National Electrical Installation Standards

Standards as High as Your Own

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

Do you know all the ins and outs of the Code? NECA and Electrical Contractor magazine are pleased to present their daily online feature, “Code Question of the Day.”

  • ?
    Monday, November 20, 2017


    In Colorado is it required that the electrical panel for a residence be installed on the exterior of the building or can it be installed inside the residence?

    Jean Ouellette

  • ?
    Friday, November 17, 2017


    Example: A group of 3 vfd's of the same rating 1 feeder/branch ckt to supply all 3 How is ampacity determined? Assume pu vfd input current = 1 430.24: 1.25 x 1 + 1 + 1 = 3.25 or 430.122: 1.25 x (1 + 1 + 1) = 3.75

    Thank you, Arthur Brower



    Hey Arthur thanks for your question. Parts I through IX of Article 430 apply unless something in Part X changes those rules. Your first example is correct, but only if the input current rating is 1 ampere.

    Conductors supplying three adjustable speed drives would be sized at 125% of the largest plus 100% of the other two as stated in 430.24 using the drives rated input current as stated in 430.122(A). So if the adjustable speed drives are all the same size and have input current rating of 10 the minimum feeder conductor ampacity would be 32.5 amperes [10 x 1.25 = 12.5 + 10 + 10 = 32.5]

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 for more information. NEISÔ 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 to view or purchase NECA safety products.

CONTACT US: To submit a Code question, subscribe, or unsubscribe from this list, please e-mail: