Dave I looked up the trip curve for a basic Sqaure D 250 amp 240 volt 2 pole breaker.
It looks like it may carry 1.25 it's rated current from about 8 minutes to forever. (About 312 amps) It will carry 500 amps from 1 to 5 minutes.
I just do not see this as a worry, UL and the NEC are well aware of all these issues.
I doubt highly that properly terminated 250 amp condutors are going to burst into flames at 312 amps forever or 500 amps in 5 minutes.
Don't get me wrong if I see readings approaching 80% of a feeder or branch circuit rating I will be trying to get the customer to upgrade.
Bob
