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?
Protecting Conductors, Part 2
If you’re working with 800A or less, don’t worry if an OCPD rated at 700A won’t accommodate your loads, but one rated at 800A exceeds the ampacity of your conductors [240.4(B)].