Parallel conductors

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newtdog

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Greeting everyone!
Been following some posts and have finally got around to joining.
I'm involved in a discussion with a fellow worker about paralleling conductors, understanding the five requirements of the NEC. My thoughts were that the conductors cmill area had to match the cmill area of the determined wire. My opposite says the conductors ampacities have to meet the determined size.
Any help here?
 
Re: Parallel conductors

Newtdog, welcome to the forum. :)
Originally posted by newtdog:
My thoughts were that the conductors cmill area had to match the cmill area of the determined wire. My opposite says the conductors ampacities have to meet the determined size.
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Can you rephrase your question? Maybe a different word for 'determined'?

Basically, as far as ampacities go, your two conductors ampacity as shown in 310.16 added together must equal or better your calculated load. After that step, you must size your overcurrent protection. Does that help?
 
Re: Parallel conductors

Assuming that the single conductor ampacity is the correct size, the parallel conductors must equal this ampacity. In most cases(maybe all) the sum of the cmils of the 2 conductors will be less than that of the single conductor. This is due to skin effect.
 
Re: Parallel conductors

Newtdog, welcome to the forum.

Now the bad news, your opponent is right.

Roger
 
Re: Parallel conductors

This "discussion" centered around 500 kcmill conductor, I assumed it took two 250 kcmill conductors to meet the size, but 310-16 says that two 4/0's meet the ampacity.
Bob, never thought skin effect had much of an influence on a 60 hz. system, but I'll try get some more info.
 
Re: Parallel conductors

Appreciate the replies, hard thing, this admitting to my "fellow worker" that he is right.
I'll get over it. Thanks again.
 
Re: Parallel conductors

newtdog
Skin effect causes a significant increase of resistance for larger conductors. The increase over the DC resistance for 3/0 is 1.01, 4/0 1.02
500 1.13 and 750 1.22 to 1.29 depending on the cable configeration.
 
Re: Parallel conductors

Peter
You can because the NEC table has already taken it into account. I brought it up to answer regarding the question about parallel conductors and single conductor ampacity.
 
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