So I’ve had a hard time fully understanding which column to go by and when.. from my understanding the 60degree column is always for romes? 75 degree column would be for 100a or less, and 90 degree is for over 100a and adjustment factors?
Romex (i.e. NM cable) is a special case, where you're limited to the 60 C column for terminations, regardless of conductor or terminal ratings. SE cable is governed by a similar rule, when routed in thermal insulation.
#10Cu and less, you have the small conductor rule.
100A and less, there is a rule that is more academic than practical, which is that 60C is the default. You have a burden of proof to show that terminations on both ends can handle anything higher. Most modern equiment meets this burden of proof, so 75C most commonly governs terminations.
Anything over 100A, and 75C is the default.
In all of the above cases, you can use a higher rating that comes with the wire type, as the starting point for derate calculations, which is the primary value in 90C rated wire, despite a 75C terminal limitation.
