Sizing a parallel run in one conduit

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gatorfrost

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Winter Haven. FL USA
I have one 4" conduit feeding a 400 amp panel. I would like to run 6 - 250 AL with no neutral. I am on the understanding for derating purposes you can use the 90' column. That would be 230 amps times 2 = 460 amps. I have to derate by 80%, bring me to 368 amps. The next biggest breaker size is 400 amp. Am I correct in my thinking. The total load of the panel is 308 amps. Please let me know one way or the other if I am right or wrong. Thanks
 
Engineer disagrees

Engineer disagrees

He is telling me you can only use the 75' column and that the amp on the wire has to bethe same or higher than the breaker size. Can you help me with where I can dispute him in the NEC, please.
 
gatorfrost said:
He is telling me you can only use the 75' column and that the amp on the wire has to bethe same or higher than the breaker size. Can you help me with where I can dispute him in the NEC, please.
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I don't recall offhand where it says you can use the 90 deg. column for derating purposes even when connecting to 75 deg. lugs, but the section that says you can use the next higher breaker size is 240.4(B).
 
gatorfrost said:
He is telling me you can only use the 75' column and that the amp on the wire has to bethe same or higher than the breaker size. Can you help me with where I can dispute him in the NEC, please.
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110.14(C) says "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."

This permits you to use the 90deg ampacity rating of a 90deg conductor on 75deg terminations when applying the 0.8 adjustment factor for 4-6 current carrying conductors.

240.4(B) says that "The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided the following conditions are met:"

It appears you meet each of the 3 conditions. This would permit you to protect your conductors with an ampacity of 368 with a 400A OCPD.
 
gatorfrost said:
He is telling me you can only use the 75' column and that the amp on the wire has to bethe same or higher than the breaker size. Can you help me with where I can dispute him in the NEC, please.
Click to expand...

Whoever he is, he's wrong. :)

David gave you the applicable code sections and the proof that he's incorrect.
 
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