Great question — answering as the UphillTech team:
It's impossible to tell 100% from a picture, but comparing the size of wires landing on those 60A breakers vs. the size of wires landing on the 90A, plus seeing the feeder cable labeled as 4/3 SER, one can reasonably assume that those are all 4 AWG on the 2-pole breakers. With wire size assumed to be 4 AWG, that 90A breaker would not be OK. 4 AWG Al XHHW is rated for 75A @ 90C (65A @ 75C). Additionally, you'll rarely see terminals rated higher than 75C in residential or light commercial panels, which means the usuable ampacity of that wire can be assumed to be 65A. Those ratings are from NEC table 310.16. It's a WAG, but I suspect that the 90A was just what was at hand in the van at the...
Full answer by ThisOneGoesToEleven: https://diy.stackexchange.com/a/333556 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
