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