I am planning to run two sets of four-wire cables (two hots, one neutral, and one ground) through the same conduit to feed two subpanels from a single main breaker panel. I understand that I need to derate the ampacity of the cables to 80% because I am running them in conduit without maintaining spacing, but I was wondering this table, the very last section shows the 100% rated ampacity for SE cable as a feeder, but that seems to assume 90°C terminal ratings, which the subpanels (see page 6 here) are not rated for. They have a 75°C terminal rating.
Does this mean that I need to take 80% of 75°C rating presented in the first table to get my conductor size (ex. 1GA Cu conductors for 100A capacity) or can I take 80% of the feeder wire specs (ex. 2GA Cu conductors for 100A capacity).
I understand the difference is literally one wire size, so it's probably best to oversize than undersize it, but I want to know how I should be reading those tables properly.