Code Question of the Day

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Code Question Of The Day

Monday, March 26, 2012

Question:

Charlie,

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

Answer:

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

ANSWERS: Answers are the author’s opinions and do not represent formal interpretations of the National Electrical Code.

All answers are based on the latest edition of the NEC, unless the question requests a response based on a specific edition.

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ABOUT THE AUTHOR: Charlie Trout is a nationally-known NEC expert and author. He served on three different National Electrical Code-Making Panels and is past chairman of CMP-12. He is also a member of the NECA Codes & Standards Committee. In 2006 Charlie Trout won the prestigious Coggeshall Award, given to recognize outstanding contributions to the electrical contracting industry in the technical and training area.

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