2005 nec 250.122(f) on high resistance grounded systems

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rcw91765

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The question concerns the installation of five 3/c 500KCMIL with #2 EGC, paralleled feeder cables on a 2500KVA, 480V/277V transformer secondary switchgear feeder breaker.

The transformer is equipped with high resistance grounding that limits ground fault current to 5A. The system is set up to alarm when a ground fault occurs but does not trip.

The ampacity of the 5x500KCMIL feeder (not taking derating into account) is 1900A (5X380A=1900A). The ampacity of a single #2 EGC is 115A. The breaker OCPD is currently set to trip at 1600A phase overcurrent and 500A ground fault current (OCPD minimum ground fault setting).

The system is in an industrial facility with full time qualified electrical maintenance personnel.

Owing to the High Resistance Grounding System protecting the 1/c #2 EGC from current greater than 5A, does this arrangement have any effect on 2005 NEC 250.122(F) sub paragraph (2)(2) requirement for the provision of a ground fault protective device set to trip the breaker if ground fault current exceeds the EGC ampacity?

Does providing a high resistance grounding system that limits ground fault current to 5A (well below the 1-1/c #2 EGC ampacity of 11 ) and short circuit protection for the EGC conductor meet the intent of the code
 
I don't see anything in 250.122 that would let you use less than a 4/0 EGC with each set of 500s. While the system limits the ground fault current to 5 amps for a single fault, it does nothing to change the ground fault current if a second fault would occur on a different phase. This second fault would require the larger sized EGC to clear the fault. I don't see anything that would let you use the provisions of 250.122(F)(2) as there is no ground fault equipment that is listed for the purpose of protecting the EGC as required by 250.122(F)(2)(3).
 
rcw91765 said:
Owing to the High Resistance Grounding System protecting the 1/c #2 EGC from current greater than 5A, does this arrangement have any effect on 2005 NEC 250.122(F) sub paragraph (2)(2) requirement for the provision of a ground fault protective device set to trip the breaker if ground fault current exceeds the EGC ampacity?
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I think so. I work with similar installations every day. I'm embarassed I can't just rattle the code sections right off the top of my head:confused: But I can't.

I didn't have enough time to go clear through the code, so here is where I would start:

Assumption: your transformer is a separately derived system - service point is on the xfm primary. This is pretty normal for the stuff I work on - we own the xfm; utilities aren't generally interested in owning an impedance grounded xfm.

References are 2005

So, start with 250.30. The exception takes you to 250.36. In particular look at 250.36.E,F,G.

Depending on how you are connected, You end up at 250.66 or at 250.36.B

Take a look at my references - see if this addresses your questions. Let us know.

If my assumption is wrong, let me know - I'll take another look at it.

cf
 
Curiosity Question:

480V, 2500kVA is about 3000FLA. How come the designers limited the load to <2000A?

I'm guessing the switchgear was existing 2000A - new xfm.

cf
 
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