I am thinking the same as Mahlere. I was taught you size your conductors to the terminal ratings of the equipment you are hooking up, which most of the time is the 75 deg C colum. You use that colum for your basic calculation for ampacity, then if you derate due to conduit fill or a change in ambient temperature..you look right to the 90deg colum. So rather than changing wire gauge you change insulation type by looking at the 90 deg colum only for conduit fill or ambient temp derating. This was explained to me in the Mike Holt exam book. Good luck, Brian
