Wire Sizing Between Disconnect and Sub-Panel

[kp542]

I am looking to determine the appropriate sized wiring between a disconnect in a feeder panel and a sub-panel. Both the disconnect and the sub-panel are rated for 400A and I would like to know whether I would size the next size up (#600) or next size down (#500) wire.

 

[charlie b]

Is there a load calculation? If not, then you need the feeder to handle every bit of the 400 amps. So a 500 MCM copper conductor would not suffice. But if a load calculation has a result of 380 amps or lower, you can use the 500's.