In article ,
LRod wrote:
On Fri, 03 Dec 2004 16:56:53 GMT, "John Moorhead"
wrote:
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?
So far the answers you've gotten are either wrong or way wrong.
Agree. Even after this excellent post, a whole stream of wrong
answers kept coming. Fortunately, there were also a few right answers.
Rule #1: DO NOT TRUST SOME RANDOM POSTER ON USENET. For all you know,
we might be all homicidal maniacs. Furthermore, not all localities
use just the NEC, some have additional electric codes. Call either
the building inspector in your locality, or talk to a real
(experienced and licensed) electrician in your area.
That having been said ...
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.
Completely agree. More discussion of GFCI below, though.
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.
Partially agree, partially disagree. Agree: If you use them to feed
the two halves of a duplex receptacle, I think they are dangerous.
Because (a) there are 240V across components that are physically very
close together, and (b) all you need is one wire to come loose, and
you have 240V across the load.
On the other hand, if you use them to feed separate receptacles, I
think they are acceptable - they end up being a way to save wire and
time (it is easier to string one 12-3 than to string two 12-2 next to
each other). Just use extra care in wiring the neutrals (*) together,
to make sure they don't come undone.
Even better: They make raceways with built-in outlets (wiremold is a
popular brand). This stuff looks like a metal tray, about 1" square,
and it has an outlet ever few inches or every foot. This is ideal for
feeding power at counter height in a shop. And you can get it
pre-wired for two circuits: One neutral conductor, one ground
conductor, and two hot conductors. This makes a perfect match to an
edison- or multiwire circuit with a dual-pole 20A breaker: With very
little work, you have two 20A circuits everywhere in your shop, and
you'll never run out of outlets. This type of multioutlet assembly
(technical term for the wiremold stuff) has to be fed from an edison-
or multiwire circuit. This is what I have installed in my basement
shop.
The only real problem is how to add GFCIs. One option is to use two
(or more) individual GFCI outlets at the endpoint of the edison
circuit. The problem is: if you want to add the "load" connection of
the GFCI outlet, you must never merge the neutrals back together. So
behind the GFCI, the circuit is no longer an edison- or
multiwire-circuit. If you want to use multi-outlet assemblies (like
the wiremold stuff), or a split duplex receptacle (one outlet on each
of the circuits), there is no simple way to add GFCIs. You can't just
put two GFCIs next to each other, and feed through them, because
merging the two neutrals back together is a no-no (not just illegal,
it will just plain not work). In this case, you have to use stronger
medicine: A 2-pole 240V GFCI 20A breaker in the panel. Those exist,
but they are very rarely used, and therefore pretty hard to find (the
borg doesn't stock them). The real bad news is the cost: I paid a
little over $100 for the GFCI breaker. So if you need GFCI protection
(basement, garage, outdoors, etc.), this negates any potential cost
saving.
(*) The term "neutral" is not used in the code for 240V circuits, or
edison- or multi-wire circuits. Instead, it is called the "grounded
conductor" (a.k.a. the white wire). Don't confuse that with the
"grounding conductor" (a.k.a. the green wire). The colloquial term
"neutral" is much clearer, and in common use. Unfortunately, to
further confuse the issue, the code uses the term "neutral" for
certain cases of three-phase circuits.
--
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Ralph Becker-Szendy