I'm installing a new 100A subpanel, about 10' away from the main panel (so no concerns about voltage drop). I believe all wire and terminals on the main + subpanels are rated for 75° or higher, so AFAICT 3-3-3-5 copper SER would be the easiest cable to use here. However, due to local availability issues with small amounts of this cable, I'm wondering if there are other options besides copper SER.
Can I use 3x #3 AWG copper THHN and 1x #5 AWG copper THHN, run in 1-1/4" metal flex conduit, to feed the subpanel?
Per this conduit fill table, it seems that 1-1/4" flex conduit can take up to 5x #3 AWG conductors. But I'm not sure if there are other issues with using flex conduit or THHN wires for a feeder. I'm also not sure if derating would apply here since it's just 1 feeder circuit.
I'm also open to other cheaper or easier (or otherwise relevant) suggestions.
I'm in a NEC 2020 state.
p.s. I'm planning to run a load calculation before doing this, and will use a subpanel with lots of spaces but control circuit size via the breaker on the main panel. So need to spend time debating this part here :-)