Code Q&A: Calculating Feeder or Service Loads

Aug. 8, 2014
NEC requirements for calculating feeder or service loads

Q. How can you determine the feeder or service load for an existing installation? (Answer based on 2011 NEC.)

Find the Answer

A. The calculation of a feeder or service load for existing installations can be based on the actual maximum demand under all of the following conditions [220.87]:

1)  The maximum demand data for one year [220.87].

Exception: If the maximum demand data for one year isn’t available, the maximum power demand over a 15-min. period continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase, based on the initial loading at the start of the recording is permitted. The recording must be taken when the building or space is occupied based on the larger of the heating or cooling equipment load.

2) The maximum demand at 125% plus the new load doesn't exceed the ampacity of the feeder or rating of the service.

3) The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.

Let’s clarify these points by analyzing an example situation.

What would the service load calculation be for an existing installation, where the greatest kWh usage consumed for any 15-min. period was at the rate of 23.66kW per hour? The service is 120/240V single phase, 200A, and the owner wants to add a 50A load.

As was stated, the maximum demand recorded for the year was 23.66kW. To find the amperage, divide the demand wattage by the service voltage.
23,660W ÷ 240V = 98.58A

Next, take this amperage and multiply it by 125% [220.87(2)].
99A x 125% = 123A

Then, add the additional new load.
123A + 50A = 173A

Since the existing service is 200A, you have room to add the additional load to this service [220.87(2) and (3)].

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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