Branch-Circuit, Feeder and Service Calculations, Part LX

The National Electrical Code (NEC) contains nine chapters. The second chapter, Wiring and Protection, is one of four that apply generally to all electrical installations. Some of the articles in Chapter 2 include Article 200—Use and Identification of Grounded Conductors, Article 210—Branch Circuits, Article 230—Services, Article 240—Overcurrent Protection and Article 250—Grounding and Bonding.

Article 220—Branch-Circuit, Feeder, and Service Calculations provides requirements for calculating branch-circuit, feeder and service loads. Article 220 is divided into five parts. Part I contains general requirements for calculation methods. Part II covers calculation methods for branch-circuit loads. Part III contains feeder and service load-calculation methods. Although not official, Part III is sometimes referred to as the standard method. Part IV contains optional feeder and service load-calculation provisions. The optional methods are alternative load-calculation procedures for one-family dwellings, existing dwelling units, multifamily dwellings, two-family dwellings (where supplied by a single feeder), schools, existing installations and new restaurants. Sections in Part IV include 220.80 through 220.88. The last part in Article 220, Part V, contains calculation methods for farms.

Last month’s column concluded by covering feeder and service load calculations for schools in accordance with the optional method. This month, the discussion continues with provisions for determining existing loads.

220.87 Determining existing loads
While most of the load calculation provisions in Article 220 are for new installations, there are some that pertain to existing installations. Section 220.16 covers loads for additions to existing installations. This section is divided into dwelling units and other than dwelling units. Part III (220.80 through 220.88) of Article 220 covers optional feeder and service load calculations. Article 220.83 covers optional method load calculation specifications for an existing dwelling unit. Also in Part III is a section for determining existing loads, regardless of the type of occupancy. In accordance with 220.87, when calculating a feeder or service load for existing installations, it shall be permissible to use actual maximum demand to determine the existing load where meeting three conditions. The first condition states that the maximum demand data must be available for a one-year period. If the electric utility company has installed a peak-demand meter at the service and it has been in place for more than a year, the maximum demand data should be made available. For example, a business owner wants to add equipment to an existing building. The new equipment will not replace any of the existing equipment. The existing service has a rating of 1,200 amperes (A). A peak-demand meter has been connected to the service for more than a year. The maximum demand data from the peak-demand meter shows a demand of 743A. In this example, the first condition of 220.87 has been met (see Figure 1).

It is not always possible to obtain the data required to meet the first condition. If the first condition in 220.87 cannot be met, there is an exception. The calculated load can be based on the maximum demand of the feeder or service by using a recording ammeter or power meter. This maximum demand must be continuously recorded over a minimum 30-day period and the instrument must provide an average measurement of amperes or kilovolt-amperes (kVA) over a 15-minute period. The recording ammeter or power meter must be connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. Ensure the meter is recording the demand of the feeder or service when the building or space is occupied so as to measure the maximum demand. The maximum demand 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.

Once the maximum demand is known, multiply the amperes or kilovolt-amperes by 125 percent. The maximum demand at 125 percent must now be added to the new loads that will be installed on the feeder or service [220.87(2)]. The new loads must be calculated in accordance with standard load-calculation procedures. Make sure to include the noncontinuous loads at 100 percent and the continuous loads at 125 percent. If the calculations were performed using kilovolt-amperes, convert to amperes by dividing the kilovolt-amperes by the total voltage. Once amperes are known, check the rating of the existing feeder or service. The rating of the existing feeder or service must be equal to or higher than the minimum required ampere rating calculated in accordance with the first two conditions in 220.87. For example, the 1,200-ampere service in Figure 1 had a maximum demand of 743A. The new equipment that will be added to that service has a calculated load (in accordance with standard load calculation procedures) of 251A. Does this existing service have an ampere rating high enough for the existing loads and the new loads? First, multiply the maximum demand from the first condition (or the exception to the first condition) by 125 percent. The maximum demand at 125 percent is 929A (743 125 percent = 928.75 = 929). Now add the new loads that will be installed on the feeder or service. The maximum demand at 125 percent plus the new loads is 1,180A (929 + 251 = 1,180). Since the existing service is rated 1,200A, this installation will be Code-compliant (see Figure 2).

The third and last condition states that the feeder must have overcurrent protection in accordance with 240.4, and the service must have overload protection in accordance with 230.90. Section 240.4 covers the protection of conductors. Unless otherwise permitted or required in 240.4(A) through (G), conductors shall be protected against overcurrents in accordance with their ampacities as specified in 310.15 [240.4]. Section 240.4 does not apply to flexible cords, flexible cables or fixture wires. Section 230.90 covers overload protection of service conductors. Each ungrounded-service conductor shall have overload protection. Such protection shall be provided by an overcurrent device in series with each ungrounded-service conductor that has a rating or setting not higher than the allowable ampacity of the conductor. For example, the existing 1,200A service, including the new loads, in Figures 1 and 2 met the first two conditions. The service conductors consist of four parallel sets of 350 kcmil XHHW copper conductors. Each parallel set of conductors is in a separate raceway. Does the existing service in this example meet the third condition in 220.87? As required by 240.4(C), the ampacity of the conductors must be equal to or greater than the rating of the overcurrent device. The ampacity rating of one 350 kcmil XHHW copper conductor at 75°C is 310A [Table 310.15(B)(16)]. The combined ampacity of four 350 kcmil XHHW copper conductors in parallel is 1,240 amperes (350 4 = 1,240). Since the overcurrent device is rated 1,200A, this installation is Code-compliant (see Figure 3).

Next month’s article continues the discussion of optional feeder and service load calculations.

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, and

About the Author

Charles R. Miller

Code Contributor
Charles R. Miller, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615....

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