Services & DistributionQuestion 102 of 521
A 75 kVA, 208Y/120 V transformer has an impedance of 2%. Using the infinite-bus (worst-case) method, the approximate available fault current at the transformer secondary is about:
a.10,400 A
b.4,160 A
c.20,800 A
d.2,080 A
Explanation
Secondary full-load current = 75,000 / (1.732 x 208) = 208 A; dividing by the per-unit impedance gives 208 / 0.02 = 10,400 A. Using the wrong impedance or forgetting to divide gives the incorrect values; this infinite-bus method assumes an unlimited primary source.
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