'nuther Electric wiring query

[John Moorhead]

Folks -

I'm back... Now it's a wiring question. I'd like to run a string of outlets at benchtop height, and run two circuits. I'll put a pair of outlets in each box, 1 on each circuit. Can I safely run 12/3 for a pair of 20 amp circuits using a common neutral (white) and ground? I'd run the red wire (marked black) to one outlet and the black to the other. The two outlets would share the white wire, along with the grounds.

At the panel, I'd have the red (marked black) wire going to one CB and the black one to a separate CB. If I can do this, I'd have to string way less NM... Would this be a violation of code? Is this safe?

I know, I know... call an electrician or an EE... I'm just trying to get a little help here....

Thanks in advance!

John Moorhead

[U-CDK_CHARLES\Charles]

Are you Canadian? Something similar to that--perhaps exactly--is standard for kitchens in Canada. I think they may require seperate jackets, but perhaps not.

I don't think it meets code in the US. At the very least, I've never seen it recommended for US practice.

Why not just alternate curcuits?

[John Moorhead]

Nope - I'm in Northern California... Well, I *do* want alternate circuits... The reason for using a single run of 12/3 is that I wouldn't have to run an additional separate length of wire - everything would be in the one cable. Sorry if my post didn't make that clear...

John

[U-CDK_CHARLES\Charles]

I'm reasonably certain that's not allowed in the US.

In any case, the lone neutral lacks the capacity for both circuits. Also if it's in a garage, the GFCI will sense the imbalanced current and trip.

[Swingman]

What you are looking at is a "split circuit" receptacle. It's been perfectly legal in a number of locale's, but I am not sure what the latest NEC says about it.

Two caveats that are generally required to do it:

1. You must cut or remove the metal bridge on the duplex receptacles

1. It is generally required that you must use two, two pole circuit brkr's, with the two circuit brkr's "tied" together so that if one trips, they both trip.

In any event, the terminology you want to use with your building inspection department is "split circuit receptacle".

[Swingman]

Sorry, I left out the word "hot": That should read "cut or remove the metal bridge on the HOT side".

The neutral wire metal bridge in the receptable remains intact.

[Mike Marlow]

Seems to me that shared neutrals are allowed under some circumstances, but I can't recall if you have to downgrade the cirucuit when doing so. Wait for some of the electricians to weigh in on this one.

I don't see how this would happen. The potential between neutral and ground are the same as if you had run a separate neutral.

[john]

It is simply a multiwire circuit. As long as you use a 240v breaker for the two circuits, it fine; maybe.

If you need a GFCI, such as in a garage or a finished basement, you will have problems. They have 240v GFCIs, but I have never used one and don't know if they work on multiwire circuits or not.

[LRod]

So far the answers you've gotten are either wrong or way wrong.

Shared neutral is certainly legal in the U.S. For those pontificating on how it's allegedly unsafe, the two hots MUST be fed from a duplex breaker, which does two things: it ensures that both feeds are on opposite buses which means that the neutral currents for each branch are 180° out of phase and thus subtractive, and it ensures that when the breaker is tripped (intentionally or otherwise) there are no hot leads in a box with two separate branches in it.

So the short answer is, yes, you can do that.

The long answer is I don't like shared neutral circuits and avoid them like the plague. Call it personal prejudice but you have to make sure you have a continuous neutral (all pigtails to the receptacles) so that you don't wind up with 240V across the devices if the neutral is lost.

Wire is relatively cheap. No matter how big your shop (well, up to a reasonable size, anyway), you'd be hard pressed to add $100 to the bill by wiring two completely separate circuits. Of course if you do it that way I wouldn't recommend putting receptacles fed by two different circuits in the same box. Same issue as with the shared neutral circuit if a duplex breaker weren't used.

- - LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

[Mike Marlow]

Look at it again Swingman - he doesn't want to split an outlet, he wants two duplexes together in a double box that have their own hots, but share a neutral and a ground.

[Bob]

No.

The neutral is a current carrying wire in a single phase circuit. You should run 12/3 for each circuit. I think you may be trying to be too symmetrical and neat by putting two branch circuits in the same box. It also crowds the box and makes connections more difficult. But I'm assuming that you are placing outlets at some reasonable spacing for a shop (Lots of 'em).

Its only an opinion, but my philosophy is to use multiple outlets for connectivity, as opposed to load carrying capability. My shop has three heavily interactive walls (bench and portable tool areas). I put one branch ciruit in each area and populated it with an embarrassingly high number of outlets. I also ran one 220 ciruit to each area with 3 outlets each.

Bob

[Frank Ketchum]

I do not believe this is allowed by the NEC. If you think about it, each of the two hots will be on 20 amp breakers and will be current limited to 20 amps. The neutral will have to carry the sum of the two circuits, or potentially 40 amps back to the panel. 12 AWG wire is not rated for 40 amps! You may be allowed to do this if you derate the circuit, but I doubt you will want to do this.

Frank

[Frank Ketchum]

A GFCI works by detecting a difference in the amount of current flowing though the hot conductor and the neutral. I think this is what he is referring to. The neutral will carry the sum of the two branch circuits.

Frank

[Doug Winterburn]

The answer is "yes". Run 12-3 with ground as you said, but use a ganged breaker since if you need to work inside any box, you want both circuits shut off. This is exactly what the electrician did for my gar^H^H^Hshop. All the left hand duplex outlets in each double box are on one circuit, and all the right handers on the other. The 'trician used surface mount boxes and conduit with individual wires rather than romex on mine and ran a seperate ground even though the conduit can be used for ground. I also had him run 2 240V circuits through another single conduit with every other box containing a single 240V outlet and alternating between circuits.

Make sure you have the bottoms of the outlets at least 40" above the floor so you can put standard 36" high benches under.

-Doug

[Bob]

I disagree. You are allowing 40 amps (2 * 20) across a single conductor (the neutral) by sharing it with two single phase 20 amp circuits.

Bob

[Robert Bonomi]

First question: *WHERE* are you? Code _does_ vary, by locale, on whether things like this are allowed.

Comment: this 'two hot, one neutral' wiring is called an 'Edison circuit'.

*IS* generally allowed, _as_long_as_ you make sure that the two hot wires go to *different* phases of the main supply.

You do -not- need to 'tag' the red wire as black, btw. anything _other_ _than_ one of the 'reserved' colors (white, green, bare) is presumed to be a 'hot' lead.

I would *NOT* recommend running "12/3" wire. Pull _individual_ wires -- even when pulled at the same time, they go around corners *easier* than the multi-wire cable. You'll likely find that the individual wires are less expensive than the multi-wire cable, too.

[Doug Winterburn]

Also, make sure the ganged breaker is mounted like a 240V breaker with each half on a different phase so your neutral will never carry more than the load for the biggest drawing circuit.

-Doug

[Doug Winterburn]

I disagree. As long as each breaker is on the opposite hot as in any

240V circuit, the total neutral load will never exceed the current draw of the largest circuit load. If each circuit was drawing an equal load, the neutral load would be zero.

-Doug

[Swingman]

Granted he is us>>I'll put a pair of outlets in each box, 1 on each circuit.

[Swingman]

You left out an important step ... on most duplex receptacles you MUST remove, or cut, the HOT bridge between the two outlets if you are going to split a circuit.