National Electrical Installation Standards

Standards as High as Your Own

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Monday, April 23, 2018

Question:

An existing installation has (3) separate oil-filled single phase transformers (pad-mounted) wired together as delta-wye in an overhead bus structure. All (3) transformers are protected with (1) overcurrent relay. My question is: does each transformer require it's own independent overcurrent protection? How does NEC-450.7 relate to this?

Barry Calloway

A

Answer:

Hey Barry thanks for your question. What you are describing is not operating transformers in parallel as covered by 450.7. If you connected the three separate single-phase transformers in parallel, to have multiple sources or for additional capacity, then they would need to be switched as a unit as stated in 450.7. If they were not then deenergizing only one transformer would still leave the others supplying power.

For overcurrent protection multiple single-phase transformers operating as a unit is considered "a transformer" for overcurrent protection as stated in 450.3.

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