Suppose your calculated load is 720A and the ampacity of the conductors being protected is 780A. Now look at this excerpt from the standard breaker sizes listed in Sec. 240.6 of the 2014 NEC: “...500A, 600A, 700A, 800A, 1000A....” Do you see the problem, here?
An OCPD rated at 700A won’t accommodate your loads, but one rated at 800A has a rating that exceeds the ampacity of your conductors.
But if you’re working with 800A or less, you don’t need to worry about this [240.4(B)]. You can use the next higher standard OCPD, if you meet these three conditions:
1) The conductors being protected aren’t part of a branch circuit supplying more than one receptacle for cord-and-plug connected portable loads (check this receptacle issue carefully, if working with a branch circuit).
2) There’s not a standard-size OCPD at that rating.
3) The next higher standard rating OCPD is less than 800A.
If you can’t meet those conditions, you’ll need to use larger conductors or move some loads to another circuit.
