30Amp Sub in a garage?

[Alchymist]

Is that because you should not wire a circuit with more than 80% of the breaker capacity. This said, a 60A breaker should not have more than 48A of juice and anything between 55-60 is probably going to be momentary and pop it as it'd be a failure of some component?

Not quite. A 60 amp breaker will pass 60 amps indefinitely. (With the possible exception of one located in a very hot environment). The circuit derating is an exercise in conservative use - allows for peak currents for inductive loads, etc. The 60 amp breaker allowed in this case is because there is no commercially available 55 amp breaker. In most calculations the NEC allows rounding up to the next readily available size.

 

[Charles (in GA)]

Is that because you should not wire a circuit with more than 80% of the breaker capacity. This said, a 60A breaker should not have more than 48A of juice and anything between 55-60 is probably going to be momentary and pop it as it'd be a failure of some component?

While certainly good practice, the 80% rule is NOT a code requirement, except for continuous loads, which the code defines as loads of 3 hrs or more, and then it also defines lighting and electric heating as continuous loads.

General purpose circuits and other types of loads are not restricted by the 80% rule.

In the aviation world, the 80% rule is the rule for every circuit.

Charles