Use two 12/2s for 240v?

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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.
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300.3 Conductors.
(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,
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...unless otherwise permitted in accordance with 300.3(B)(1) through (4).
Nick
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No
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I looked 300.3(B) up in my electrical book and it gave the example below.
http://www.frontiernet.net/~toller/circuit.jpg
The text says this is forbidden if the wires pass through any metal, but allowed otherwise.
If this example is okay, wouldn't mine also? Mine is rather better, since all the wires are at least in close proximity.
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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.
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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.
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toller wrote:

Unzip the cable (pretty easy to do with NM; almost impossible to do with UF) and run the individual conductors in a 1/2" conduit.
Bob
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Yeah, great idea. Now he needs to buy and install conduit, rip apart cable, pull wires, all to save buying a roll of wire to do the job the right way. When did copper wire become so expensive?
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Chet Hayes wrote:

It was his idea to not buy the wire. Putting in conduit *is* the right way. So is using the proper cable. Now he has 2 choices.

about a year ago.
Best regards, Bob
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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.
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Greg wrote:

I think they are marked, but I'm not sure. I'll have to check this afternoon.
Bob
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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.
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toller wrote:

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. -- Tom H
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I doubt it, at 60 Hz, with balanced currents in the two hots.
Nick
Nicholson L. Pine System design and consulting Pine Associates, Ltd. (610) 489-1475 821 Collegeville Road Fax: (610) 831-9533 Collegeville, PA 19426 Email: snipped-for-privacy@ece.villanova.edu
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
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snipped-for-privacy@ece.villanova.edu wrote:

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. -- Tom H
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Tom, is this a danger also with the ungrounded and ground-ED (i.e. hot and neutral) conductors run in the same raceway, and the ground-ING conductor in a different raceway some ten feet away?
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Doug Miller wrote:

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 possible. -- Tom H
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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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