Difficult service question

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Red Wiggler

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I have a situation where a college owns the medium voltage service equipment on their facility. The PCO has completed their service to the 2 separate 2000 amp 12.47kv medium volt switchgear. These two sections of gear then feed two pad mounted 3750 12.47kv/480/277v transformers also owned by the college. So all service equipment is owned by the college. The Utility only has provided the service drop.

Now here is the area where I am unclear about...

The two pad mounted transformers then each feed switchgear in the building. There are 12 parallel 600 KCMIL (4 per conduit) feeds from each xfmer to the swithgear at which point the neutral will be grounded. This seems like a typical commercial service arrangement but...

We are being told to add equipment grounding conductors to the service conduits (24 total) entering into the building. I am confused to why I need to do this. Besides there is no place in the service transformers to terminate the ground conductors. We are being told to refer to NEC 2011 Articles 230.200 and 230.31. They are also refering to NEC Article 225 Part II because we have parking garage and another building being served by this service.

Any help here?
 
Red Wiggler said:
The two pad mounted transformers then each feed switchgear in the building. There are 12 parallel 600 KCMIL (4 per conduit) feeds from each xfmer to the swithgear at which point the neutral will be grounded. This seems like a typical commercial service arrangement but...
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You have pointed out that the service is the 12.47kV from the utility. The transformer secondary feeds to the building are separately derived systems, not services.

Red Wiggler said:
We are being told to add equipment grounding conductors to the service conduits (24 total) entering into the building. I am confused to why I need to do this. Besides there is no place in the service transformers to terminate the ground conductors. We are being told to refer to NEC 2011 Articles 230.200 and 230.31. They are also refering to NEC Article 225 Part II because we have parking garage and another building being served by this service.

Any help here?
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230.200 and 230.31 would not apply to the transformer secondaries. See 250.30 for the grounding of the separately derived systems. 250.30 permits you to bond the grounded conductor (neutral) to the equipment grounding conductors at any point from the source to the first disconnecting means. The grounding electrode conductor connection must be made at the same point as the system bond.

What you have described with bonding the neutral at the the switchgear in the building sounds perfectly compliant to me.
 
david luchini said:
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What you have described with bonding the neutral at the the switchgear in the building sounds perfectly compliant to me.
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I agree with your reply up to the quoted part above, though it is compliant... once, but not twice. If the pad mount transformers' secondaries are "tapped" more than once ("we have parking garage and another building being served by this service"), IMO it would be best to connect SBJ and GEC(s) at the transformers. Keep in mind, though always subject to AHJ interpretation, that pad mount's are not part of the structure(s) served. The GES/GEC does not have to connect to metal water pipe or building steel electrodes, as there are none for the pad mount structure. Each building or structure served will have its own GES, but not connected directly to the Neutral.
 
Smart $ said:
I agree with your reply up to the quoted part above, though it is compliant... once, but not twice. If the pad mount transformers' secondaries are "tapped" more than once ("we have parking garage and another building being served by this service"),
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I do not read the OP as "tapping" the secondaries more than once. He says he has two 12.47kV switches, two 3750kVA transformers, and twelve parallel sets of 600mcm from each transformer to switchgear in a building. I read the feeders to the parking garage and other building as originating in the 480V switchgear.
 
I just went thru that scenario at a local college. The engineers had approached the feed from the outside, transformer as a service even thought the college owns and maintains the 12 kv distribution system.
It seems to me that exception 2 in 250.30 in the '08 Code pretty much allows you to treat the SDS as a service as long as you establish no parallel return paths (earth excluded).


I have not studied it in depth, but it appears 150.30(A)(2) in the '11 Code may remove that ability.
 
david luchini said:
I do not read the OP as "tapping" the secondaries more than once. He says he has two 12.47kV switches, two 3750kVA transformers, and twelve parallel sets of 600mcm from each transformer to switchgear in a building. I read the feeders to the parking garage and other building as originating in the 480V switchgear.
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You may be correct. I have re-read the OP. It is not [absolutely] clear where the supply for the parking garage and other building originate...???
 
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