Im about to run the feeders to a 100 amp sub panel in a detached garage. I already roughed in the garage and got it inspected. The next inspection will be for the finish and feeders. This is my plan...
The difference between 90 and 100 amp breaker on #2 AL is the outter insulation on your ser or seu they require a 90 amp breaker. It's the direct burial cable that is allowed to be on a 100 amp breaker. It has no outter insulation.
In the south we use direct burial cable to the garage. It comes with 4 wires colored. Then we sleeve it when it comes up to the panel or j-box with schedule 80 pvc. no expansion fitting. Connector with bushings on both ends. A lot less work then running conduit the whole way. Isolate the neutral from the ground and use a 8 ft'. rod.