Something I ran into...

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This is one of the many areas where the words "shall be permitted" are used incorrectly in the code. If you look at the NEC Style Manual you will find that these words are to be used like an exception. That is to permit an option to the main rule. There is no main rule here as there is nothing in the NEC that says you cannot parallel smaller conductors. The act of speciffically permitting something by the use of the words "shall be permitted" does not directly prohibit other things.
 
don_resqcapt19 said:
This is one of the many areas where the words "shall be permitted" are used incorrectly in the code. If you look at the NEC Style Manual you will find that these words are to be used like an exception. That is to permit an option to the main rule. There is no main rule here as there is nothing in the NEC that says you cannot parallel smaller conductors. The act of speciffically permitting something by the use of the words "shall be permitted" does not directly prohibit other things.
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Can anyone explain - technically - it to me why smaller counductors should not be paralleled? Especially in the case when the paralleling is done for voltage drop purposes, not necessarily to achieve higher current for the circuit.
 
don_resqcapt19 said:
The act of specifically permitting something by the use of the words "shall be permitted" does not directly prohibit other things.
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Clearly, you are far more well in tune with the code making process, with the people who make the codes, and with their intentions, than I will ever be. All I have to work with is the actual language as written and published in the book. By ?the book,? I can only mean the NEC, since the NEC Style Manual is not itself an enforceable document.

Speaking from the perspective of ?symbolic logic,? when there are two events under consideration, and when between the two you cover all possible events, such as in the case of (1) Paralleling conductors below 1/0 and (2) Paralleling conductors 1/0 or above, and when the words say, ?(1) shall be permitted,? then I think it appropriate to infer that ?(2) is not permitted.? Otherwise, there would have been no need to describe one item as allowable; they could have simply said that paralleling conductors is OK regardless of size.

I suspect that the intent of the code writers was in keeping with my interpretation. I also think there are enough words in the code, as written, for me to infer that the NEC indeed does forbid paralleling conductors under 1/0 in circumstances other than those addressed in the exception.
 
weressl said:
Can anyone explain - technically . . . .
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Not I. Not in objective, technical terms.

But I suspect it has to do with the probability of damage occurring to one of the paralleled wires. Smaller wires have significantly less structural strength. A slight, errant bending force exerted during the process of pulling wires might put a kink in a smaller wire, increasing its resistance at that point. That would certainly alter the current divider calculation. And if one wire does break, then the other wire will carry all the current. That may or may not lead to a failure of the other parallel wire. If you paralleled for VD considerations only, the wire would be adequately rated for ampacity. But if the load is a motor, and if the resulting VD (i.e., from one of the two wires breaking) is severe, then you may get an excessive amount of current in that wire. Not a good situation, whether or not the breaker acts to terminate the event.
 
iwire said:
Yes, all over the place, the NEC has been changing the wording of the section to try to clear up the confusion.

Only the conductors that supply 100% of the dwelling units power can be sized using table 310.15(B)(6).
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Where does it say that it must supply 100%?

310.15(B)(6) says .....the main power feeder shall be the feeder(s) between the main disconnect and the lighting and branch circuit panelboard(S)

How does this not apply?:-?
 
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