National Electrical Installation Standards

Standards as High as Your Own

Wednesday, June 12, 2019


Raceway Fill and Adjustment Factor Part 2

I need to know how many #10 awg thhn wires and #18 awg dimming wires derated I can fit in a 1" conduit? How would you derate and split up the different wires to know how many we can have in each pipe.

Michael Henson


Michael the other part of your question was about derating. The term "derating" has been replaced with the term "adjustment factor" and is not about how many conductors can be installed in conduits or tubing but about how we determine conductor ampacity. Using the 10 AWG THHN (copper) from your example it has an allowable ampacity of 40 in Table 310.15(B)(16) using the fifth row in column one and then going to the 90 degree C column for THHN. That table heading indicates it is based on not more than 3 current carrying conductors in a raceway, cable or earth. If more than 3 current carrying conductors are installed, the adjustment factors (formerly derating) in Table 310.15(B)(3)(a) need to be applied. Using 9 THHN 10 AWG conductors from your example gives us an adjustment factor of 70 %, 40 amperes X 70% = 28 which is the ampacity of each of those conductors based on that adjustment factor. If 10 through 20 conductors are installed the adjustment factor is 50 % which results in an ampacity of 20 for the 10 AWG THHN conductors, that is why we only chose 9 in yesterday's explanation. The ampacity of the 18 AWG conductors of 6 amperes is in Table 402.5 which is referenced in 725.49(A) if the dimming circuits are Class 1 as they are defined in 725.2. If the Class 1 dimming circuit conductors do not carry more than 10 % (0.6 amperes for the 18 AWG) they do not need to be counted for applying the adjustment factor as stated in 725.51(B). Consider using more and likely smaller conduits to keep the adjustment factors low. Or better yet use metal wireways because the adjustment factors do not apply until more than 30 current carrying conductors are installed as stated in 376.22(B) but the conductor area cannot exceed 20% as stated in 376.22(A).

ABOUT CQD: The Code Question of the Day (CQD) is NECA and ELECTRICAL CONTRACTOR Magazine’s flagship National Electrical Code (NEC®) public forum for the industry, sponsored by EATON. The daily distribution of Q&A generates a lively dialogue and shares relative Code-based practical responses.

SUBMIT YOUR CODE QUESTION: Click here to submit a question to for inclusion in an upcoming edition of the Code Question of the Day, or email

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.

NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. NEIS can be purchased in the NECA Store in three formats: a printed or PDF download of a standard or, as an annual subscription service.

NECA SAFETY PRODUCTS & PUBLICATIONS: NECA produces electrical safety publications and products for the industry including jobsite safety guides, handbooks and resource kits. View a full listing of available resources and products »

IMPORTANT NOTICE: Unless the question requests a response based on a specific edition, all answers are based on the latest edition of NFPA 70® National Electrical Code®.

This correspondence is not a formal interpretation of the NEC® and any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, the NEC Correlating Committee any Code-making panel or other electrical technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services. 

UPDATE YOUR SUBSCRIPTION PREFERENCES: Subscribe or Unsubscribe from this list.