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Tuesday, December 27, 2022

Question:

Good morning, Charlie. If I have 75 KVA, 480 volt delta/208 volt wye transformer with 4-4 wire secondary sets of conductors that terminate in 4 individual 208 volt wye panelboards on a 100 amp main circuit breakers, does this meet the requirements of NEC 450.3(B) note 2? Thanks, David Evans
A

Answer:

Hi David, Section 450.3 and Table 450.3(B) establish the minimum primary and secondary overcurrent protection for protecting the transformer itself.

 

Based on the information provided, the secondary rated current of the transformer is 75,000/(277 x 1.732) = 208A. When required, the maximum secondary OCPD rating is 208 x 1.25 = 260A.

With 4–100 Amp OCPDs for protection of transformer secondary, the total protection equals 400A which exceeds the maximum OCPD rating of 260A. This installation would not meet the restrictions to apply Note 2.

 

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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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