Expanding electrical systems for existing residential, commercial and industrial facilities can be an expensive and difficult endeavor, unless the electrician, electrical contractor or design engineer can apply an optional calculation rule in Part IV of Article 220. Section 220.87 of the 2008 National Electrical Code (NEC) provides an optional calculation method that will often permit smaller feeder or service conductors or permit the use of existing services or feeders without upgrading them based on demand loads. There are conditions that must be satisfied before this optional calculation can be used.

The first condition that must be satisfied is the availability of the maximum peak load demand for a one-year time period. Most commercial and industrial facilities have a meter that monitors the peak demand for a minimum 15-minute time period. A 15-minute demand meter will register any peak demand that lasts for a 15 minute or longer time frame, charging the customer for any peak over a certain set value. Single-family, two-family and multi-family dwellings also may be subject to demand meters but may have one-hour peak demand meters installed.

Where the peak power usage lasts for an hour or more, the user receives a surcharge for that peak demand. The utility company keeps monthly records of the peak demand for each facility, and the load is usually available from the electrical utility company by requesting the electrical meter records for the particular facility in question. The month with the highest peak demand may vary depending on whether the facility is located in a part of the country where the weather is cold in the winter, hot in the summer or both.

The maximum peak demand is used to satisfy the second condition for the optional calculation by multiplying the peak demand by 125 percent. The existing peak demand calculated above is added to any new load calculated at 100 percent for a noncontinuous load and 125 percent for any continuous load that is being added to the feeder or service. Care must be taken to ensure the peak demand that is being used for this calculation is the demand reading during normal operation of the building. Taking a reading during a planned shutdown of the plant or facility would provide a lower than normal meter-demand rating and could result in a future overload of the facility electrical system.

The third condition that must be satisfied is that the existing overcurrent protection for the feeder or service must comply with 240.4 for protection of feeder and service conductors and 230.90 for protection of service conductors. A careful study of the overcurrent protective devices in the feeder circuit and at the service is necessary to ensure the proper sizing of the overcurrent-protective device compared to the conductors. Special care must also be taken to ensure that the proper ambient temperature correction factor and any adjustment factor in Table 310.15(B)(2)(a), based on the number of current-carrying conductors in the raceway or cable, has been applied to the conductors.

For where the one-year maximum demand data mentioned above is not available, an exception has been added to 220.87 that permits the calculated load to be based on the maximum demand (the measure of average power demand over a 15-minute period) that is continuously recorded during a minimum of a 30-day period. The measuring device for recording the power demand must be a recording ammeter or a power meter connected to the highest loaded phase of the feeder or service and must be based on the initial load at the start of the recording. The recording from the ammeter or the power meter must reflect the maximum demand of the feeder or service, so the recording is being taken when the building or space is occupied.

The recording must include the actual measurement of the larger heating or cooling equipment load and other loads that may be periodic due to seasonal or similar conditions. If an actual measurement cannot be made, a calculation of the seasonal loads, such as large boilers, large motors or similar loads, and the larger of the heating or cooling load must be made and then added to the peak demand load from the recording. Use of the larger of the heating or cooling load is based on 220.60 for noncoincident loads. Noncoincident loads are two or more loads that are unlikely to be in use simultaneously, so only the larger of the two loads must be used for the calculation.

Careful use of this optional calculation can save customers money and still provide a very safe and effective job.

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and mark.c.ode@us.ul.com.