National Electrical Installation Standards

Standards as High as Your Own

Thursday, December 14, 2023


Good morning, Charlie, we have a transformer in a non-industrial setting, mounted 40feet in the air. There is a non-fused disconnect on the secondary side of the transformer, and the circuit extends to a fused disconnect mounted on the wall at about 5 feet high. My co-worker says that this meets the code, because it has a disconnecting means within 25ft, and overcurrent protection at floor level (below 6'7") for the wire. I suggested that the OVERCURRENT protection must be within 25ft, not just a disconnecting means. So, seeing as the overcurrent protection can be no higher than 6ft 7in, both switches would need to be fused, with the one on the transformer being considered a Supplementary Overcurrent Protection in order to meet the code. What do you think? All the best, Curt S.


Hi Curt, thanks for the great question. The overcurrent protection requirements for transformer secondary are some of most misapplied rules in the NEC®. Secondary overcurrent protection is not always required as permitted in Section 240.21(C)(1)-(6). For example, per Section 240.21(C)(1), a 3-phase, delta-delta transformer with a single voltage secondary, would not require any secondary overcurrent protection if the primary overcurrent protection device is sized in accordance with Table 450.3(B). See Sections 240.24(A)(4) and 408.36(B) for more guidance on the location of OCPDs.

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CHARLIE TROUT: Charles M. Trout, better known as Charlie, was a nationally known NEC® expert and author. He served on several NEC® technical committees and is past chairman of CMP-12. In 2006 Charlie was awarded the prestigious Coggeshall Award for outstanding contributions to the electrical contracting industry, codes and standards development, and technical training. Even though Charlie passed away in October of 2015, his work continues in spirit. NECA continues to maintain this question forum for its many subscribers in memory and recognition of all his significant contributions to making the NEC what it is today.

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