One addendum to 334.80 and NM (which I know I didn't start out asking about, but which has since been brought up). And that is that, yes, you start in the 90 degree column for NM....but only for derating purposes. Otherwise, as pointed out by 334.80, you would use the 60 degree column, regardless of the fact that 334.112 rates NM at 90 degrees. Furthermore, you can use the new derated ampacity(and conductor) from the 90 degree column only if that ampacity does not exceed what is given in the 60 degree column for that conductor size.
