NEC Service Entrance Equipment

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timm333

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Minneapolis, MN
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Electrical Design Engineer
Just need some clarification for the NEC service entrance equipment for industrial facility. The utility supply at the point of interconnection (POI) is 12.47kV. After point of interconnection (POI), the scope of utility ends and the scope of plant starts. Then there is a 12.47kV fused disconnect switch (owned by plant) which feeds a 12.47kV-480/277V transformer, which then feeds a 480V switchboard. All the plant loads are fed by this 480V switchboard.

The neutral is to be connected to the ground at the service equipment.

What would be the service equipment in this case? Will the service equipment be the 12.47kV fused disconnect switch on the primary of the transformer, or will it be the 480V switchboard on the secondary of the transformer. Thanks.
 
timm333 said:
What would be the service equipment in this case? Will the service equipment be the 12.47kV fused disconnect switch on the primary of the transformer, or will it be the 480V switchboard on the secondary of the transformer.
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If the plant owns the transformer, I'd say the former. It seems this answers your question:
timm333 said:
After point of interconnection (POI), the scope of utility ends and the scope of plant starts.
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Services only come from utilities, see the NEC definition.

If you own the equipment, everything is a feeder. For the most part feeders have the same requirements as services.
 
That makes sense. But the neutral has to be connected to the ground at the service equipment. There is no neutral coming from the utility. So if we take the 12.47kV fused disconnect switch as service equipment, there is no neutral at this equipment. The first neutral is at the star point on the 480/277V secondary of the transformer. Should we connect the neutral to ground at the 480V switchboard?
 
In your case the transformer is simply a SDS and 250.30 applies to the secondary. It is lilely that 250.30 (A)(1) Exception 2 will apply.
 
We have had this debate at my plant also. To aid in the discussion, I have compared the NEC requirements for services to those for feeders, and separately derived systems. In many cases the language is almost identical, but service requirements are located in a single article where the requirements for feeders and SDS's are spread out over multiple articles.
 

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  • Services vs. Bldgs Supplied by Feeder with text (NEC 2020).pdf
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  • Service Definitions.pdf
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timm333 said:
That makes sense. But the neutral has to be connected to the ground at the service equipment. There is no neutral coming from the utility. So if we take the 12.47kV fused disconnect switch as service equipment, there is no neutral at this equipment. The first neutral is at the star point on the 480/277V secondary of the transformer. Should we connect the neutral to ground at the 480V switchboard?
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Are the utility conductors concentric type with the grounded conductor as the outer conductor? that is what would be normal around here, that outer conductor is the MGN conductor at your service equipment.

I'd have to otherwise check if NEC would allow not having a grounded conductor, I know it won't for under 1000 volts.

Other thing to consider is if it is a grounded system, that could change things also. You would still need to bond frame of the service equipment to a grounding electrode though.

After that the 480/277 is a separately derived system to NEC.
 
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