Home #1:#2 copper conductor to main panel w/125 Amp main breaker. #2 is allowed for 100 Amp max. breaker (60C rated assumed).
Is #2 ok for 125 Amp 75C rated breaker?
Home #2:#2 aluminum conductor to main panel w/100 Amp main breaker. #2 is allowed for 90 Amp max. breaker (60C rated assumed).
Is #2 ok for 100 Amp 75C rated breaker?
I don’t recall breaker ratings being discussed in the NACHI electrical course and am assuming the wire to breaker sizes mentioned in the course are for 60C rated breakers.
Most breakers are 75C rated but if this is the main feeder or service conductor for a dwelling you can use 310.15(B)(6) to size the breaker. #2cu is OK for 125a and #2al is ok on a 100.
As Greg stated, for the purpose of sizing residential service conductors temp ratings do not come into play. Wire types do, but all “typical” wire types are listed in 310.15(B)(6).
I also agree…chances are the terminals you are dealing with ( because they are service terminations ) are 75 degree rated. Just use the table 310.15 (B)(6) for your sizing and you will be fine.