"Ground-fault protection of equipment" test question

[RonEl]

I'm preparing for NEC Exam, on practice questions I always get the following question wrong, can someone help me understand why ....

Q: Ground-fault protection of equipment shall be required for a feeder disconnect if _____?

a) the disconnect is rated 800A, 208V
b) the disconnect is rated 800A, 480V
c) the disconnect is rated 1,000A, 208V
d) the disconnect is rated 1,000A, 480V

I go to the correct code section 215.10, but always answer "C". I think I'm associating 208 with a wye system and 480V with a delta system.

 

[HEYDOG]

The voltage to ground on the 208 if it is a wye is 120. If the 480 is also a wye the voltage to ground is 277 volts. In that case d would be the correct answer.

 

[RonEl]

Thanks Heydog - so simple. Makes me feel rather pathetic that I missed such a simple reason. Of course, they're always more simple when you know the answer. Thanks again, have a good weekend.