Looking for a little help. I need to size conductors for a 400 amp 480v 3wire feed from a buss bar fused tap box to a transformer. The lugs on the tap box are rated for 75 degrees the lugs on the transformer are 75 degrees also. The wire will be thhn. Am I right in figuring I need to run 600’s to satisfy the 75 degree column even though the thhn is rated for 90 degrees and I could size it at 500’s? I think so, but my second question has me questioning myself.
Second question. I will the be going from the transformer to a 400 amp 480v 4 wire panel. Same question as above, what wire size? 75 degree lugs on transformer, however lugs in main breaker in panel maxes out at 500, the 600’s won’t fit. I believe the breaker is also rated for 75 degrees. Because the breaker is 400 amps and only has lugs big enough for 500’s it’s leading me to this question of is it 500’s or 600’s I need to run? I think 600’s, am I missing something that lets me run 500’s?
Thanks for your help.
Mic.
