Derating? What, me worry?
NEMA did a study that concluded that simply having Romex encased in foam was no realson to have to derate the conductors. Go ahead, use the 75 degree table.
THEN the NEC decided that if the cables pass through a framing member that needs to be firestopped (like the top plate, you need to derate. Hello 60 degree table.
The reality> The NEC already massively derates the table values for the smaller wires (#'s 14, 12, 10), so the effect should be minimal. In the usual house, this means #6 instead of #8 to the range, and maybe #8 to the air conditioner.
