Okay, this is the engineer in me trying to understand something that just seems counterintuitive. I think I understand that according to NEC that the allowable ampacity for a wire that is somewhere between what the POCO supplies and what we lay people like to call a main panel, but which is apparently properly referred to as the service panel, can be found in table 310.15(B)(6) but for downstream panels the ampacity must come from table 310.16
What this means effectively is that for a given current you want a wire to handle, you need a bigger wire downstream where you go through your house than you need coming into your house.
The fact that code says this seems pretty well explained here:
So for a 100 amp "main" panel, wiring it from a meter, table 310.15(b)(6) says I can use 2 gage aluminum wire, but if I wire a "sub" panel that same wire could only handle 90 amps and I would need 1 gage or heavier aluminum wire to handle 100 amps.
So my question is why is 2 gage aluminum okay upstream but not okay downstream? I get it for overhead wires where air flow can better cool the wire than in enclosed installations, but your main panel feeder goes through the same walls other wires go through...