Branch-Circuit, Feeder and Service Calculations, Part II

By Charles R. Miller | Apr 15, 2006
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An essential part in the life of an electrician is performing load calculations. Determining what size conductors and overcurrent protective devices to install is something most electricians do on a daily basis. Specifications for calculating branch-circuit, feeder and service loads are in Article 220 of the National Electrical Code (NEC).

This article is divided into five parts. Part I covered general requirements for calculation methods (Electrical Contractor, March 2006). Branch-circuit load calculation methods are covered in Part II. Calculation methods for feeders and services are covered in Parts III and IV. Part V provides calculation methods for farms. This month’s column is the second part in a series dedicated to clarifying the load calculation requirements stipulated in Article 220.

Sections 220.3 and 220.5 are included in the general specifications in Article 220. New to the 2005 edition of the NEC is 220.3 and Table 220.3. Other articles contain load-calculation requirements in specialized appli-cations. These references are in Table 220.3. These requirements are in addition to, or modifications of, those within Article 220.

For example, while load-calculation requirements for electric welders are not listed in Article 220, they are in Article 630. Table 220.3 provides the specific sections in Article 630 pertaining to the ampacity calculations for electric welders. [630.11, 630.31]

Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347 and 600 volts shall be used. [220.5(A)] When performing calculations in Article 220, it is permissible to round a fraction of an ampere to the nearest integer or whole number. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped. [220.5(B)]

For example, after performing a load calculation, the ending result is 122.32 amperes. Because of 220.5(B), the fraction of an ampere that is less than 0.5 can be dropped. Because this fraction can be dropped, the new result is 122 amperes. If the ending result of the calculation is 122.5, the fraction must be rounded up. Because 0.5 must be rounded up to the next whole number, the new result is now 123 amperes.

Part II (220.10 through 220.18) of Article 220 covers branch- circuit load calculations. Although Part I provides general requirements for the article, 220.10 provides general requirements for calculating branch-circuit loads in Part II. Branch-circuit loads shall be calculated as shown in 220.12, 220.14, and 220.16. [220.10]

220.12 Lighting Loads for Specified Occupancies

The actual specifications for load calculations begin in 220.12. Table 220.12 provides unit loads for a variety of occupancies. Unit loads are provided in both volt-amperes per square foot and volt-amperes per square meter. Unit loads range from ¼ to 3½ volt-amperes per square foot. Some examples of general lighting unit loads include 1 volt-ampere per square foot for churches, 2 volt-amperes per square foot for hospitals, 3 volt-amperes per square foot for dwelling units, and 3½ volt-amperes per square foot for banks and office buildings (see Figure 1).

The floor area for each floor shall be calculated from the outside dimensions of the building, dwelling unit or other area involved. [220.12] Regardless of the wall thickness, the floor area for the occupancy must be calculated from the outside dimensions.

For example, the general lighting load is needed for a dwelling unit. The inside dimensions, measured from inside wall to inside wall, are 25 by 40 feet. Each exterior wall measures 6 inches deep. Because the exterior walls must be included, the calculated floor area is 1,066 square feet (26 x 41 = 1,066). Table 220.12 stipulates a minimum unit load of 3 volt-amperes per square foot for dwelling units. The minimum general lighting load for the dwelling in this example is 3,198 volt-amperes (1,066 x 3 = 3,198, see Figure 2).

As stated in the last sentence in 220.12, certain areas in dwelling units can be omitted from the calculated floor area. It is not necessary to include open porches, garages, or unused or unfinished spaces not adaptable for future use.

For example, the general lighting load is needed for a dwelling unit. The outside dimensions are 30 by 50 feet. This dwelling includes an open front porch that is within the boundaries of the outside dimensions. The front porch measures 5 by 8 feet. The total area, including the open porch, is 1,500 square feet (30 x 50 = 1,500). The porch’s area is 40 square feet (5 x 8 = 40). After deducting the area of the open porch, the calculated area is 1,460 square feet (1,500 – 40 = 1,460). After applying the minimum unit load per square foot, the minimum general lighting load for this dwelling is 4,380 volt-amperes (1,460 x 3 = 4,380, see Figure 3).

Calculating specialized areas separately is not permissible in one-family dwellings and individual dwelling units of two-family and multifamily dwellings. [Table 220.12]

Table 220.12 contains unit loads for areas within the main occupancy. These areas include assembly halls and auditoriums at 1 volt-ampere per square foot; halls, corridors, closets and stairways at ½ volt-ampere per square foot; and storage spaces at ¼ volt-ampere per square foot. It is permissible to calculate these specialized areas at lesser unit loads.

For example, a bank has a total area of 20,000 square feet. The following are part of the bank’s total area: hallways, corridors and stairways that total 1,600 square feet; storage spaces that measure 400 square feet; and a 3,000-square-foot assembly hall (see Figure 4).

First, calculate the specialized areas within the bank. The general lighting load for this bank’s hallways, corridors, and stairways is 800 volt-amperes (1,600 x 0.5 = 800). The general lighting load for storage spaces is 100 volt-amperes (400 x 0.25 = 100). The general lighting load for the assembly hall is 3,000 volt-amperes (3,000 x 1 = 3,000). The total area for hallways, corridors, stairways, storage spaces and the assembly hall is 5,000 square feet (1,600 + 400 + 3,000 = 5,000).

Next, calculate the general lighting load for the remaining area in the bank. After deducting the specialized areas in the bank, the remaining area is 15,000 square feet (20,000 – 5,000 = 15,000). The general lighting load for the main area of the bank is 52,500 volt-amperes (15,000 x 3.5 = 52,500).

Finally, add together all the general lighting loads in this bank. The general lighting load (not counting receptacle loads) for this bank is 56,400 volt- amperes (800 + 100 + 3,000 + 52,500 = 56,400, see Figure 5).

Had the specialized areas within the bank not been calculated separately, the general lighting load would have been 70,000 volt-amperes (20,000 x 3.5 = 70,000).

Calculating the general lighting load is one step in a multistep process when determining branch-circuit, feeder and service loads. Although the lighting load in this bank is a continuous load, it has not been multiplied by 125 percent.

There is no requirement in Article 220 to multiply continuous loads by 125 percent. When sizing conductors and overcurrent protection for feeders and services, the general lighting load will be combined with other steps. At that point, all continuous loads must be multiplied by 125 percent.

Next month’s column continues the discussion of load calculations.     EC

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 615.333-3336, [email protected], or


About The Author

Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.” He can be reached at 615.333.3336 and [email protected]. Connect with him on LinkedIn.





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