A three-phase, 480-V generator powers a load via single-phase feeders and 25-kVA, single-phase, 480/120-V step-down transformer with series impedances of 1 + j5% as shown below. The single-phase feeder on the generator side has 0.002 + j0.006 Ω impedance. The generator is much larger than the 25-kVA transformer rating, making the generator bus infinitely stiff with zero internal impedance. Determine the symmetrical rms current rating of the transformer circuit breaker on the 120-V load side, assuming the fault location is close to the breaker LV terminals.

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Gen 1-phase cable 0.002 + j0.006 Fault 1-ph, 25-kVA Transformer 480/120 V, Z= 1 + j5% 3-ph 480-V infinite bus 120-V loads Other loads