Code Question of the Day with Charlie Trout

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The National Electrical Code is the bedrock of the electrical construction business.

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Code Question Of The Day

Monday, March 26, 2012


Thanks for the CQD. I depend on this information. I’m installing a large dry-type transformer separately derived system and need to verify the correct size system bonding jumper. I have 2000 kcmil copper ungrounded phase conductors for each of the three phases and the neutral of the system. (4) 500 kcmil copper conductors in parallel per phase. According to Table 250.66 is 3/0 copper the size required for the system bonding jumper? Thanks in advance for your help.

Reggie Gerard



Hey Reggie,

Thanks for participating and thanks for the follow up. The requirements for sizing a system bonding jumper for separately derived systems are found in 250.30(A)(1) which refers to 250.28(A) through (D). Section 250.28(D)(1) addresses system bonding jumper sizes. Note that if the supply conductors are larger than 1100 kcmil copper, the system bonding jumper must be at least 12.5% of the total cm. Based on the information in the question, 2000 x .125 = 250 kcmil and is the minimum size required for the system bonding jumper. Remember Table 250.66 is used to size more than just grounding electrode conductors. It is used to establish the minimum sizes for grounded conductors, supply-side bonding jumpers, main bonding jumpers, and system bonding jumpers. The 12.5% rule is in effect for all but the grounding electrode conductor. Hope this is helpful.


Section: 250.30(A)(1), 250.28(D), Table 250.66

ABOUT CQD: NECA’s Code Question of the Day (CQD) is Charlie Trout’s flagship National Electrical Code® forum for NECA and the industry. The CQD continues to generate a lively dialogue and relative Code-based and practical responses to an ever-increasing and interactive audience.

ABOUT CHARLIE: Charles M. Trout, better known as Charlie, is 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.

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