A journeyman electrician is installing a new 480-volt, 3-phase, 75 kVA transformer to supply a 208Y/120-volt panel. The transformer primary conductors are protected by fuses. What is the maximum rating of the primary overcurrent protection if the transformer's impedance is less than 6%?

Correct answer: A. 125 amperes

First, calculate the primary full-load current (FLA): FLA_primary = (75 kVA * 1000) / (480 V * √3) = 75000 / (480 * 1.732) = 75000 / 831.36 = 90.21 amperes. According to NEC Table 450.3(B) for transformers with impedance less than 6%, the maximum primary overcurrent protection for fuses is 250% of the primary FLA. Maximum OCPD = 90.21 A * 2.50 = 225.525 A. NEC 450.3(B) Exception No. 1 allows the next higher standard overcurrent device rating to be used if the calculated value does not correspond to a standard size. However, it also states that the overcurrent device shall not exceed 125% of the transformer rated primary current for over 600V, or 167% for 600V or less, if 250% is too high, or if the 250% value does not correspond to a standard fuse size. Let's re-read 450.3(B) which specifically states that for transformers 600V or less, with primary protection only, the maximum is 250% of the primary current. If 250% of the primary current does not correspond to a standard rating, the next higher standard rating shall be permitted, not to exceed 167% of the primary current. This is a common point of confusion. The note c) under Table 450.3(B) applies to 'primary and secondary protection', not 'primary protection only'. Let's stick to the 250% rule for primary-only protection. Calculated maximum OCPD = 225.525 A. The standard fuse sizes (NEC 240.6(A)) are 100, 110, 125, 150, 175, 200, 225, 250... Since 225.525 A is not a standard size, and we can go up to the next higher standard size, 250 A would be permitted. However, 250 A is not among the options. Let's re-evaluate 450.3(B) step-by-step. NEC 450.3(B) states: Each transformer 600 volts, nominal, or less, shall be protected by an individual overcurrent device on the primary side, rated or set at not more than 125 percent of the rated primary current of the transformer. Exception No. 1: Where the 125 percent of the primary current does not correspond to a standard overcurrent device rating, the next higher standard rating shall be permitted. This is the *general* rule. The 250% rule applies to *secondary* protection, or where *both* primary and secondary protection is provided. The question asks for primary protection only. Let's re-calculate using 125% for primary protection only, as per the main rule of 450.3(B). FLA_primary = 90.21 A. Maximum OCPD = 90.21 A * 1.25 = 112.76 A. Standard fuse sizes are 100 A, 110 A, 125 A. Since 112.76 A is not a standard size, according to Exception No. 1, the next higher standard rating is permitted. The next higher standard rating after 112.76 A is 125 A. This fits one of the options. Therefore, the maximum rating of the primary overcurrent protection is 125 amperes.