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What is NEC Code 220.87?

The National Electric Code (NEC) exists to protect both people and property from potential hazards. Before adding equipment to your electrical system, it is recommended that a 30-day load / power study is performed to protect you from potential system overload. The NEC Code 220.87 Determining Existing Loads specifies that the calculation of a feeder or service load for existing installations shall be permitted to use actual maximum demand to determine the existing load under all of the following conditions:

  1. The maximum demand data is available for a 1-year period.
  2. The maximum demand at 125% plus the new load does not exceed the feeder’s ampacity or service rating.
  3. The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.

What happens if the maximum demand data for a 1-year period is not available?

At first glance, it appears one would need to have a pre-installed data logger for the 1-year period of data. However, there is an exception to the rule that electrical contractors and engineers will appreciate. The rule states:

If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (a measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period. This 30-day period is to be measured using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording.

The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load and other loads that may be periodic in nature due to seasonal or similar conditions.

Where 1-year data is not available, a 30-day load study will suffice. Easy enough, right? If not, PMC is here to help with everything from application assistance to equipment purchases and rentals.

How can PMC help?

30 day power study

If you’re running short of meters, take a deep breath – PMC Rentals can help supplement your project needs big or small.  With a large selection of metering equipment for projects large and small, PMC Rentals can supplement your project needs and even help with some of the grunt work including:

Take a look at our Dent Instruments EliteProXC Power and Energy Logger. This simple yet elegant solution is a cost-effective power logger that can be rented for 2 days up to months. Flexible rental periods also allow PMC to meet all budget requirements. For more information about purchasing or renting equipment to meet your power / load study needs, contact us today.