I was taught that there are very few cases where a conductor with 90 degree C rated insulation can actually be used at the ampacity listed in the 90 degree column. The only 90 degree rated equipment that I know of are certain terminal lugs and split bolt style connectors. I have never seen equipment such as disconnect switches, panelboards, ect. that have over a 75 degree ratings.
The only case where I can possibly see that you could ever use a 90 degree conductor at its full ampacity is if you had a junction box on both ends of the run, and then spliced it to larger wires at each end to meet the 75 degree rating of the equipment. I doubt there would ever be much of a need for this type of installation.
The 90 degree ampacities are almost exclusively used for derating calculation purposes only.
Matt
