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I am in the process of rewiring my 100 year old house.

I would like to use 14/3 wire from the breaker box to run 2 separate circuits, one side will be just lights, the other side will be just for the ceiling fans. Both sides together will be about 1200 watts if all fans are on high and all the lights are on and if I use incandescent bulbs (but who does that anymore).

My thinking is this, some day I would like to put in a backup/alternative power source, I don't want everything to run off the backup power source, just lights and a few other select circuits. In trying to keep the demand on my backup power low.

Anyways is it an option to run those 2 circuits on a single 14/3?

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No, you need to keep neutrals separate.

YOU CANNOT RUN SEPARATE CIRCUITS ON 14/3

That thing you're thinking of, is called a "multi-wire branch circuit" (singular). It is one circuit. Note that they share a neutral, and that's what makes them one circuit.

The only way you could conceivably ham-bone 2 circuits onto 14/3 is by using the ground wire as the second circuit's neutral, but that's so outrageously reckless that I mention it only to assassinate it.

Actually, you have a bigger prob^H^H^H^H challenge

Speaking of assassinating, I am assuming you will NOT be using one of those atrocious 6-8-10 circuit transfer switches that cost a fortune, aren't well built, are by shabby suppliers like FPE->Republic, cost 4 times as much, and have spaghetti wiring. Their only virtue is ease of retrofit, and that's no virtue at all on a ground-up rewire.

Generally the way smart people do it, is with a separate subpanel for the circuits to be switched between sources. Then you have a simple 2- or 3-pole transfer switch or interlock; this can be as simple as a $25 strap that ties together 2 backfed breakers in the panel. That, with a

And another alternative is DC power. For a modest "lights, refrigerator, few other things" supply, it may be better to "stay" in DC power, if your auxiliary source is battery plus solar/wind/microhydro/simple-gen. This avoids the significant energy overhead of an inverter running 24x7 merely to run loads that can work on DC. You can still use mains wiring and mains panels, however, the challenge I'm about to mention will become even bigger, because these systems must be rigidly separated from Mains.

The challenge

When you are in auxiliary-power mode, or when switched to it, you will have two separately-derived power sources, and certainly, two separate panels. Because of this, wiring must be carefully separated, in particular, neutrals cannot cross. That means things like the 14/3 MWBC trick are right out.

In fact, you'll need a fan-light which has two separate neutral wires: one for fan and one for light.

Low voltage would need to be in different boxes or with listed partitions, and separate cables. If both sides are mains power, even if they're separate panels or sources, they can commingle in the same box or even the same cable. But still, neutrals need to be separated. And there's the trick. You need some way to "mark" which cable is coming from which service.

For instance, you could do a really diligent job of marking cables with colored tape (and leaving a bit past the cable clamp). (in particular, tape panel 2's white wires with gray tape). Or you could wire the alternate service in UF or MC cable instead of NM. Anything like that would be acceptable, but you'll need to do it, or you'll lose your mind trying to work inside boxes. I work in conduit and have as many as 4 circuits in a conduit at a time, and you have to color-code or you will make mistakes!

  • Yep, this guy knows his MWBC and the nightmare scenario created by generators and aux power when dealing with them. In my mind a MWBC should be the last available option and only used when you can't wire them properly without ripping out walls and floors, every other option should be analyzed and attempted first. – Bob the builder May 6 at 19:46
  • They are dangerous and they're usually installed incorrectly (my current rental with an open ground and a MWBC in the kitchen with no GFCI). Sharing the neutrals between circuits means you can't separate them, one single misplaced splice and your entire circuit becomes a dangerous potential to feed current through the grounds across into the actual grounds... Now there's a current carrying conductor attached to the wall plate screws and the metal chassis of everything plugged in. Reverse the polarity of an outlet and now the live isn't switched and the metal toaster becomes a death trap... – Bob the builder May 6 at 19:49
  • @bob the builder, they are permitted per code so not sure why you think they are dangerous. – Kris May 6 at 22:01
  • Yeah, the MWBC is going to cause serious issues here if he wants to set up to use a standby generator "out of the box", as that requires a switched neutral transfer switch, which will not play nicely with a "half standby" MWBC – ThreePhaseEel May 7 at 1:01
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    @Kris you can't serve one leg of an MWBC out of a different panel than the other leg. Wouldn't matter if they were separately derived, but, think about a shared neutral utility and generator connection with one MWBC leg on each, what happens to neutral, when power is at a slightly different frequency (they drift in and out of phase).... – Harper May 7 at 13:47
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Yes, it's possible. The 14 gauge wire will handle 1200W (about 10A) but check your fan wiring closely. Most ceiling fans I've seen have separate hot wires for the fan and light but a common neutral. Running the fan from the house wiring and the light from emergency power (e.g. a generator) could energize the house neutral, relative to ground, which is unsafe for electrical workers. You'll probably need a whole house shutoff to keep from back feeding the grid when you're on emergency power.

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    The disconnect from the main or whole house shutoff is a requirement for generators weather it is a transfer switch or a mechanical lockout of the main that allows the panel to be fed by a breaker. The listed mechanical lockouts are the least expensive route but not automatic like some transfer switches, some of the breakers usually need to be turned off in this case so the generator is not overloaded. – Ed Beal May 6 at 14:45
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Multi-wire branch circuit "MWBC" is technically the term.

They are generally only preferred to cut cost on materials and labor.

They require a double pole ( more than certain a AFCI type in your case ) breaker to de-energize the ungrounded conductors.

What you're trying to do is a lot of work for nothing much when all you need to do is make sure the fan motors are in the off mode when using the back up power.

  • A MWBC is only permitted under very strict circumstances and if they are intending to split the loads to be supplied by another source then a MWBC is the last thing you want to deal with due to the shared neutrals. The breakers would be required to be common trip and you couldn't supply only one half of the branch, this is horrible advice. – Bob the builder May 6 at 19:29
  • No bad advise was given. . Also, cite a code reference to the "strict requirements". – Kris May 6 at 21:58

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