The short answer is voltage drop will usually make all the other computations useless since you will generally end up with a wire far exceeding anything you could possibly get to from the derating. Basically when you spread that heat out along a long conductor, to get to a temperature capable of hurting the insulation, there won't be a useful amount of power getting to the load.
110.14(A) basically says in equipment less than 100a assume it is 60c termination (1 & 2) unless it says so (3) or it is a design B, C or D motor(4) but still limited to the rating of the conductor.
