12-3 Wire: is it one circuit, or two? Wattage capacity? - AVS Forum
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post #1 of 18 Old 07-12-2007, 07:08 PM - Thread Starter
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Hope I'm asking this right..

Electrician put up one run of 12-3 Romex today to power the front speakers (2 250W DefTech BP2006TLs, 1 150W DefTech CLR2300 Channel) AND my HSU VTF-3 MK3 (450W) front Subwoofer. This run goes back to one (I believe) 20 AMP circuit on the panel. (Although it could actually go to two, as he explained the single cable can handle 2 circuits..more on this below).

I questioned one run for two separate duplex outlets, with this much power on it, as I originally told him I wanted (2) 20 AMP circuits powering the front of the room - one for the DefTechs, 1 for the VTF-3.

The way he explained it, the 12-3 (apparently) carries 2 circuits in a single cable. He also said he would run the single 12-3 into one gang box, and jumper it to the second, so I'd have two separate outlets as I'd wanted. Is this true and good Theater electrical design? (I haven't heard of such a thing, and it sounded a bit fishy to me). He also said each one can support up to 1850 watts (I think), so I should have more than ample capacity.

Is he being straight up here, or looking to shortcut?

We're about ready to drywall next week, so if I need new runs, I need to get em up there now vs later.

Thanks (from an Electrical noob).

- J
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post #2 of 18 Old 07-12-2007, 07:39 PM
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If he runs it to a ganged (double) breaker, it's good for two independent 20A circuits, each of which can do something over 2000 watts in practice, though 1850 may be the "design limit".

He's right, provided he terminates it to a double 20A breaker. I don't think it's a shortcut at all, personally, and in fact is exactly what I'm about to do this weekend to feed my two racks in our new house.

Disclaimer: I'm an engineer, but not electrical, and not an electrician, but I understand house electricity pretty damn well. I don't know the NEC, but I believe this meets NEC.
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post #3 of 18 Old 07-12-2007, 07:42 PM
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Yep 12-3 has (4) wires and can have 2 different circuits - 12 gauge is used for 20 amp circuits. This will work and you do get the advertised wattage out of it. However, like all things in life, there is no free lunch. The downside for AV is that the two circuits you get out of this are 180 degrees out of phase. Generally you want all circuits to be on the same "leg" or phase to reduce the likely hood of ground loop and other issues. This setup is fine for non-AV equipment, but personally I would not go this route for AV stuff.
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post #4 of 18 Old 07-12-2007, 08:16 PM
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He was doing this to save on copper, which has risen 3-4x in cost over the past two years. It is within the National Electrical Code to do it this way. Bear in mind that if this needed to be a ground fault circuit (due to being in a basement, garage, or potentially wet area), this would not meet code.

I agree that this causes the potential for ground potential differences, so not ideal for AV implementation.

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post #5 of 18 Old 07-13-2007, 09:47 AM
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The reason this works is that the two circuits can share the neutral wire, each one only needs its own hot wire, thus the black wire is one circuit, the red the other, and they share the white. You can avoid the 180 degree out of phase issue by attaching the second circuit to the second down breaker from the first. However, it's not clear in your post whether he is using a breaker that snaps on to two positions in the box, or only one, but provides two circuits. Some areas allow that, but I think for 15 amp circuits only, so I'm guessing the breaker he put in takes two slots. If so, attach the second circuit two slots down from the first, not one. If you don't have another 20 amp circuit to make use of the other side of the double breaker he put in, tell him not to put in a double breaker, but two single breakers with a slot between them for something else.
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post #6 of 18 Old 07-13-2007, 10:52 AM
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Quote:
Originally Posted by chiguy View Post

You can avoid the 180 degree out of phase issue by attaching the second circuit to the second down breaker from the first.


NO! NO! NO! You cannot safely have the black and red fed from the same phase of power in a 12-3 circuit, or you will overload the white neutral wire.

Doing that, or using two breakers that are not mechanically interconnected, is both not to code AND quite unsafe. (I mention both because some people don't care about NEC and only about safety.)

Why is this? If you hook it up to opposite phases and draw 18A from each circuit, the neutral wire is carrying 0 Amps (18A from one leg and -18A from the other). If you have them hooked to the same phase, and draw 18A each side, the neutral has to carry 36A (18A + 18A) and will overheat, without the benefit of a circuit breaker to protect you.

Don't do it.
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post #7 of 18 Old 07-13-2007, 10:56 AM
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OK, maybe I'm an idiot, but I thought everything on the same side was on the same phase, not every other one going down the side. Can someone enlighten me?

Tom

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post #8 of 18 Old 07-13-2007, 11:01 AM
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Nope. In most panels, every other breaker down each side is on opposite phases, the reason being, otherwise a 220V breaker (for a dryer, electric heat, etc) would have to be shaped like a DVD case (to hit both sides of the breaker panel) instead of shaped like they are.

If you take the cover off and look at the spots that don't have breakers (pull breakers if needed to see), you'll see how it works.
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post #9 of 18 Old 07-13-2007, 11:37 AM
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Why are you bothering with multiple 20 amp circuits for that much gear?
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post #10 of 18 Old 07-13-2007, 11:47 AM
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Good question. I have all my gear (save an SVS sub) on one 20 amp circuit. That one circuit powers 7 class AB amplifiers, 1 class D amplifier (for a sub), 1 tube amplifier (class unknown), a preamp, a 57 inch RPTV, a CD/DVD transport, a Replay, a HD cable box, and some other miscellaneous stuff.

Bob
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post #11 of 18 Old 07-13-2007, 11:50 AM
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Sokoloff -

Makes sense now that I think of the 220 breaker. I've added circuits, just haven't had to be concerned about phases. yet. Next time I take the cover off, I'll pay attention.

I wasn't planing to use the single wire method, but I was plannig to run two individual 20 amp circuits to the equipment closet. So, to be clear, to eliminate the phase problem among equipment, when adding two 20amps circuits for my equipment closet, they should be on the same side of the panel, right next to each other, or a "double 20amp" within a single space?

Thanks for the help,
Tom

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post #12 of 18 Old 07-13-2007, 12:15 PM
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They need to be on a double-wide breaker, such that each gets a different phase. This is to avoid the safety problem of unprotected overcurrent on the neutral.

Do not put them on a "two breakers in one space" breaker.

There is no way to use a 12-3, get all equipment on the same phase (which you don't/shouldn't care about anyhow), and have it be safe. Punt on the having them both on the same phase and everything falls into place.
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post #13 of 18 Old 07-13-2007, 12:54 PM
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Quote:
Originally Posted by tlogan6797 View Post

So, to be clear, to eliminate the phase problem among equipment, when adding two 20amps circuits for my equipment closet, they should be on the same side of the panel, right next to each other, or a "double 20amp" within a single space?

Thanks for the help,
Tom

Tom,

Something you need to understand is if your panel uses full or 1/2 space breakers. If you got the 1/2 space breakers you can add the 2 HT Equip circuits breakers side by side within one space. Its only with full space breakers that you must skip a space to stay on same phase.

Cheers,
Mark

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post #14 of 18 Old 07-13-2007, 12:57 PM
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True, but you can't do that with a single run of 12-3 without creating a fire hazard, so that works for Tom's case, but not OP's case...

(I hadn't fully realized there were two people asking questions, one that matched the thread title, and one that was just a related question, as I was reading the posts one at a time, not the whole thread each time...)
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post #15 of 18 Old 07-13-2007, 12:57 PM
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Brit -

THAT'S what I talkin' about! OK, I got it, and that's what I was trying to clarify/ask in my previous post.

Thanks all for the help!
Tom

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post #16 of 18 Old 08-01-2007, 08:22 AM
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Sokoloff -

Better late than never so I don't know if you'll ever see this, but thanks on the explanation of overloading the neutral wire. I thought it was funny to share a neutral wire, but now I see why it can work on opposite phases with your explanation. I'd have never done it anyway, but good to know. My electrical knowledge is pretty good, but not exhaustive, so I tend to avoid anything esoteric (like sharing a neutral wire on two circuits). Looks like that's a good idea.
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post #17 of 18 Old 08-01-2007, 11:32 AM
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How long is this wire run? Is all this discussion really worth the $50 (I'm guessing), especially when you already knew what you wanted?
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post #18 of 18 Old 08-01-2007, 12:34 PM
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The only issue here is Phase, which as previously stated is not an issue. The way he wired your circuits is how almost every commercial building in this country is wired, it is called "multi-wire branch" circuit, look it up. It saves wire, is a MUCH cleaner install at the box, and allows for MUCH greater flexibility throughout the house/building (you can have 220v anywhere you might ever need it)....saftey is a non-issue as both types of circuits are equally safe. although it is a money saver, there is no downside to it except a few more junction boxes as needed. You can do just as many dangerous things with a single wire 120v circuit as you can with a muti-wire branch circuit.


Further, someone mentioned you cant use GFCI with 12/3 MWB circuits????? I've never heard of this or ever heard of anyone suggesting this. If you try to wire it to the branch circuit or make a branch circuit downstream of a GFCI, OF COURSE it wont work!!!! if you understand what a GFCI does, then thinking it would work in those situations is not even possible, lol...You must tap off a 2 wire circuit from the 12/3 and then install your gfci (making sure the neutral goes back to the neutral of the 12/3).

You also dont need to have the breakers tie-barred together in the box if it is only 120v cirucits and NO split recepticles....there is no way for 120v to go from one circuit to the other (leave that to your local code enforcer/electrician though) and generally it doesnt hurt anything to kill both branches (in some commercial installations you may not want to kill both room 1 and room 2, as an example), but just sayin.
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