Services, Grounding & BondingPregunta 43 de 120
A 45 kVA, three-phase transformer has a 480-volt primary. If only primary overcurrent protection is provided at 125 percent, what is the maximum standard primary device rating?
a.90 amperes
b.60 amperes
c.80 amperes
d.70 amperes
Explicación
Primary full-load current is 45,000 / (1.732 x 480) = 54.1 A, and 54.1 x 1.25 = 67.6 A. Because the calculated value does not correspond to a standard rating, the next higher standard size of 70 amperes is permitted. The 60-ampere answer rounds down and would nuisance trip, while 80 and 90 amperes skip past the next standard size.
Referencia Legal: 2023 NEC §450.3(B)Practica las 120 preguntas gratis — sin registro.
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