National Electrical Installation Standards

Standards as High as Your Own

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Tuesday, December 3, 2019

Question:

A client owns a 12.6 KV X 480Y277 transformer. This transformer primary is supplied by the utility and is the sole power source for the building. The project electrical engineer shows bonding and grounding of  X 0 at the transformer, then just 3 phase conductors to the first disconnecting means. We objected. He replied the the grounded conductor will not serve any neutral load loads, and seems to be treating this like a separately derived system instead of what we think it is, a service entrance. We cited the code requiring that the grounded conductor was required to be brought to the first disconnecting means regardless, and grounded and bonded there in addition to the bonding at the transformer. He is adamant, and we are not willing to knuckel under. The argument seems to center on the private ownership of the transformer. In our opinion, who owns the transformer doesn't matter. Your thoughts might help us bring some sanity.

Ron Robertson

 

A

Answer:

Hey Ron thanks for your question. Actually, ownership makes a considerable difference. If the customer owns the transformer then the secondary is not a Service as that term is defined in Article 100. The service ends at the point where the primary conductors terminate, which should be the Service Equipment (disconnecting means) as that term is defined in Article 100. See also the definition of Service Point in Article 100.

The transformer in your example is a Separately Derived System as that term is defined in Article 100. The word "if" in 250.30(A)(3) allows a grounded conductor but does not require one for a separately derived system. But in your case a supply side bonding jumper must be installed between the transformer and disconnecting means it is supplying as stated in 250.30(A)(2), which can be a metal nonflexible raceway.

 

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