Unsure on parallel feeders

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601hand

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I'm having a hard time understanding feeder calculations on a current project, and looking for some advice. Weare installing a 480v/3ph/1400A service. What is proposed is 4-4" IMC from the outdoor Xfmr to our meter base/ main DC aproximately 25' away. My question is, the contractor says that 4 runs of 500 MCM thhn will suffice, but with my calculations (no derating, and 75degree terminations) that we will not reach 1400a. I believe him, but would like to know for my own piece of mind where this calculations is found.
 
Since the feeder will be in the ground you will need to derate the cable ampacity because the ground acts like insulation and the heat generated in the cable will not be easily dissipated. Annex B discusses this and gives guidelines for the cable installation. This section is for information only and is not a requirement of the NEC. IEEE 390 also discusses this. Depending on the conduit configuration in the soil the derating factor will vary.
 
misinformation

misinformation

My problem will probably have a lot more relevance with the correct info. I am hooking up a 1600 A switch not a 1400A (checked this morning after feeling foolish with the obvious answer to my earlier post). So, to correct, is it OK to feed a 1600A/480v/3ph main breaker from the utillity Xfmr with 4 runs of 4" IMC each having 4 500mcm copper feeders. we have the grounds in there also. Just concerned with wire sizing, the 310.16 tables tell me no. Thanks for the help, sorry for the confusion.
 
You say you have grounds in the conduit. Is this a 3 wire system or do you have neutrals in each raceway?
 
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