Question on home wiring

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I would like to run some new outlets in my garage. There is currently a subpanel located there, the run will be fairly short. I have a section of armored cable which was left over from work at my office that I'd like to use. I'm thinking of using a setup which I have seen used in other houses, but want to run past you first.
The armored cable has two 10-gauge (white & gray), 4 12-gauge "hots" (various colors), and two 12-gauge grounds. I'd like to use it to run 4 20-amp circuits. Circuits A and B would come off of opposite phases, and share one of the neutrals and one of the grounds. C and D would be run similarly. That way, neither neutral would ever carry more than 20 amps.
I know that I would never exceed rated ampacity for any conductor, but I do not know the NEC well enough to say whether that will meet current code or not. Anybody want to chime in?
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On Dec 18, 10:44 pm, snipped-for-privacy@gmail.com wrote:

What about GFI outlets, since this is in a garage. I don't know the code that well, but I think GFI's may be required for the outlets since the garage floor could be wet.
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Yes, they are, I have that part covered.
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On Thu, 18 Dec 2008 20:44:26 -0800 (PST), snipped-for-privacy@gmail.com wrote:

As Bob says you need GFCIs on the circuits but you can put them in after you split out the multiwire circuits (NEC speak for those shared neutral circuits). That "MC cable" (not AC) is designed for exactly what you are doing. The upsized white and gray (the other acceptable neutral color) is because of the possibility of neutral harmonics although that is really a 3 phase thing. Just be sure to keep your colors straight so you keep them on opposite phases. The new code requires that you group each pair with a cable tie or something, along with it's associated neutral in the panel and use a 2 pole breaker for the pair. This is to tip off the next guy what you are doing. I would put another tie on each pair of hots, closer to the breaker, after you split out the neutrals. The other tip is to be sure you get the right box connectors for the MC cable. At the load end put a red wirenut on the #10 along with 2 whites, one for each circuit and treat it like 2 regular circuits from there. GFCIs will work fine at that point.
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Thanks a million. I'm glad to know that I'm not doing something I shouldn't.
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On Dec 18, 11:44 pm, snipped-for-privacy@gmail.com wrote:

Two hots (opposite each other) using the same neutral is commonly called an 'Edison circuit'. AFAIK it is still OK to to wire new circuits this way but there are problems.
I'm not an electrician but as I understand it if one breaker is shut off the neutral can still carry current if the other breaker is on. The outlets should be wired in such a way so that if an outlet on circuit A is disconnected (for replacement or other reson) the neutral will not be broken.
Do some googling on 'edison cicuit energized neutral' and you will come up with many examples of how this type of circuit can be very dangerous. Here is just one link: http://www.hanford.gov/rl/?page=968&parent=334
Ganged breakers can prevent problems but think about the guy down the road or yourself in 10 years when you're not exactly sure what you did. Wire is cheap don't take the chance.
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wrote:

Two hots (opposite each other) using the same neutral is commonly called an 'Edison circuit'. AFAIK it is still OK to to wire new circuits this way but there are problems.
I'm not an electrician but as I understand it if one breaker is shut off the neutral can still carry current if the other breaker is on. The outlets should be wired in such a way so that if an outlet on circuit A is disconnected (for replacement or other reson) the neutral will not be broken.
Do some googling on 'edison cicuit energized neutral' and you will come up with many examples of how this type of circuit can be very dangerous. Here is just one link: http://www.hanford.gov/rl/?page –8&parent34
Ganged breakers can prevent problems but think about the guy down the road or yourself in 10 years when you're not exactly sure what you did. Wire is cheap don't take the chance.
*The 2008 code requires a two pole breaker for this set-up.
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Does NEC require a de-rating for the neutral wire current?
If you have 20 Amp breaker on phase A and a 20 Amp breaker on phase B and both phases share a neutral, you would think that the max current the neutral could ever see is 20 Amps, but that is wrong. Due to power factor issues there can be more than 20 Amps flowing through the common neutral wire.
I don't think this will be a problem for the OP in his garage but it was a problem in large office buildings with large numbers of early PCs. I am curious if the NEC codes address this question?
Mark
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Does NEC require a de-rating for the neutral wire current?
If you have 20 Amp breaker on phase A and a 20 Amp breaker on phase B and both phases share a neutral, you would think that the max current the neutral could ever see is 20 Amps, but that is wrong. Due to power factor issues there can be more than 20 Amps flowing through the common neutral wire.
I don't think this will be a problem for the OP in his garage but it was a problem in large office buildings with large numbers of early PCs. I am curious if the NEC codes address this question?
*The code does address nonlinear loads, but I forget specifically in what chapters. Article 210.4 does mention them as a FPN.
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No, it requires a simultaneous disconnect, e.g. a handle tie. It does not require a simultaneous trip.
Cheers, Wayne
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Quote: "I'm not an electrician but as I understand it if one breaker is shut off the neutral can still carry current if the other breaker is on".
Suggestion: You use a double pole breaker. This disconnects 'both' the hots associated with that Edison circuit and its' single neutral. This is how they are wired in our house.
The two hots are the two 'legs' (sometimes and often incorrectly called 'phases') of the supply. The 'plus and minus' ** 115 volts, usually being the two ends of the single phase 230 volt centre tapped output winding of the local distribution transformer.
** Being AC this + and - business is, strictly speaking, incorrect. But have found it a way of explaining three wire (and ground of course) 115/230 volt systems business to certain people (usually my neighbours!) who don't understand electricity very well. Certainly not those here present of course.
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You will need to use larger outlet boxes to accommodate the larger and additional conductors especially if you will be going from box to box. 4 11/16" x 2 1/8" square boxes will work best. Check tables 314.16(A) and (B). The number ten wires will be difficult to attach to wiring devices.
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I'm planning on either using two two-gang boxes, or a single four-gang box. I'm only entering 2 10's, and 6 12's, I think that just about any two-gang has enough cubic inches to qualify for those.
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*A deduction of two conductors must be made for each wiring device based on the largest conductor. A deduction of one conductor is made for clamps inside the box. 2.25 cubic inches is required for each #12. 2.5 cubic inches is required for each #10.
4 x 2.5 = 10 cubic inches for the devices. 2 x 2.5 = 5 cubic inches for the two #10's. 6 x 2.25 = 13.5 cubic inches for the six #12's 1 x 2.5 = 2.5 cubic inches for the clamp. Total = 31 cubic inches.
If you use 3.5" deep gem boxes ganged together you should be fine. Don't skimp on cubic inches to save a buck. It will be a pain pushing all those wires into the box with GFI's attached and #10 wire.
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I'd not want to do that. Later electricians will find mauve, purple, fuscia and olive wires. And not know which one is hot, or neutral. For the one time expense, I'd suggest to buy new wire, and stick to the accepted colors for hot, ground, and neutral.
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Stormin Mormon wrote:

Everything in the post was "accepted colors". Nothing suggested colors that are not "accepted".
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Yeah. I'd be pretty leery of an electrician that didn't understand that setup... but then again, I'm used to dealing with electricians that come and work on our 3-phase, 440V stuff.
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On Fri, 19 Dec 2008 10:19:41 -0800 (PST), snipped-for-privacy@gmail.com wrote:

Residential electricians deal with black, white, red and green. ( or shades there-of) So grey and mauve could be black and red?
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On Dec 20, 6:25 pm, snipped-for-privacy@snyder.on.ca wrote:

Agree. But have met electricians who know all about current practice of how to wire, how much slack, frequency of stapling etc. etc. but haven't got a clue about the ratio between RMS and peak voltage, or why phase to phase voltage is different from phase to neutral voltage, for example in 3 phase service in a supermarket lighting sytem and have no idea how a GFI works.
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On Sat, 20 Dec 2008 16:59:28 -0800 (PST), terry

And they need to know this theory to effectivey and safely wire a house? Residential electricians do not need to know 3 phase. And they need to know how to wire a FFI, but do not need to know the nuts and bolts of how it works.
That's why there are different licences - a lot of industrial electricians would have a heck of a time wiring a house. No conduit? No BX? The wires go THROUGH the 2X4 studs? No raceways?
How the heck am I supposed to rout the wires?
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