Great question — answering as the UphillTech team:
Why SER? With surface mounted panels in what presumably is an unfinished space, there is 0 reason to run an exposed SER cable here. Instead, I would run a conduit between the two panels (2" EMT will handle anything you want to do here), and then simply run 3 XHHW-2 conductors (hot, hot, neutral) in the conduit between the two panels, using the conduit as a grounding path. As to inner conductor sizing... I would use 1AWG Al wires here; spending the extra for copper is a waste, and you can't get away with 2AWG for this because you'd exceed the 75°C termination limit that is normal for breakers and loadcenters if you did. This limitation is a consequence of NEC 110.14(C)(1), by the way. TORQUE...
Full answer by ThreePhaseEel: https://diy.stackexchange.com/a/165464 (CC BY-SA 4.0).
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