Standards as High as Your Own
NECA Online Help Desk Please leave a message
Re: CQD answer published Wednesday, July 10, 2019 -NM Cable Adjustment Factor
Hi Charlie 2.0, Regarding the answer provided to the question for Tuesday 7/9/2019, just wanted to point out the 80% adjustment factor mentioned in the answer is applied to the ampacity allowed for the maximum conductor temperature. For NM-B Cable, the conductor insulation is rated 90C therefore the ampacity adjustment is applied to the 90C ampacity. In this situation (12/4 or 12/2/2 NM-B), that would give an allowable ampacity of 0.8 x 30 Amps = 24 Amps. Since this calculated result is higher than the 60C ampacity, we must use the lowest of the two values which is the 60C ampacity. The net result is that a 4 conductor 12 AWG NM-B Cable (whether it is one circuit or two circuit) has an ampacity of 20 Amps – just like the 12/2 and 12/3 NM-B Cables. If multiple cables are bundled together, further ampacity adjustment may be required.
ABOUT CQD: NECA’s Code Question of the Day (CQD) is a leading National Electrical Code® forum for NECA and the industry. The CQD generates a lively dialogue and relative practical and Code-based responses to an ever-increasing and interactive audience.
IMPORTANT NOTICE: All answers are based on the latest edition of the National Electrical Code®, unless the question requests a response based on a specific edition. This correspondence is not a formal interpretation of the NEC®. Any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, or any technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services.
ABOUT CHARLIE: 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. Charlie was also a member of NECA’s Academy of Electrical Contracting. Charlie’s experienced team of industry experts keep the CQD dialogue and discussions active and informative in the spirit of the man himself, as he wanted.
NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. Visit NECA-NEIS.org for more information. NEISÃ” can be purchased in three formats: as paper books, on CD, or as electronic downloads.
NECA SAFETY PRODUCTS: NECA publishes valuable electrical safety books and CDs for the industry. Visit necanet.org/store to view or purchase NECA safety products.
CONTACT US: To submit a Code question, subscribe, or unsubscribe from this list, please e-mail: firstname.lastname@example.org.
© 1999 – 2014 National Electrical Installation Standards. All rights reserved.
Brought to you by NECA. NECA is the voice of the electrical contracting industry NATIONAL ELECTRICAL INSTALLATION STANDARDS, NEIS and the NEIS logo are registered trademarks of the National Electrical Contractors Association.
Created by Matrix Group International, Inc.