Question on home wiring

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?

Reply to
bafenator
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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.

Reply to
hrhofmann

Yes, they are, I have that part covered.

Reply to
bafenator

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.

Reply to
gfretwell

Thanks a million. I'm glad to know that I'm not doing something I shouldn't.

Reply to
bafenator

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:

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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.

Reply to
Limp Arbor

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.
Reply to
John Grabowski

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.

Reply to
Stormin Mormon

Everything in the post was "accepted colors". Nothing suggested colors that are not "accepted".

Reply to
bud--

For code questions, only your local code enforcement office can give you the last word on this. Often they make changes/additions to the NEC and that is what you'll have to work to. The NEC is MINIMUMs: Local offices often add to them. Give them a call; they're usually more than happy to discuss it with people. You can also make certain of the permit situation at the same time; you may need to pull a permit but in your case it'll be a very simple process as will the inspection.

Twayne

Reply to
Twayne

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.

Reply to
bafenator

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.

Reply to
bafenator

buffalo ny: the code is constantly revising for home safety. reconsider and hire an electrician, and first think of maximum heat/ air conditioning electric that you may want there in the future. if the homeowner is permitted to run low voltage in your area, the 12 volt stuff, it is less lethal for the next guy if you make a mistake in the wiring. for the electrician lay out the outlets and appliances on a sketch. number your outlet covers to match the breaker numbers in the panel at the end of the job. and add all audio, remote doorbells, tv, phone, video, cable, internet, intercom extra future wiring, as well as sink, shower, water, sewer, and gas, in today's job.

Reply to
buffalobill

*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.

Reply to
John Grabowski

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:

formatting link
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.
Reply to
John Grabowski

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

Reply to
makolber

I could be wrong, but I think you may also need to derate the wire sizes because of multiple circuits in the cable, i.e. treating the 12 ga cables like 15 amp, regarding loads and the breaker.

Reply to
bill allemann

yeah some derating will be necessary,

besides which copper price is down, run a new heavy whatever cable like a 6 gauge to a sub panel and save yourself a lot of hassle, and lots of questions at home resale where nervous home inspectors will jump all over your cabling.

I ran some 10 gauge once for outdoor lights left over from my sheds power line run, and regretted it . how miserable to work with for lowly lighting

needed 10 gauge with 20 amp breaker for shed cause of long run, and possible use of heavy load table saw out there in future.

Reply to
hallerb

No, it requires a simultaneous disconnect, e.g. a handle tie. It does not require a simultaneous trip.

Cheers, Wayne

Reply to
Wayne Whitney

It is necessary to derate whenever you have more than 3 current carrying conductors in a bundle or conduit. However, the ampacity of small conductors with 90 degree insulation is higher than the conventional maximum OCPD. For example, for most branch circuits, #14 can not be fused at higher than 15 amps, but for purposes of derating the ampacity of #14 with 90 degree insulation is 20 amps. So as long as your derating factor is at least 0.75, you still get an ampacity of at least 15 amps. The upshot is that for #10 and smaller, with 6 conductors, there is no need to decrease the OCPD from the usual.

Yours, Wayne

Reply to
Wayne Whitney

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