The National Electrical Code (NEC)began as a 54-page document in 1897. In contrast, the 2023 NEC has 911 pages. In the 126 years in between, a huge amount of expertise and knowledge has been used to create the world’s most awesome electrical document.

The NEC provides a prescriptive method and inspection process for ensuring that our electrical systems are among the safest installations in the world. The purpose of the NEC is stated in 90.2(A) as the practical safeguarding of persons and property from hazards arising from the use of electricity. It also states that the NEC is not intended as a design specification or instruction manual for untrained persons. Anyone familiar with the NEC can certainly understand it takes many years to become familiar with the Code’s intricacies. I firmly believe that the NEC can be considered the most complex of all the building codes and standards, but then, having been studying it for many years, I am rather opinionated on that subject.

Much of the information in the NEC originates from local electrical issues which, when proven useful and accurate, is eventually inserted into the Code. Such is the case of 220.87 when determining the calculation of feeder and service loads for existing installations.

The concept for this section actually originated in the local Phoenix Electrical Code around 1975, was submitted to the NEC in 1978 and was finally accepted into the 1981 NEC. I was a member of the 1975–1989 Phoenix Electrical Code Panel and used this local amendment for quite a few service and feeder calculations before the method was inserted into the NEC.

Keep three conditions in mind

The introductory text in 220.87 states that the calculation of a feeder or service load for existing installations is permitted to use actual maximum demand to determine the existing load, but only where complying with three conditions. The first condition is to have access to the maximum demand data for a 1-year period. This demand data is usually available from the serving electrical utility company. Electric utility companies have become much more restrictive in releasing this information, so it might require written permission from the property owner.

Where this demand data is not available at all or there is not a 1-year time frame, an exception has been added to the first condition. This exception is fairly long, but is very descriptive in how to determine a demand value: “If the maximum demand data for a 1-year period is not available, the calculated load is permitted to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period 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 must reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and must include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that might be periodic in nature due to seasonal or similar conditions. This exception is not permitted if the feeder or service has a renewable energy system (i.e., solar photovoltaic or wind electric) or employs any form of peak load shaving.” 

A recording ammeter, data analyzer or data logger can be used as a means of providing the authority having jurisdiction with a written or electronic record of this demand. A key issue here is to ensure that the period of time required, such as the 1-year or 30-day periods, actually reflects the normal work time, and not holidays or long time periods where the work is not consistent with normal load.

The second condition requires the resulting maximum demand, provided by the utility company or the 30-day recorded demand, be calculated at 125% plus any new load. The new load must be determined as continuous at 125% and noncontinuous at 100%, and that calculated load cannot exceed the ampacity of the feeder or the rating of the service. This 25% extra load value added to the existing demand load provides a buffer to any abnormal existing load peaks that might be felt.

The third condition requires feeder overcurrent protection in accordance with 240.4 and that the service have overload protection based on 230.90.

The additional load to an existing service must not overload the current feeders and the conductors supplying the existing service. This is a great addition to the NEC, keeping costs down for existing systems.