Standards as High as Your Own
NECA Online Help Desk Please leave a message
I want to use 90C rated 8 AWG wire for wiring to circuit breakers in a location not expected to exceed 40C with 3 current-carrying wires in a conduit. By Table 310.16 the wire has ampacity 55A with 40C temperature derating to 50A. However, NEC 110.14(C)(1) suggests that the 8AWG wire is limited to the ampacity of 60C 8AWG wire in Table 310.16, which is 40A at 30C. Do I then need to derate the 40A ampacity for 40C, which would bring it to 33A? If instead I have 4 current carrying wires in the conduit, do I need to derate the ampacity of the 8AWG 90C wire further to 26A, following Table 310.15(B)(2)(a)? If I follow this logic, if I need to carry 50A at 40C with 4 current carrying conductors in a conduit, I would need to use at least 3AWG wire, even though 3AWG 90C wire is rated to 110A. If I do not need to do any of the derating of the 60C ampacities, it seems that I could use 6AWG wire. If I only need to do the conduit derating, it seems that I can use 4 AWG wire. I hope you can clarify this for me. Thank you in advance for your assistance.
Hey Finley thanks for your question. The heading of Table 310.15(B)(16) indicates that it is based on not more than 3 current carrying conductors and an ambient temperature of 86 degrees F. If the installation is in an ambient temperature of 40 degrees C (104 degrees F) then the correction factor in Table 310.15(B)(2)(a) must be applied to the values in Table 310.15(B)(16). If there are more than three current carrying conductors the adjustment factor in Table 310.15(B)(3)(a) must be applied to the values in Table 310.15(B)(16). If both conditions of use exist then both factors must be applied.
Using 90 degree C insulated copper conductors at an ambient temperature of 104 degrees F (40 C) with 4 current carrying conductors for a load of 50 amperes would need 6 AWG conductors. The 75 ampere value [from the 90 degree C column in Table 310.15(B)(16)] X 0.91 [from the 90 degree C column in Table 310.15(B)(2)(A)] X 80% [from Table 310.15(B)(3)(A)] = 54.6 amperes.
Conductors cannot be used above the terminal temperature limitations [55 amperes for 6 AWG copper in the 60 degree C column in Table 310.15(B)(16) in this example, unless they are listed and marked otherwise] but the 90 degree C values can be the starting point as stated in the second paragraph of 310.15(B).
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.