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Wednesday, August 28, 2019
Question:
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
A
Answer:
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.