I dont understand what you mean. I thought you sized the conductor according to ampacity required, then adjusted the selected conductor according to ambient temp, # of conductors in conduit, etc. I am not trying to sound obtuse, but I guess I was tuaght differently.
nope...you size the conductors based on the terminal temp rating...but for derating purposes only, you can use the 90deg column. because you are worried about the insulation rating, not the connection rating.
without derating, 4-350's or 4-400's have enough ampacity in the the 75deg table to carry the load. but for derating only, you can use the 90deg table.
maybe someone else can explain it better.