National Electrical Installation Standards

Standards as High as Your Own

Wednesday, August 28, 2019


Re: CQD answer published Thursday, August 22, 2019 - Demand Factor

Hello Mr. Trout, I read the questions and answers every day, and it is greatly informative. Regarding the question by Rick Gardner, "How can a main breaker of 1200 amps support feeders of 400 amps, 800 amps and 1000 amps (2200 total amps)?

I think there are many novice non-electricians, such as myself, who are interested in the theory of why this is allowed. I understand to an extent that the NEC has instituted certain calculations to determine proper sizing of breakers, and the proper sizing of wiring is straightforward enough that I understand that. What is confusing is that, contrary to the NEC, it is possible to run every light, the A/C, water heater, stove/oven, and operate three hair dryers simultaneously. I always thought the NEC erred on the side of ultimate safety, but in this case that doesn't seem to be true.  I suppose should such an unusual overload occur, the main breaker would trip, but I thought that was also a situation to try to avoid, as breakers have such limited number of on/off tripping before they're suspect to failure.

Thank you for your help.

Kevin Kayden


Hey Kevin thanks for your follow up question and the compliment. The concept this is based on is Demand Factor as defined in Article 100. It is extremely unlikely that everything in a building (total connected load) will be operating at the same time so demand factors can be used and are provided in Article 220. Many of these have been determined over time based on utility data and research studies. Single appliances such as household electric ranges can also have demand factors as shown in Table 220.55 because even at holidays or other occasions it is extremely unlikely that every burner and the oven is on at full setting. Some lighting load values are being changed for the 2020 NEC partially due to the use of more energy efficient lighting such as LEDs. If conductors were all required to be sized based on the total connected load they would be much larger without a real need. As you mention if the very unusual situation occurred and every electric load was on, the overcurrent protective device is expected to operate to deenergize circuit caused by the overcurrent condition.

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.