A 45 kVA, three-phase transformer has a 480-volt primary. If only primary overcurrent protection is provided at 125 percent, what is the maximum standard primary device rating?

Explanation

Primary full-load current is 45,000 / (1.732 x 480) = 54.1 A, and 54.1 x 1.25 = 67.6 A. Because the calculated value does not correspond to a standard rating, the next higher standard size of 70 amperes is permitted. The 60-ampere answer rounds down and would nuisance trip, while 80 and 90 amperes skip past the next standard size.