Article 220—Load Calculations
220.14 Other Loads—All Occupancies
Understanding how to perform load calculations is an important part of an electrician’s professional career. While knowing how to work load calculations could be the difference between pass and fail on a journeyman or master electrician’s exam, it is also valuable knowledge in everyday life. Conductor sizes must be known prior to installation. Ratings for overcurrent protective devices must be determined before installing panelboards and switchboards. Calculating branch-circuit, feeder and service loads in accordance with Article 220 of the National Electrical Code (NEC) is the first step. Next, conductor sizes and ratings for overcurrent protection can be calculated. For example, branch circuits for lighting loads must be calculated in accordance with Part II of Article 220; results can then be used in calculations to determine branch-circuit conductors and overcurrent protection. Provisions for branch-circuit conductors are covered in 210.19. Specifications for sizing branch-circuit overcurrent protection are in 210.20. This series explains and illustrates the load-calculation requirements in Article 220. Last month’s Code in Focus concluded with dwelling-occupancy receptacles 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). Requirements for all receptacle outlets—except for receptacles in dwellings, banks and office buildings—are covered in 220.14(I). Section 220.14(J) covers certain outlets in one-family, two-family and multifamily dwellings and in guest rooms or guest suites of hotels and motels. Receptacle loads for banks and office buildings are covered in 220.14(K).
In banks and office buildings, receptacle loads are selected based on the larger of two calculations or methods. The first method, covered in 220.14(K)(1), is the calculated load as determined by 220.14. With this method, the actual number of receptacle outlets and lengths of fixed multioutlet assemblies (if any) must be known. For example, 80 receptacle outlets and ten 5-foot sections of fixed multioutlet assembly will be installed in a bank. The receptacle outlets in each section of multioutlet assembly will not be used simultaneously. What is the total branch circuit load for receptacles? In accordance with 220.14(H)(1), each 5-foot section is calculated at 180 volt-amperes. Because there are ten 5-foot sections, the load for the fixed multioutlet assembly is 1,800 volt-amperes (10 x 180 = 1,800). In accordance with 220.14(H), each receptacle outlet must be calculated at 180 volt-amperes. The load for 80 receptacles is 14,400 volt-amperes (80 x 180 = 14,400). In this bank, the calculated load for known general-use receptacles is 16,200 volt-amperes (14,400 + 1,800 = 16,200) (see Figure 1).
The second method for banks and office buildings, covered in 220.14(K)(2), requires calculating a load from the total floor area. In accordance with 220.12, the floor area for each floor should be calculated from the outside dimensions of the building. Multiply the area by 1 volt-ampere per square foot (11 volt-amperes per square meter). For example, a bank has a calculated area of 9,300 square feet on the first floor and 7,200 square feet on the second floor. The actual number of receptacles is unknown. What is the branch-circuit load for receptacle outlets in this bank? First find the total floor area. The total area of this bank’s first and second floor is 16,500 (9,300 + 7,200 = 16,500). The formula requires 1 volt-ampere per square foot. Therefore, the branch-circuit receptacle load for this bank is 16,500 volt-amperes (see Figure 2).
If the floor area and the number of receptacle outlets are known, calculate by both methods in 220.14(K), compare the two, and select the larger as the general-use receptacle load. For example, 80 receptacle outlets and ten 5-foot sections of fixed multioutlet assembly will be installed in a bank. The receptacle outlets in each section of multioutlet assembly will not be used simultaneously. The calculated floor area for this bank totals 9,300 square feet on the first floor and 7,200 square feet on the second floor. What is the branch circuit load for general-use receptacles? Since the receptacles and fixed multioutlet assemblies are identical to those in Figure 1, the calculated load for known receptacles is 16,200 volt-amperes. Since the total square-foot area is the same as in Figure 2, the receptacle load for 1 volt-ampere per square foot is 16,500 volt-amperes. The minimum branch-circuit receptacle load is the larger of the two, 16,500 volt-amperes (see Figure 3).
Banks and office buildings are the only two occupancy types that must compare the receptacle load to 1 volt-ampere per square foot (11 volt-amperes per square meter). Since either method could yield the larger number, calculate by both methods and then select the larger load. For example, 130 receptacle outlets and 21 5-foot sections of fixed multioutlet assembly will be installed in an office building. The receptacle outlets in each section of multioutlet assembly will not be used simultaneously. The total area of this office building is 25,000 square feet. What is the total branch-circuit receptacle load? The load for 130 receptacles is 23,400 volt-amperes (130 x 180 = 23,400). The load for the fixed multioutlet assembly is 3,780 volt-amperes (21 x 180 = 3,780). The calculated load for known receptacles is 27,180 volt-amperes (23,400 + 3,780 = 27,180). Since the area of the office building is 25,000 square feet, the receptacle load at 1 volt-ampere per square foot is 25,000 volt-amperes. The minimum receptacle load is the larger of the two. The branch-circuit general-use receptacle load for this office building is 27,180 volt-amperes (see Figure 4).
The last specification in this section covers any remaining general-use receptacle outlets. Other outlets not covered in 220.14(A) through (K) should be calculated based on 180 volt-amperes per outlet [220.14(L)].
220.16 Loads for Additions to Existing Installations
Section 220.16 is divided into dwelling units and nondwellings. When the floor area exceeds 500 square feet (46.5 square meters) in structural additions to existing dwelling units or for previously unwired portions of existing dwellings, loads must be calculated in accordance with 220.12 and 220.14 [220.16(A)(1)] (see Figure 5). Installing new circuits or extending existing circuits in previously wired dwelling units must be calculated in accordance with either 220.12 or 220.14 as applicable [220.16(A)(2)].
Loads must also be calculated when adding new circuits or tying onto existing circuits in nondwelling occupancies. Loads for new circuits or extended circuits in other than dwelling units should be calculated in accordance with either 220.12 or 220.14 as applicable [220.16(B)].
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.