Equipment & SystemsQuestion 262 of 521
A 30 kVA, three-phase transformer has a 480 V primary (primary current ~36 A). With primary-only protection and no supervised condition, what is the maximum primary overcurrent device using the 125% factor of Table 450.3(B)?
a.36 A
b.45 A
c.60 A
d.72 A
Explanation
For a transformer with primary current of 9 A or more, Table 450.3(B) limits primary-only protection to 125%: 36 x 1.25 = 45 A (with the note allowing the next standard size where 125% does not correspond). 36 A omits the factor, 60 A is ~167%, and 72 A is 200%.
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