I am sure this is a code violation, but why wouldn't it be safe?

[Toller]

I have one 240v/20a circuit in my shop and it looks like I need another. If I ran them both to a subpanel, I would have 40a! That should be rather more versatile than 2 20a circuits. Doing it properly with #8 cable is much more difficult (I just ran 8/3 from my generator to a transfer switch, and it wasn't fun) and much more expensive as I already have nearly free 12/3.

I know you cannot run a circuit over multiple wires because if one breaks you have an overload on the other, but each cable would be on its own properly sized breaker, so that can't happen.

I probably won't be doing it because it is too much trouble for something that will kill my resale value, but I don't see anything wrong with it. What am I overlooking? All I can think of is that if one conductor fell off there would be an unbalanced load between the two cables, but that could be made moot by running both through all the same holes.

If I was smart I would have put a subpanel in the first time. If I was really smart I would have figured out how to make the subpanel work as my generator hook up; but those boats have sailed.

[RBM]

No matter how you slice it, you're still paralleling conductors. Nec requires 1/0 awg as the minimum size you can parallel

[Toller]

Sure, I don't argue it is a code violation; but would it actually be unsafe?

[pawlowsk002]

T:

Well, if both circuits originated from their own 2-pole breakers, and only one was switched off, the wires from the breaker, seemingly dead, would

remain live, fed back from their connection to the still-live circuit at the subpanel. Of course, if the breaker was a 4-pole this situation would not be possible. I don't know if you can even get a 4 pole this small, but

theoretically they'd keep the above situation from happening. :)

[Doug Miller]

Of course it would. It's a short circuit -- just ask Richard J. Kinch.

[buffalobill]

[Toller]

[Toller]

That's a good point. They make quad breakers for my box that nest two 240 circuits. Perhaps it would be possible to put a tie through all the handles. Unless of course that stopped the breakers from tripping properly; then it truely would be dangerous.

[Bob]

OK, at the risk of admitting a code violation, here's a different variation of the problem. I ran a double run of 12/2 w/g to my shop (because I had it available vs buying new wire). But it is fed from a single 20A breaker. The double run was for voltage drop rather than additional current. Technically I guess that's a violation, but I don't see a safety issue, do you?

Bob

[RBM]

I'm not that technical, but even with running 1/0 in parallel, both conductors must be the same length or there will be an imbalance. Possibly that type of imbalance would be accentuated in a smaller conductor. If for no other reason, the NEC tries to standardize methods and materials, so people in the industry can more easily diagnose problems that occur.

[Toller]

I did the same thing when I ran wires 120' to my boat house because I could buy 14/2 for almost nothing, but I thought the VD would be excessive. I used one cable for the hot and one for the neutral; they can't hardly get separated that way.

No, I don't see anything wrong with it; but mine is technically an extension cord...

[kevin]

How bout this:

Someone comes along to fix or remodel something. They see two wires coming off the same breaker. That is normally just two runs of wire off the same circuit, and is done all the time. They shut the breaker off, disconnect one set of wires, and turn the breaker back on. Poof, they are holding a live pair of wires, because the two are connected downstream (in violation of code and all normal practices).

The same problem is even worse if there were two separate breakers, even if they are tied.

Electricians and repairmen simply do not (and should not have to) expect conductors to be connected downcircuit, because parallel conductors are forbidden (with the exception of the much larger wires that *are* allowed to parallel).

All this is in addition to the possibility for inductive heating (especially when you are using romex, people are not used to worrying about that), and different length cables causing an imbalance.

[Chris Lewis]

According to Toller :

I would not expect a quad breaker of that type to operate correctly with the two halves tiebarred.

Really the question an understanding that "safe" and "unsafe" aren't absolutes, and the real question is "how unsafe are you willing to tolerate"?

There are several unpleasant possibilities with such a circuit. Is it beyond your toleration level?

I think it should be. But that's an opinion, not an absolute fact.