The National Electrical Code (NEC) is divided into nine chapters. Chapter 2, titled Wiring and Protection, contains 10 articles. Article 220 contains requirements for calculating branch-circuit, feeder and service loads. Requirements in Article 220 are divided into five parts. Part I covers general requirements for calculation procedures. Part II provides calculation provisions for branch circuits. Feeder and service calculation requirements (sometimes referred to as the standard method) are in Part III. Optional feeder and service load calculations are specified in Part IV. Farm loads must be calculated in accordance with Part V.
Results from load calculations in Article 220, along with provisions from other articles, can be used to find conductor sizes and ampere ratings for overcurrent protection. For example, results from calculations in Parts III, IV and V of Article 220 are used with the provision in 215.2(A)(1) to find the minimum feeder conductor size. Likewise, the results along with provisions in 215.3 are used to find the minimum size fuse or breaker permitted for feeders. Last month’s article concluded by covering track and show-window lighting loads in 220.43. This month, the focus shifts to demand factors for nondwelling loads as specified in 220.44.
Although branch-circuit receptacle loads calculations specified in Part II of Article 220 must be included in feeder and service-load calculations, it may not be necessary to include the total calculated receptacle load. It is permissible to derate or reduce certain receptacle loads in nondwelling occupancies. Receptacle loads calculated in accordance with 220.14(H) and (I) shall be permitted to be made subject to the demand factors given in Table 220.42 or Table 220.44 [220.44]. In addition to using Table 220.42 to reduce certain lighting loads, it also is permissible to reduce receptacle loads with the demand factors found in this table. In accordance with 220.44, the receptacle load can be included with lighting load before applying demand factors in Table 220.42. Table 220.44 can be used only to reduce receptacle loads greater than 10,000 volt-amperes. Applying this table’s demand factors to lighting loads is not permissible (see Figure 1).
Two options are available for receptacle loads calculated in accordance with 220.14(H) and 220.14(I). If the occupancy is one of the types (other than dwelling units) listed in Table 220.42, the receptacle load could be added to the general lighting load and made subject to the demand factors. For example, the general lighting load, before applying demand factors, for hospital patient rooms is 100,000 volt-amperes. The receptacle branch-circuit load, calculated in accordance with 220.14(H) and (I), is 144,000 volt-amperes.
What is the lighting and receptacle load after demand factors? Start by adding the receptacle load to the lighting load (100,000 + 144,000 = 244,000). Multiply the first 50,000 by 40 percent (50,000 × 40% = 20,000). This leaves 194,000 volt-amperes (244,000 – 50,000 = 194,000). Multiply the remaining 194,000 volt-amperes by 20 percent (194,000 × 20% = 38,800). The lighting and receptacle load, after applying Table 220.42 demand factors, is 58,800 volt-amperes (20,000 + 38,800 = 58,800) (see Figure 2).
The second option available for receptacle loads, calculated in accordance with 220.14(H) and (I), is to apply the demand factors listed in Table 220.42. For example, instead of adding the hospital receptacle load (from the previous example) to the lighting load and applying Table 220.42 demand factors, multiply the receptacle load by the demand factors listed in Table 220.44. The receptacle branch-circuit load from the previous example is 144,000 volt-amperes.
What is the receptacle load after applying the demand factors in Table 220.44? The first 10 kVA (10,000 volt-amperes) is multiplied by 100 percent; therefore, it remains at 10,000. After subtracting 10,000 from the total receptacle load, the remainder is 134,000 volt-amperes (144,000 – 10,000 = 134,000). Multiply the remainder by 50 percent (134,000 × 50% = 67,000). The receptacle load, after applying Table 220.44 demand factors, is 77,000 volt-amperes (10,000 + 67,000 = 77,000) (see Figure 3).
If the occupancy is not one of the types listed in Table 220.42, there is only one option available for reducing the receptacle load. A store has a total calculated multioutlet assembly and receptacle load of 18,000 volt-amperes. What is this store’s receptacle load after applying demand factors? The first 10,000 volt-amperes remain at 10,000. After subtracting 10,000 from the total receptacle load, the remainder is 8,000 volt-amperes (18,000 – 10,000 = 8,000). Multiply the remainder by 50 percent (8,000 × 50% = 4,000). The receptacle load, after applying Table 220.44 demand factors, is 14,000 volt-amperes (10,000 + 4,000 = 14,000) (see Figure 4).
If the number of receptacles is known but the total volt-amperes are not known, multiply the number of receptacles by 180 volt-amperes before applying any demand factors. For example, a store will have a total of 80 duplex receptacles. What is the receptacle load after demand factors? First, multiply the 80 receptacles by 180 volt-amperes. This store has a receptacle load before applying demand factors of 14,400 volt-amperes (80 × 180 = 14,400). The first 10 kVA (10,000 volt-amperes) is multiplied by 100 percent; therefore, it remains at 10,000. After subtracting 10,000 from the total receptacle load, the remainder is 4,400 volt-amperes (14,400 – 10,000 = 4,400). Multiply the remainder by 50 percent (4,400 × 50% = 2,200). The receptacle load, after applying Table 220.44 demand factors, is 12,200 volt-amperes (10,000 + 2200 = 12,200) (see Figure 5).
If the occupancy is not one of the types listed in Table 220.42 and the receptacle load is not greater than 10,000 volt-amperes, the receptacle load will remain at 100 percent. For example, a store has a total of 55 duplex receptacles. What is the receptacle load after demand factors? First, multiply the 55 receptacles by 180 volt-amperes. This store has a receptacle load before applying demand factors of 9,900 volt-amperes (55 × 180 = 9,900). Since this load is not greater than 10,000 volt-amperes, it will remain at 100 percent. The receptacle load for the store in this example is 9,900 volt-amperes (see Figure 6).
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, charles@charlesRmiller.com or www.charlesRmiller.com.