National Electrical Installation Standards

Standards as High as Your Own

Monday, September 17, 2018


Re: CQD answer published Wednesday, September 5, 2018 - Load Calculations

Load calculations per 220.83 existing building. I'm curious how additional loads should added?

I have been told by chief electrical inspectors at least 3 different ways.

1. Additional loads at 40%

2. Additional loads at 100%

3. Additional loads at 125%

I understand the misunderstanding. I've been told to use the 40% rule because that's what we'll be used the next time then additional loan is added.. makes sense. So I understanding the two other additional load calculations are extrapolated from other parts of the code book to make it more difficult to add additional loads to the existing structure? Are any of these methods correct or following the code as it is written? I am aware of how the rest of the calculation is done the first 10000 volt amps at 100% and the remainder at 40%. But the NEC does not specifically say additional loads at 40% or what percentage it should be added in at? Any help in this matter would be greatly appreciated. And as always Charlie thank you for the code question of the day and all of your great work answering our questions!

Thanks, Oliver Allen



Hey Oliver thanks for your question and the compliment. The answer to your question is found in the first sentences of 220.83(A) and (B) - "The following percentages shall be used for existing and additional new loads." For example, if no air conditioning or electric space heating loads are supplied the first 8 kVA of existing and proposed new loads are taken at 100 % and the remainder at 40% as stated in 220.83(A). If air conditioning or electric space heating loads are supplied the percentages in 220.83(B) are used.

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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.

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