Help wiring light/fan/heater

I am trying to install a combination light/fan/heater in a bathroom. There are already 2 sets of wires going from the switch box to the ceiling location, but I need 3 (one for each light, fan, heater). Would it be OK to share a single neutral for all three devices and use the extra white wire in the second cable as an additional hot? The two cable are different colors (black and white) and the black seems to have larger gauge wire than the white. Any help appreciated.
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It's ok, that's what is usually done. Just make sure you know which wire is which. Are you sure the black wire is larger than the white? Both are usually the same guage in a romex or BX wire. You should also have a ground wire. And this device should be on its own circuit since a heater is involved. At least 12 guage wire.
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First of all, you need a dedicated 20 amp circuit for the unit. The easiest way to wire the unit is to run the #12/2 feed into the switch box, then get either a piece of 12/4 cable to go from the switch to the unit, or use flexible metal, or non metallic conduit, and pull 4 #12 conductors, plus a ground, from the switch to the unit. Most of these units now also include a night light, if yours does you'll need one more conductor from switch to unit
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Bill McKenzie wrote:

nope. what you need to do is run either a single 12/4 if such a thing exists, or a piece of greenfield with six conductors in it (black, red, blue, ?? (orange?) white and green. Otherwise you need to split the neutrals and replace one of your 12/2wg's with a 12/3wg. Reason being that all the currents in a given cable assembly need to sum to zero.
nate
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Nate Nagel wrote:

Oops I meant five conductors. I was getting your post confused with one from very recently. That poster was using a fan/heater/light/night light combo.
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The return currents only sum to zero when opposite phases are involved. With multiple switch conductors the sum of the neutral current will be equal to the total load in use at any given time.
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Correct.
Also correct. What is required is that the neutral current and the sum of the "hot" currents be equal and opposite within a cable assembly. If the OP has two cables going to the fan and repurposes one of the neutrals to serve as a hot, and then combines all neutrals into one, then one of the cables will have all of its current flowing in the "hot" direction. (it helps to visualize this better if you think of this as a DC circuit, even though it's not.)
nate
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Thanks for the responses. Your explanation makes sense, Nate, but can I ask what the risk is of having unbalanced loads in the two cables? Is it possible this would have been allowed 30 years ago but not now? Reason is, we have an older house (about 30 yrs) with 6 bathrooms. 5 of them have an older heater/fan/light combo, all identical and when I take the plate off the switches they all have the same cabling: one 10/2 coming in, and a 10/2 and 12/2 going out. About 10 years ago (before we bought it) someone took the fan out of one of the bathrooms, leaving the old wiring in place and I want to put a new one in. I'd really like to avoid re-wiring (it's fastened to the studs in the wall), but I dont want to burn my house down either.
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Bill McKenzie wrote:

I don't believe that it's fundamentally horribly dangerous; certainly not as dangerous as, say, a splice outside a junction box or such like, but it *is* against code. I believe that the reason for this particular code is that there exists a possibility of inductive heating which could, in a rare but not completely impossible circumstance, cause a fire or other undesirable occurrance.
It's up to you what you want to do, but if it were I that was doing this, if it wouldn't be too difficult I would try to run some Greenfield between the wall box and the ceiling unit so that you could "do it right." Not seeing your particular bathroom I have no idea if this could be easily done with making only one or two holes or if it'd turn into a near re-do.
nate
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Thanks for the responses. Your explanation makes sense, Nate, but can I ask what the risk is of having unbalanced loads in the two cables? Is it possible this would have been allowed 30 years ago but not now? Reason is, we have an older house (about 30 yrs) with 6 bathrooms. 5 of them have an older heater/fan/light combo, all identical and when I take the plate off the switches they all have the same cabling: one 10/2 coming in, and a 10/2 and 12/2 going out. About 10 years ago (before we bought it) someone took the fan out of one of the bathrooms, leaving the old wiring in place and I want to put a new one in. I'd really like to avoid re-wiring (it's fastened to the studs in the wall), but I dont want to burn my house down either.
It's gonna be real fun trying to squeeze a 10/2 into that tiny junction box. The code requires all the circuit conductors be run in the same cable. You have three devices, so you need neutral and three hot conductors, plus ground. What you have isn't correct and hasn't ever been correct. At the very least, you'll need to scrap the 10/2 and use a 12/3, plus the existing 12/2. You'll run three hots and two neutrals, plus grounds
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I'm curious if he really has 10/2; wouldn't that be unusual for a bathroom which I would assume would be a 20A dedicated branch circuit (hopefully GFCI protected?)
Also, I *thought* but may be mistaken - someone correct me if I am - that current code prohibited using two different size wires on a given branch circuit, even if the smaller size used was appropriate for the overcurrent protection device.
nate
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N8N wrote:

10/2 does sound odd.

You can use any size wire as long as the overcurrent protection is for the smallest branch circuit wire.
Mixing may or may not be a good idea. It can cause confusion. But on a 20A circuit you might run #10 to a garage for voltage drop and #12 within the garage.
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OK, I'm probably full of it there. It's probably 12/2 and 14/2, but one is definitely bigger than the other. The big one has black outer coating and the smaller one is white, but I doubt that means anything. I'll crawl up in the attic tonight and see if I can read the writing on the cable. In any case, I think i'm going to skip the heater and just go with fan and light. Gotta figure out which of the 80-odd circuits it's using.
Supplemental question: Any problem with using expandable foam insulation to seal the box around the hole in the drywall (top down, from the attic)?
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One more thing. How can it be both dedicated and GFCI protected? Is there a standalone interrupt/reset somewhere? There is a GFCI- protected plug in the bathroom, but I'm assuming its a different circuit. And I guess current code may not be relevant, since this was all done in the 70s.
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There's an either/or in the code; if the GFCI outlet is on a 20A circuit that is only used for bathrooms, the lights etc. can be on another circuit. However if the fan is in the shower area it also must be GFCI protected.
rather than crawl around the attic, why not just get some scraps of 12AWG and 14AWG and see if they match the sizes of the wires in the box? I suspect you will find that the larger cable is 12AWG in which case I would replace the 14/2 with a 12/3.
nate
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wrote:

One more thing. How can it be both dedicated and GFCI protected? Is there a standalone interrupt/reset somewhere? There is a GFCI- protected plug in the bathroom, but I'm assuming its a different circuit. And I guess current code may not be relevant, since this was all done in the 70s.
It could be fed from a GFCI breaker, or the circuit can be broken through a GFCI outlet, separate from the one by the sink, or protected by a faceless GFCI mounted in a closet or other obscure location. Most likely, given when it was installed, there is no GFCI protection on it, and none is required unless it's installed over a tub or shower
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