Article 220 of the National Electrical Code (NEC) provides requirements for calculating branch-circuit, feeder and service loads. An essential part of an electrician’s professional career is performing load calculations. Knowing how to calculate loads in accordance with the NEC is only the first step. Knowing what to do with the loads is the next step in the practical safeguarding of persons and property from the hazards arising from the use of electricity. To find minimum conductor sizes and maximum ratings of overcurrent protection, the results from Article 220 must be inserted into calculations from other articles.
For example, results from specifications in Part II of Article 220 inserted into calculations from Article 210 will provide sizing for branch-circuit conductors and overcurrent protection. Last month’s column concluded by covering luminaires (lighting fixtures) in 220.14(D). This month, the discussion continues with more requirements for general-use receptacles and outlets not used for general illumination.
Outlets for heavy-duty lampholders must be calculated at a minimum of 600 volt-amperes [220.14(E)]. For example, a store will be installing 16 outlets for heavy-duty lampholders. The heavy-duty lampholders will be supplied by 20-ampere, 120-volt branch circuits and they will remain energized for more than three hours. How many branch circuits are required for these 16 heavy-duty lampholders?
In accordance with 220.14(E), each outlet for heavy-duty lampholder must be calculated at a minimum of 600 volt-amperes. Because the load is continuous, multiply 600 volt-amperes by 125 percent (600 x 125 percent = 750 volt-amperes). A 20-ampere, 120-volt branch circuit will carry 2,400 volt-amperes (20 x 120 = 2,400). The maximum number permitted on a 20-ampere branch circuit is three (2,400 ÷ 750 = 3.2 = 3). Six 20-ampere branch circuits are required for 16 outlets for heavy-duty lampholders (16 ÷ 3 = 5.3 = 6, see Figure 1).
Finding the total volt-amperes of all the heavy-duty lampholders first and then dividing by the volt-amperes per 20-ampere branch circuit can lead to an incorrect answer. For example, by multiplying the 16 heavy-duty lampholders by 600 volt-amperes, the result is 9,600 volt-amperes (16 x 600 = 9,600 volt-amperes). Because the load is continuous, multiply 9,600 volt-amperes by 125 percent (9,600 x 125 percent = 12,000 volt-amperes).
Dividing the continuous load by the volt-amperes per 20-ampere branch circuit will show that only five circuits are required for 16 heavy-duty lampholders (12,000 ÷ 2,400 = 5). Performing the calculation in this manner provides an incorrect answer. This calculation shows three and two-tenths (3.2) heavy-duty lampholders will be installed on each branch circuit (16 ÷ 5 = 3.2, see Figure 2).
Sign and outline lighting outlets must be calculated at a minimum of 1,200 volt-amperes for each required branch circuit specified in 600.5(A) [220.14(F)]. Article 600 covers the installation of conductors and equipment for electric signs and outline lighting. At least one sign or outline lighting outlet is required at each entrance to each tenant space in each commercial building and commercial occupancy accessible to pedestrians [600.5(A)].
Service hallways and corridors are not considered accessible to pedestrians. For example, a retail store in a shopping mall has two entrances from the mall corridor and one entrance from a service hallway. What is the calculated load for the three entrances? Although there are three entrances, only two are accessible to pedestrians. The entrance to the service hallway is not considered accessible to pedestrians, and therefore can be omitted. The minimum calculated load for this example is 2,400 volt-amperes (1,200 x 2 = 2,400 volt-amperes, see Figure 3).
An outlet for electric signs and outline lighting must be supplied by a branch circuit that supplies no other load, also the circuit must be rated no less than 20 amperes [600.5(A)]. In the previous example, because there are two entrances, at least two branch circuits having a rating of at least 20 amperes are required. The 20-ampere branch circuits feeding the sign or outline lighting system must be dedicated to the sign or outline lighting system it supplies (see Figure 4).
Branch-circuit loads for show windows must be calculated in accordance with either of the following: 1. the unit load per outlet as required in other provisions of this section, or 2. at 200 volt-amperes per 1 foot (300 mm) of show window [220.14(G)]. There is an option for calculating branch-circuit loads for show windows. Unlike the requirement in 220.14(K), this section does not specify that the calculated load must be the larger of 1 or 2. as listed above. If the load is based on the unit load per receptacle outlet, it must be calculated at not less than 180 volt-amperes for each single or each multiple receptacle on one yoke [220.14(I)]. For example, a show window in a retail store has three duplex receptacles installed directly above the show window.
What is the branch-circuit load for the three receptacles? Since the linear measurement of the show window is not given, there is only one method for calculating this load. Multiply three receptacle outlets by 180 volt-amperes. The branch- circuit load for this show window is 540 volt-amperes (3 x 180 = 540, see Figure 5).
Article 210 contains specifications for required receptacle outlets above show windows. As specified in 210.62, at least one receptacle outlet must be installed directly above a show window for each 12 linear feet (3.7 linear m) or major fraction thereof of show window area measured horizontally at its maximum width. Although a receptacle is required if the show window measures 12 linear feet, a receptacle outlet could be required even if the show window is less than 12 feet. For example, a small retail store has a show window measuring 6 feet across. Because 6 linear feet (1.8 linear m) is a major fraction of 12 linear feet, at least one receptacle outlet is required (see Figure 6).
The branch-circuit load for a show window can also be calculated at 200 volt-amperes per 1 foot (300 mm) of show window. For example, what is the branch-circuit load calculation for a show window measuring 8 feet across? Multiply 8 feet by 200 volt-amperes. The load calculation for this show window is 1,600 volt-amperes (8 x 200 = 1,600, see Figure 7).
Next month’s Code in Focus 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.
Figures mentioned in article available upon request.