I need to run a 20a/240v 4wire circuit. I have a lifetime supply of 12/2 on
hand but would have to buy the 12/3.
Can I run two pieces of 12/2, using only one of the neutrals? I can't see
any reason it would be any less reliable than 12/3; but expect it probably
violates some code or other, though I don't know why it should.
(B) Conductors of the Same Circuit. All conductors of the same circuit and,
where used, the grounded conductor and all equipment grounding conductors and
bonding conductors shall be contained within the same raceway, auxiliary
gutter, cable tray, cablebus assembly, trench, cable, or cord,
I looked 300.3(B) up in my electrical book and it gave the example below.
The text says this is forbidden if the wires pass through any metal, but
If this example is okay, wouldn't mine also? Mine is rather better, since
all the wires are at least in close proximity.
This example is not doing what you are saying. It is not passing separate
black conductors to the same circuit using different cables. It is using a
two wire cable instead of a three wire one on a three way switch. It's not
the worst thing I've seen, but I wouldn't recommend that either.
For those who are interested in the worst thing I've seen, I'll mention it,
although it won't help with this problem. There was one house I came across
where somebody decided to use two switches to control a light but had only a
two wire cable connecting them. He ran a standard two wire cable into the
first switch box. It passed the white wire through to the next switch, and
the other lead running to the other switch switched between black and white.
The second switch passed that switching lead directly to the light, and the
second terminal of the light connected to a three way switch that switched
between the white coming from the other switchbox and a new black that came
in from somewhere else. This had been working fine until the homeowner
replaced the switch with a lighted mercury switch, and when he turned the
light on or off, the circuit breaker tripped (or it might have been a fuse
back then.) It turned out that in a mercury switch, as the mercury flows, it
might still be in contact with one pole momentarily when it contacts the
other pole before the mercury all runs down. So what seemed like a simple
task of changing a switch resulted in a circuit that would not stay on and
there was no sane explanation. I suppose who ever did that could have made
it even more confusing by using only one of the conductors. Perhaps he
did-- it was at least three decades ago.
Any time you do something that may confuse the next homeowner who assumes
that it's a straightforward circuit, you are asking for trouble. In the
example you showed, a homeowner could easily take a switch out to replace
it, not pay much attention to the wires until it's time to put the new
switch in, and he'd be left scratching his head trying to figure out what is
going on, especially with an unused lead in there.
It violates code. Code requires all conductors for a circuit to be in the
same "raceway", which in the case of romex, means a single sheath. With
conduit, it means the same hunk of pipe. Etc.
Principle of least surprise for renovations and circuit alteration, plus
probably a few other reasons.
May also be classified as poor workmanship - wrong materials.
Sure you need to make it four wire?
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
Using discrete conductors from a cable in a raceway isn't the "right" way.
Will it work? sure.
Is it safe? probably
Can you prove it? no.
These conductors are not marked so there is no way to tell exactly what they
are and they are not tested outside of the cable they were made in.
Wires in cables are not marked. It is assumed that since they are 90c rated
they are THHN but there is really no way to prove it, nor is there any
obligation for the manufacturer to actually use a wire with THHN specs. It
might turn out that the wire in Romex lacks some abrasion resistance or other
property you would find in THHN.
When they are making thousands of miles of Romex a year, shaving a small
fraction of a penny off the cost of the wire will add up quickly so you don't
know what you are getting. Cables are tested as an assembly and you can be sure
they don't add anything extra beyond what it takes to get listed.
There is a practical reason not to do that. If the conductors are not
in the same cable or raceway they will produce an electromagnetic field
of sufficient amplitude to cause interference with radios and hearing
aids. If at any point along the pathway a metallic object is between
the two sets of conductors an AC current can be induced into the
metallic object and inductive heating can result.
I doubt it, at 60 Hz, with balanced currents in the two hots.
Nicholson L. Pine System design and consulting
Pine Associates, Ltd. (610) 489-1475
821 Collegeville Road Fax: (610) 831-9533
Collegeville, PA 19426 Email: email@example.com
Computer simulation and modeling. High performance solar heating and
cogeneration system design. BSEE, MSEE, Sr. Member, IEEE. Registered
US Patent Agent. Web site: http://www.ece.villanova.edu/~nick
Were do we get the guarantee that the two hots will remain balanced in
the four wire circuit? The circuit is four wires because a neutral is
needed to carry the imbalance current that is the difference of the load
on the two ungrounded conductors. If no difference were going to occur
no neutral would be required in the circuit and it could be run in a
single two wire plus ground romex cable.
Maybe I'm not following you but the idea is to put all of the circuits
conductors in the same cable or raceway. So that their respective
magnetic fields will cancel each other out to the greatest degree
Yep, understand that -- but how much of an issue is this, if the conductor
that's separated is the equipment grounding conductor (which should never
carry any current under normal circumstances)? It's a 240V circuit, with the
neutral running in the same raceway as the two hot conductors, so the magnetic
fields there should cancel out.
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