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What are the rules as per the NEC providing overcurrent protection when utilizing autotransformers?

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Electrical Engineering

What are the rules as per the NEC providing overcurrentprotection when utilizing autotransformers?

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4.3 Ratings (744 Votes)
Over Current protection for autotransformer is similar to that of transformer Protection at the primary side of the transformer Transformer primary protection Overcurrent protective devices OCPDs are required as primary protection for transformers when the primary voltage is greater than 1000V Tables 4503A and for transformers with primary voltage of 1000V or less Table 4503B with no Exceptions However a fire pump has a special requirement for its transformer primary and secondary protection which is considered a modification to Art 450 If a transformer serves a fire pump Art 695 its primary OCPD shall be sized per the lockedrotor current of the fire pump and transformer secondary protection shall not be permitted 6955B Transformer primary feeder protection The NEC does not list any specific Exception for transformer primary feeder protection An OCPD is required for the transformer primary conductor The only Exception is found in Sec 2404A If overload protection of the OCPD would create a power loss hazard such as fire pump application and some processes at refinery facilities the overload protection shall not be required However short circuit protection is still required As a reference Sec 6954B2a1 and 2 requires the overcurrent device for a fire pumps to be sized per lockedrotor current of the largest fire pump motor and shall not open within 2 minutes    See Answer
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