Change in size of parallel conductors along cable route

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Shells

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I have a circuit powered by a 250A, 480V CB. The circuit has a splice in it with 2x500KCMIL parallel cables being used be for approx. 1000ft and transitions to 2x4/0 AWG cables in parallel for another 1000ft. The voltage drop across each is acceptable but wondering if there is any issues or code restrictions in doing something like this. The conductors would meet the requirements of 310.10 - same length, material, cross sectional area and insulation. They will also use the same termination method and be located in the same raceway. The only thing I see is that the 4/0 AWG (230A @ 75 deg C) is lower than the Overcurrent Protection (250A). Not sure if that is a requirement for conductors >1/0 AWG.
Any advice you may have would be appreciated.
 
Conductors in parallel meeting the requirement of 310.10 are permitted to operate at the total combined ampacity. So the 4/0 section almost certainly has more than 250A of ampacity. (I say almost certainly because you don't provide sufficient info to do the derating calculation, but assuming 3 phase with 6 current carrying conductors, normal ambient temperatures the net ampacity of the parallel conductors is 230*2*0.8 = 368A).

I've seen arguments that when changing conductor size in the middle of a parallel run, all conductors need to be combined in a single splice. While this is certainly necessary if there is a change in symmetry of the conductor sets, I don't think it is really necessary here.

Remember that the EGC size has to be changed when conductors are increased in size versus the baseline requirement.

-Jon
 
winnie said:
Conductors in parallel meeting the requirement of 310.10 are permitted to operate at the total combined ampacity. So the 4/0 section almost certainly has more than 250A of ampacity. (I say almost certainly because you don't provide sufficient info to do the derating calculation, but assuming 3 phase with 6 current carrying conductors, normal ambient temperatures the net ampacity of the parallel conductors is 230*2*0.8 = 368A).

I've seen arguments that when changing conductor size in the middle of a parallel run, all conductors need to be combined in a single splice. While this is certainly necessary if there is a change in symmetry of the conductor sets, I don't think it is really necessary here.

Remember that the EGC size has to be changed when conductors are increased in size versus the baseline requirement.

-Jon
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Thank you. You are correct, its 6 current carrying, normal ambient, so 368A.
 
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