Paralleling conductors

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Mike Koivisto

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When paralleling conductors, for instance, paralleling two (2) 1/0 AWG copper conductors does the ampacity of the combination now equal 300 Amps, or do you have to add the circ. mils and then the new cir. mil area determines the ampacity of the two paralleled conductors?
 
Don?t forget 310.4(D) that refers you to 310.15(B)(2)(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. If you have 2-1/0 Cu. conductors @ 75?C, you have 150 amperes permitted in each conductor. However, if you have three sets of parallel conductors in the same raceway, you have to take a reduction. You would have:
150 X 2 = 300 amperes per ?.

Then, since you have 6 current carrying conductors in the same raceway, you would have:
300 X .8 = 240 amperes per ?.

It may be well to consider using 90?C rated conductors so you start with 170 amperes per conductor and that will leave you with 272 amperes per ?. Not quite 300 amperes but 32 amperes closer. ;)
 
When you use the 90 degree ratings don't you have to be sure all terminations are also rated at 90 degrees ?
Section 310-15(b)FPN(1)
 
Ileumas said:
When you use the 90 degree ratings don't you have to be sure all terminations are also rated at 90 degrees ?
Section 310-15(b)FPN(1)
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The trick is to use the 90 degree ratings for the derating calculations and then don't exceed the 75 degree ratings for the actual load (because of the terminations).
 
Someone recently asked me the same question. When sizing the ampacity for parallel 500 kcmil conductor would you use (380 *2) or look up the ampacity of a 1000 kcmil conductor. Kind of strange when you compare the ampacities of the two.

Parallel 500 kcmil THHN (380)(2) = 760 amps
Single 1000 Kcmil THHN = 545 amps
 
infinity said:
Someone recently asked me the same question. When sizing the ampacity for parallel 500 kcmil conductor would you use (380 *2) or look up the ampacity of a 1000 kcmil conductor. Kind of strange when you compare the ampacities of the two.

Parallel 500 kcmil THHN (380)(2) = 760 amps
Single 1000 Kcmil THHN = 545 amps
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That is because of Skin Effect.
 
bob said:
That is because of Skin Effect.
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Is it really the only reason? I was under the impression at 60 Hz it is not a big issue.

It seems to me that the big issue with large conductors is the fact the the amount of copper inside as a ratio to the outside diameter increases so much that the conductor is not able to shed heat to the air as well.
 
Bob,

I think that in the largest systems that you've worked with, skin effect may have been significant.

According to one approximation, the resistance of a 1000 kcmil copper wire is 10% higher then expected because of skin effect.

But on the whole I agree, change in ampacity as conductor size changes is dominated by heat dissipation capacity and surface area versus cross section.

-Jon
 
charlie said:
Don’t forget 310.4(D) that refers you to 310.15(B)(2)(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. If you have 2-1/0 Cu. conductors @ 75?C, you have 150 amperes permitted in each conductor. However, if you have three sets of parallel conductors in the same raceway, you have to take a reduction. You would have:
150 X 2 = 300 amperes per ?.

Then, since you have 6 current carrying conductors in the same raceway, you would have:
300 X .8 = 240 amperes per ?.

It may be well to consider using 90?C rated conductors so you start with 170 amperes per conductor and that will leave you with 272 amperes per ?. Not quite 300 amperes but 32 amperes closer. ;)
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But at 272 amps, you would still have a 300 amp service.....right? 240.4 (b) (3) next standard size up rule?
 
POWER_PIG said:
But at 272 amps, you would still have a 300 amp service.....right? 240.4 (b) (3) next standard size up rule?
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Actually you would have a 272 amp service that is protected by a 300 amp breaker.

Read 230.90(A) and 240.4 (B)

steve
 
hillbilly said:
Actually you would have a 272 amp service that is protected by a 300 amp breaker.

Read 230.90(A) and 240.4 (B)

steve
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I think you catch my drift though eh smarty pants?
Or do you need further enlightment? I was simply stating that to be an example how to use the 90deg. table to your advantage.

Pig
 
iwire said:
Pig, I did not think he was being a smart guy, what he said is right on the money, it is not a 300 amp service.
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I suppose I'm the one being a smart alleck eh?,,,,,I was just funnin' no disrespect intended....
I have never addressed services by the actual, absolute amperage rating of the wire, but I guess that is the proper way. You must admit that there are a whole lot of slang terminology in the industry that may be considered technically incorrect but that is still understood by all in the trade.
Ya'll aint' gonna critique my spelling too are ya?:)
 
It's a code forum so yes it gets picky around here, don't take it personal.

POWER_PIG said:
Ya'll aint' gonna critique my spelling too are ya?:)
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With 100s or even 1,000s of people looking in someone is bound to say something about spelling. My spelling is poor so I use a spell check to try to avoid getting busted on it. :smile:
 
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