SER can't be underground. You will need xhhw, mhf or URD for that. URD can't be indoors, so I would stay away from that.
Unless it is just a short stretch that you are protecting with conduit, i would not rim ser in conduit. It will be a pain to pull through
No argument on a 90a feed. My comments were about how you breaker it. No need to change the main breaker in whatever panel you buy
So what I'm seeing is the best option might be the "MHF" aka "Mobile Home Feeder"? It is roughly $3/foot, is allowed to be buried, can be run through conduit, and is approved for indoor/outdoor use? I'm sorry for the departure from my initial garage (all interior project) posting, but I'm mirroring the garage project to my barn with the variable being the main power source for the barn being an outdoor main breaker of 200A.
Here is the 2-2-2-4 MHF I am now eyeing since it can be indoor-outdoor use:
2-2-2-4 MHF Wire
My plan for the barn power feeder is:
1. Run conduit from main breaker down into the ground making a "U" shape that is the distance between the main breaker pole and the outer exterior wall of the barn (roughly 6' in distance I'd be burying conduit). Then up the wall another 6' or so into the rafter overhang (also in conduit).
2. Run the 2-2-2-4 MHF or whatever along a rafter and drill an access hole into the wall between rafters for the wire to enter the room where the sub-panel will go.
3. Run wire down the wall into the sub-panel (in conduit or not? I'm not sure)
4. Connect the 2-2-2-4 MHF up to the sub-panel feeding the 90A main breaker on the sub-panel.
5. Run wiring for my other circuits and connect the smaller sub-panel (with the horrific wiring) with an appropriate sized wire (8/3 perhaps?) and 240V breaker (all lighting and outlets...so 20A aka 12 gauge 12/2?)
6. ?????
Thanks for the continued help, this is going to make my place so much better.