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Evaluation of Electrical Feeder and Branch Circuit Loading: Phase 1

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This month we'd like to bring your attention to a Fire Protection Research Foundation report on the NFPA website, called Evaluation of Electrical Feeder and Branch Circuit Loading: Phase 1, authored by Tammy Gammon, PhD., P.E.

The Introduction:
Interest has been growing in recent years to investigate and clarify the degree to which the feeder and branch circuit load design requirements in NFPA 70, National Electrical Code® (NEC®) need to be adjusted based on the increasing pace of technological innovation along the entire span of the electrical power chain.

There are multiple factors driving this issue and supporting the need to address this topic. For example, today's Energy Codes are driving down the electrical load presented by end use equipment and thus load growth assumptions that justify spare capacity should be re-examined. In addition, larger than necessary transformers that supply power to feeder and branch circuits expose unnecessary flash hazard to electricians working on live equipment.

This report summarizes a Phase I effort to develop a data collection plan to provide statistically significant load data for a variety of occupancy and loading types to provide a technical basis for considering revisions to the feeder and branch circuit design requirements in the National Electrical Code®. This initial effort has an emphasis on general commercial (office) occupancies, and the deliverables provide a review of the literature, and clarify the key elements of a data collection plan in support of a potential second phase (not included in the scope of this effort).

Visit the NFPA website here, or click on the image, to read the report.

Comments
  • Great job!!! Hopefully it will serve as a basis (in conjunction with Phase II) to update the current demand loads used in the NEC for commercial buildings and consequently the corresponding sizing of service and feeders and associated raceways. I believe that the same criteria employed in the above study could be extended for the Residential sector (in a future Phase 3), mainly for hi-rise multifamily buildings, where there is a lot of demand factors used for the calculation of service and feeders sizing. Just a couple of suggestions 6.2.1, Table 21: Traditional Engineering Demand Factors for Building Loads. Actually this Table includes some loads where the term Demand Factors is correctly applicable and others where this term is not applicable. For example: For Lighting, the 1.25 factor is not a demand factor, rather it´s a circuit ampacity factor. Different is the case relative to Table 220.42 of the NEC where for some occupancies, there is a demand factor applicable to lighting loads. Likewise, the figures shown for Motors and Largest Motor are not demand factors. Rather, they are factors for determining the allowable circuit ampacity. The same criteria is applicable to Other loads. With respect to Receptacle, the demand factors are correct. In the Comments column, the NEC has a requirement for the calculation of the receptacle loads for Banks and Office Buildings, according to 220.14(K) 6.2.2, Branch Circuit Sizing and Protection It's worth to note that circuit breakers rated 800A and higher (mainly applicable to service and feeders) have the option to specify 100% continuously loaded, which could result in big savings due to the use of lower size conductors and associated raceways.

    Jacob MENDELOVICI  September 21 2018, 12:36 am EDT

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