12-2-2 NM cable only has one grounding conductor?

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I need to run two new 20A circuits to the other side of the house - at Home Depot I see this "12-2-2" romex cable with 2 hots, 2 neutrals, and one ground. Is it safe to share that one 12ga. ground between two 20A circuits? (I "assume" it was a 12 ga ground, it's up in the air on the spools at HD so hard to get a close look)
Does 12/4 armored cable (MC or BX) only have one grounding conductor? or is it better because the metal jacket is an additional/supplemental grounding path?
Thanks.
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Interesting; I had never heard of 12-2-2. But reading up on it, it is specifically intended for 2 AFCI protected circuits. Since AFCIs also function as GFCIs, any current going to the ground will trip the breaker and cut off the current. There is no possible way to overload the ground.
Without the AFCI you could, in principle, overload the ground without tripping the breaker so it would not meet code. Personally, I would rather run two cables anyhow.
At least this is my reading of it; I can't wait to hear how foolish I am...
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Not [necessarily] true. *Some* (perhaps most) AFCIs are dual-listed for use as GFCIs as well. But not all of them.
And the ones that are, don't function quite the same: the trip threshold is a bit higher.

Not true.
Very unlikely, I grant, but not impossible: a low-impedance (hence high current) short from hot to ground on *each* hot leg would indeed overload the ground. This would not necessarily involve an arc that would trip an AFCI. It *is* extremely unlikely, but it's not impossible.

Even *with* the AFCI, you could, in principle, overload the ground without tripping the breaker, as I noted above.

Unless it's a really, really long run, it's probably a *lot* less expensive to use two 12-2 WG cables, and two standard breakers. AFCIs ain't cheap.
And it's a lot harder to screw up.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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AFCI's do not perform as GFCI's unless they are specifilcy built as such and labeled. This is not the general case.

How would you overload the ground, if you follow the NEC?

Learning everyday,
tom
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http://www.mikeholt.com/mojonewsarchive/AFCI-HTML/HTML/AFCI_Update~20030320.htm "all AFCI's have a built in Ground Fault Interrupter"

You couldn't, since you wouldn't be using 12-2-2. But he is talking about using 12-2-2.
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Careful, people might confuse 30 ma GFCI trip as GFCI protection for personnel. So, imho, best not to mention afci/gfci unless the item is labeled and listed as such.
later,
tom (VERY PARINOID) ;)

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Just to be clear, there is no requirement to have an AFCI protect a 12-2-2 cable. The point is that one application for 12-2-2 is running two circuits to back to back bedrooms; since the circuits must be AFCI protected, they can not share a neutral.
Cheers, Wayne
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wrote:

legs because of the shared ground, and then there is no advantage of 12-2-2 over 12-3. In fact 12-3 is better because it reduces the voltage drop. So while 12-2-2 can be used without AFCI, it would not make sense to do so.
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Why do you believe that to be the case, can you indicate a section of the NEC that requires it? I thought that if you are running two circuits in conduit, for example, it was standard to only pull one EGC. Is there a requirement to have a separate EGC for each circuit?

Admittedly it is a speciality product, but I can see other places it would make sense. For example, in a kitchen setting with two small appliance circuits, protected by separate GFCI outlets, you need to separate the neutrals downstream of the GFCIs.
Cheers, Wayne
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wrote:

However, it seems prudent to have a ground that can carry the maximum current supplied by the hots.

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Utter nonsense. If they shared a *neutral* they'd need to be on opposite legs.
Please don't give any more electrical advice until you figure out the difference between ground and neutral.

It does what?
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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It does seem true that sharing a neutral between the two circuits on separate legs reduces the voltage drop due to the wiring. The current on the neutral leg is less than on either hot, so the voltage drop due to the neutral leg is less than it would be with a separate neutral.
Cheers, Wayne
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I would run a 12-3 w/ground and put the breakers on opposite sides of the 240. I have done a similar thing with 2 electric heaters in the 2 upstairs baths ... we like warm bathrooms when showering. Wayne Whitney wrote:

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Irrelevant. This would do nothing to alter the voltage drop between the supply and the load, which is the only place it really matters. Further, the resistance of whatever load is applied to the circuit, even if it's just a single light bulb, is orders of magnitude greater than the resistance of the conductors.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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I don't believe that is correct. Given a fixed voltage available at the panel, and for example a resistive load, the hot and neutral wires are effectively resistors in series with the load. The total resistance of the circuit will be the resistance of the hot + resistance of the load + resistance of the neutral. You can't ignore the neutral just because it is after the load.
Another way of looking at things is that using a shared neutral makes the overall circuit partially parallel, and two resistors in parallel have a lower resistance than they do in series.

Well, this is always true, yet on a long run conductors are typically oversized to avoid an excessive voltage drop. So it does happen. Admittedly the benefit of reduced voltage drop due to a shared neutral may be more theoretical than of practical importance, but I do believe it is real.
Cheers, Wayne
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but the imbalanced portion of it's load at 208, 240, 480, or what have you rather than at 120, or 277. Using a single phase multi wire branch circuit supplied from a single phase 120/240 volt center tapped service with twelve amps on one leg and fifteen amperes on the other the neutral will only carry the difference or in this case three amperes. On a circuit that is fifty feet long the three amperes has a voltage drop of the round trip distance of one hundred feet the twelve remaining amperes from both legs will have the voltage drop of only fifty feet because it is using the other ungrounded conductors load to complete the trip. You get the same amount of work done for roughly half of the voltage drop. -- Tom Horne
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On Sat, 19 Mar 2005 11:47:50 -0500, "Paul"

IMHO:
Since the ECG only carries current for a very short time, in a ground fault, you should have no problems sharing the ground. I've run emt with many circuits and a single ground wire. So, imho, I don't know of any problems with the code against doing this.
BTW, now if you are interestinged in running two circuts in the same cable, why have you ruled out 12/3 ?

Personally, I like having a seperate ground, so this rules out AC as my choice. Plus, as for the MC jacket, it can only be used as a ground if listed as such. Eventhought it is grounded.
From your questions, you are doing the right thing planning out the project before you runn around buying stuff.
Might want to post what you doing(or intend on doing) to get suggestions how to do it best.
hth,
tom @ www.CarFleaMarket.com

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I had it in my head that sharing a neutral for two circuits was a big no-no, so I never considered that option....

As it turns out, I found a better route through the house and have enlisted a helper, so I'm going to KISS and run two 12/2 romex cables...
What we're doing:
Circuit #1 - 20A - To support an electric hydronic baseboard heater, 750W. Also on the circuit will be under tile electric warming mat at about 300W. A GFCI breaker in the panel to protect any faults with the baseboard heater.
Circuit # 2 - 20A - For general lighting and outlets in the bath. GFCI outlets of course.
I decided to run two circuits because I couldn't see one circuit handling baseboard heater + tile warming mat + lights + fan + hot curling iron + 1500W hair dryer.... Sure I could probably get away with it until she kicks the hair dryer into overdrive <g>...
-- Paul
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On Sun, 20 Mar 2005 17:09:03 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

You know, I couldn't understand why so many people say make sure multiconductors are on opposite 'phases' to protect the neutral conductor. Figuring, ofcouse they will be opposite when using double pole breakers. So, I wrote off the warning.
But as time goes on I hear from electricians they run into many times homeowners tossing in two breakers, and only using the spaces provided. Somtimes not having the breakers next to each(vertically) but spread out in the panel. I've even heard about one homeowner putting in mini-tandem breakers, and using that for a three conductor cable.
So very good advice!
later,
tom @ www.CarFleaMarket.com

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We were doing some wiring in my son's 95 year old house (complete rewire in the 60s) and found a common neutral where the 2 legs were fed from 2 breakers on the same phase. Of course we move things around.
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