i.e. Cable staple) that becomes the core of a transformer, but without those important laminations. – Harper Apr 5 '17 at 6:05

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