250.191 What is a substation?

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xguard

Senior Member
Location
Baton Rouge, LA
I may be terribly confused here. We have a project renovating a large ventilation building. The site will be supplied by a 1000 KVA pad mount transformer that will be owned by the utility company, I've attached a photo of the existing utility transformer, it will be larger but similar. The designer is stating that the site requires a "a fully designed grounding grid system". This would require them to perform a a "finite element analysis".

I'm not against a grounding grid but I don't see how the code sections they reference apply, as we do not have a substation at the site. The NEC (2023) section they reference is 250.191. Now, maybe we do and I just don't understand the word 'substation'. They also reference 250.194, again this seems to be for substations and concerning the proximity of exposed conductors. Our site has a typical pad mount transformer supplied by an underground utility supply. Do these code sections apply to this installation?

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If the equipment is completely enclosed in a grounded steel cabinet, like padmount transformers as in the picture, a grounding grid is not required by the NEC. Notice the word "exposed" in the cited 250.194.

The term 'substation' is not defined. People in the utility industry often think of substations as having open air terminals and and as such require ground grids as those required by IEEE 80.

Padmount style switching and transformers are typically installed with nothing more than standard ground rods.
 
infinity said:
Eaton calls many of their large dry type transformers substations.
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Actually it is a substation style transformer which mean it can be close coupled to primary and secondary equipment.

Most manufacturers call close coupled equipment "Unit Substations".
50 years ago another common phrase was to call these unit substations "Load Centers" because of the secondary feeder breakers.
 
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infinity said:
Jim, can you explain what that means?
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Close coupling usually involves manufacturer supplied conductors/bussing between components. Primary side switches are usually cable connected while secondaries are bussed. Some times all the components are from the same manufacturer sometime they aren't. They are still called substations even if the incoming or the outgoing is just a cable termination compartment.
 
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