Three or six conductors in parallel feed?

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Steve Winn

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Louisville, KY
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Electrical Contractor
We are using (2) sets of 250 MCM aluminum in conduit for a 400 ampere - three phase - three wire feed. (2) 250's per phase. The debate centers around the question: Are two conductors of the same phase counted as one conductor since they terminate on the same 2 port lug, or are they to be counted as 2 conductors in regards to 80% derating? At zero derating, each phase has a capacity of 410 amperes. For 6 conductors at 80%, it equates to 328 amps capacity. Which is correct, and where is it located in the code book? Thanks!!!!
 
You have six conductors for derating purposes if they are all in one conduit. However, you are not calculating the the adjusted ampacity value correctly. The adjusted value is 368 amps, not 328. Do you see why?
That said, if your calculated load does not exceed 368 you could still do this with a 400 amp OCPD and be compliant using the next size up rule.
 
Yup 6 CCC's.
310.15(B)(3) Adjustment Factors.
(a) More than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-
carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.
Click to expand...
 
texie said:
You have six conductors for derating purposes if they are all in one conduit. However, you are not calculating the the adjusted ampacity value correctly. The adjusted value is 368 amps, not 328. Do you see why?
That said, if your calculated load does not exceed 368 you could still do this with a 400 amp OCPD and be compliant using the next size up rule.
Click to expand...
Yes, I thought about that after sending while driving down the road!!....the 90 degree table is used for derating purposes. The main argument though is: Can two conductors of the same phase in parallel and in the same conduit be considered two conductors or one? I can see both sides of the debate and was looking for a section of code that addresses this exact situation. Thanks!
 
Steve Winn said:
Yes, I thought about that after sending while driving down the road!!....the 90 degree table is used for derating purposes. The main argument though is: Can two conductors of the same phase in parallel and in the same conduit be considered two conductors or one? I can see both sides of the debate and was looking for a section of code that addresses this exact situation. Thanks!
Click to expand...

That question was answered. They are not one conductor but are 2 conductors. You have 6 current carrying conductor's

I see you responded while I was typing.
 
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