OK, let me get the mud glasses out.
Take #12 wire in a 1/2" EMT. The 500% column is what you need to get the breaker to trip quickly (how quickly depends on the trip curve of the breaker). So we need to make 100 amps flow to get the breaker to trip (this could take 1 second based on the curve I'm looking at). Looking at the EMT column, if I'm not sloppy about tightening the screws, a run of 246 feet is the suggested maximum leangth. As noted at the bottom of the table, this is for 120 volts. At 277 volts, I'm sure the run could be longer.
I've heard that if you can make 10x current flow through a breaker, most brands will now be in the instantaneous trip part of the curve (another of those blasted rules of thumb).
Tom
