Question

An installation consists of a 30-kVA, 3-phase transformer, a 480-volt primary, and a 240-volt secondary. Calculate the largest standard size circuit breaker permitted for primary-only protection to be used without applying Note 1 of Table 450.3(B).

Answer 1

Answer:  45 A

Explanation:

Primary only protection 3-phase

I =  3 phase kVA / ( 1.723 * V)

I = 30000 / ( 1.732 * 480 ) = 36.085 A

Table 450.3(B)

Currents of 9A or more column

primary only protection  = 125%

Max OCPD pri = 125% of I = 1.25 * 36.085 = 45.11 A

 Table 450.3(B) Note 1   does not apply, use next smaller Table 240.6(A)

Next smaller = 45 A