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.
Well, if both circuits originated from their own 2-pole breakers, and
one was switched off, the wires from the breaker, seemingly dead, would
remain live, fed back from their connection to the still-live circuit
subpanel. Of course, if the breaker was a 4-pole this situation would
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. :)
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.
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
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.
Age and Treachery will Triumph over Youth and Skill
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?
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.
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
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.
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