Article 220 of the National Electrical Code (NEC) contains requirements for calculating branch-circuit, feeder and service loads. Knowing where to find load-calculation requirements is only the first step in determining conductor sizes and ratings for overcurrent protective devices. While some provisions in Article 220 are understandable and self-explanatory, others are somewhat difficult to comprehend and require some effort and research. One handy tool can be found in the back of the Code. Annex D contains examples of load calculations. It is not a part of the requirements of the NEC, but is included for informational purposes only (prior to the 1999 edition of the NEC, these examples were in Chapter 9).
Part I of Article 220 contains general requirements for calculation procedures. Part II provides calculation provisions for branch circuits. Feeder and service calculation requirements are in Parts III and IV. And finally, Part V contains requirements for farms. Knowing what to do with the calculation results is another step. Provisions from other articles will provide additional specifications in determining conductor sizes and overcurrent protective device ratings. To determine conductor sizes for branch circuits, refer to 210.19. Specifications for branch-circuit overcurrent protection are in 210.20. Last month’s column covered receptacle outlets in 220.14(I). This month, the discussion continues with more requirements for general-use receptacles and outlets not used for general illumination.
Load calculations for receptacle outlets are covered in 220.14(I), (J) and (K). Section 220.14(I) covers all receptacle outlets except for receptacles in dwellings, banks and office buildings. In one-family, two-family and multifamily dwellings, the outlets specified in 220.14(J)(1) through (J)(3) are included in the general lighting-load calculations of 220.12. No additional load calculation is required for receptacle outlets covered in 220.14(J).
All general-use receptacle outlets of 15- and 20-ampere ratings installed in dwellings are included as a part of the general lighting-load calculations. For example, a living room in a dwelling has receptacles installed in accordance with 210.52. Because of the room’s dimensions and layout, six receptacle outlets are required. Unlike receptacles installed in non-dwelling occupancies that must be calculated at 180 volt-amperes each, it is not necessary to include any additional load for these dwelling receptacles (see Figure 1). The general lighting load must be calculated in accordance with 220.12 and Table 220.12.
The outlets specified in 220.14(J)(1) through (J)(3) include the following:
■ All general-use outlets of 20-ampere rating or less, including receptacles connected to 20-ampere bathroom branch circuits as specified in 210.11(C)(3).
■ Receptacles installed outdoors, as specified in 210.52(E), and basement and garage receptacle outlets, as specified in 210.52(G). [220.14(J)(1)]
■ Lighting outlets in dwelling units, as specified in 210.70(A), and guest rooms or guest suites in hotels, motels or similar occupancies, as specified in 210.70(B). [220.14(J)(2)]
■ These receptacle and lighting outlets are included in the general lighting-load calculation of 220.12. [220.14(J)(3)]
The load calculation does not change when the number of receptacle outlets, specified in 220.14(J)(1) through (J)(3), is more than the required minimum. For example, another living room, identical to the one in Figure 1, contains twice the number of required receptacle outlets. The load for this living room is calculated exactly the same as the load for the living room in Figure 1 (see Figure 2).
The minimum number of 15- and 20-ampere branch circuits in dwellings for lighting and general-use receptacle outlets depends on the floor area. After finding the general lighting load in accordance with Table 220.12, divide by the voltage of a general-purpose branch circuit (typically 120 volts). The result (in amperes) can then be divided by either 15- or 20-ampere branch circuits. For example, a dwelling unit has outside dimensions of 55 feet by 35 feet. To find the general lighting load, the outside dimensions must be multiplied by a unit load of 3 volt-amperes per square foot. The minimum lighting load for this dwelling is 5,775 volt-amperes (55 x 35 x 3 = 5,775). With a voltage of 120/240, the general lighting load is 48 amperes (5,775 ÷ 120 = 48). To find the minimum number of 15-amperes, 2-wire circuits, divide 48 by 15 (48 ÷ 15 = 3.2 = 4). Because this result is three and two-tenths circuits, the number must be rounded up. At least four 15-amperes, 2-wire circuits are required (see Figure 3).
Calculate the minimum number of 20-ampere general-purpose branch circuits the same. For example, a dwelling unit has outside dimensions of 55 feet by 35 feet. Since this dwelling is the same as the one in Figure 3, the general lighting load is the same (48 amperes). To find the minimum number of 20-amperes, 2-wire circuits for lighting and general-use receptacles, divide 48 by 20 (48 ÷ 20 = 2.4 = 3). The minimum number of 20-ampere, 2-wire general-purpose branch circuits for lighting and receptacles in this dwelling is three. Be careful when calculating the minimum number of 20-ampere branch circuits. At least one 20-ampere laundry circuit and at least two 20-ampere small-appliance branch circuits are required. While the bathroom branch circuit supplying receptacle outlets must be rated 20 amperes, no additional load calculation is required (see Figure 4). Load calculations for small-appliance and laundry branch circuits are covered in 220.52 and will be discussed later in this load-calculation series. Caution is advised when installing receptacles on branch circuits in dwellings. Local jurisdictions could limit the number of receptacle outlets permitted on 15- and 20-ampere, general-purpose branch circuits.
Although not mentioned before Part VIII of Branch-Circuit, Feeder and Service Calculations, there are areas in another occupancy covered in 220.14(J) besides one-family, two-family and multifamily dwellings. In guest rooms or guest suites of hotels and motels, the outlets specified in 220.14(J)(1), (J)(2), and (J)(3) are also included in the general lighting-load calculations of 220.12. Therefore, no additional load calculation is required for these receptacle outlets also (see Figure 5). The unit load for general lighting in for hotels and motels is 2 volt-amperes per square foot (Table 220.12).
Next month’s column continues the discussion of load calculations. EC
MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code and NFPA’s Electrical Reference. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at 615.333.3336, or via e-mail at charles@charlesRmiller.com.