# Thread: Transformer secondary protection in panel - Mike Holt example

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## Transformer secondary protection in panel - Mike Holt example

In this Mike Holt transformer install example, it is determined that a 175A overcurrent device is required for the secondary based on:

Overcurrent Protection Device Size: Where a feeder supplies continuous loads, the rating of the (secondary) overcurrent device shall not be less than 125 percent of the continuous load [215.3] as listed in 240.6(A).

45 kVA 125A x 1.25 = 156A, 175A protection

It seems though it is assumed the continuous load in this install will use the transformer to it's full capacity, so full load current on secondary was multiplied by 1.25 to determine overcurrent protection. But if the transformer is oversized, and say continuous loads will not exceed say 120A, then a panel with a 150A main (it will serve as the transformer secondary protection) is ok right?

https://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

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Originally Posted by MechEdetour
In this Mike Holt transformer install example, it is determined that a 175A overcurrent device is required for the secondary based on:

It seems though it is assumed the continuous load in this install will use the transformer to it's full capacity, so full load current on secondary was multiplied by 1.25 to determine overcurrent protection. But if the transformer is oversized, and say continuous loads will not exceed say 120A, then a panel with a 150A main (it will serve as the transformer secondary protection) is ok right?

https://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/TransformerInstallation~20020516.htm

It is not the rating of the transformer that governs the sizing of this OCPD, it is the rating of the load connected to its load side. This OCPD has the purpose of protecting the transformer secondary conductors (from overload), the busbar of the panelboard, and if it were a fused disconnect or standalone circuit breaker instead, the feeder conductors on its load side. The primary OCPD or an OCPD internal to the transformer, would be what protects the transformer itself.

A 120A continuous load would go with a 150A breaker. At 120/208V, that adds up to 43.2 kVA, and would go with a 45 KVA transfomer.

In concept, you could have three of these setups on the secondary of the same transformer, and it would be a 150 kVA transformer. But each of the secondary OCPDs and secondary conductors would be sized for the 150A main of each panelboard, rather than the 150kVA of the transformer. 240.21(C) has context rules, some depending on the length of the secondary conductors, and in some cases you might need more than 150A worth of conductor ampacity.
Last edited by Carultch; 05-19-19 at 03:49 PM.

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