I calculate that at about 210 feet (one way circuit length), but it might be OK even for a 420 foot (one way) circuit. By contrast, if you used #10 for both the ungrounded and the EGC, you might not have trouble clearing a fault unless the circuit length exceeded 540 feet. But of course, you might be dealing with equipment performance issues with a circuit that long anyway, if you only used #10 for the ungrounded conductor.
Originally Posted by Sierrasparky
So in light of this one, limited example, there is no technical reason that we would need to upsize the EGC from #12 to #10, in that any circuit that is not so long that it causes VD issues at the end of the line would be able to clear a fault in a reasonable time, even with a #12 EGC. Did I mention already that I didn't like this rule?
Charles E. Beck, P.E., Seattle
Comments based on 2014 NEC unless otherwise noted.