Specifications for calculating branch-circuit, feeder and service loads are in Article 220 of the National Electrical Code. This article is divided into four parts: I. General, II. Feeders and Services, III. Optional Calculations for Computing Feeder and Service Loads and IV. Method for Computing Farm Loads. Part I, though titled “General,” mostly contains specifications for computing branch circuits. After completing calculations in accordance with Part I, the results can either be used to size branch circuits, or can be added to other demand factors in Article 220 to size feeders or services. The computed load of a feeder or service must not be less than the load on the branch circuits supplied, as determined by Part I of Article 220, after any applicable demand factors permitted by Parts II, III or IV have been applied [220.10].
Part II includes a variety of demand factors for various types of occupancies. While general lighting demand factors are applicable in all occupancies, other demand factors are only applicable in certain occupancies. Some of these requirements only pertain to dwelling units and some only pertain to nondwelling units. Table 220.19 applies to cooking appliances in dwelling units, and also applies to household cooking appliances (rated over 1,750W) that are used in instructional programs. Last month’s In Focus began a new series covering cooking equipment calculations. This month, the discussion continues with a detailed explanation of cooking equipment demand factors.
If the rating of the range is higher than 8 3/4kW, but not higher than 12kW, the demand load can only be found in Column C. The numbers in Column C represent kilowatt ratings. For example, the demand load for one 12kW range is only 8kW. If the nameplate rating of the cooking appliance is more than 13/4kW to 83/4kW, the demand loads can be found in Columns A or B. The numbers in these columns are percentages and must be multiplied by the kilowatt rating to find the demand load. Column C is not just for equipment rated between 83/4kW and 12kW. Column C is for equipment not over 12kW. If the kilowatt rating is within the parameters of Column A, the demand load can be calculated from either Column A or C. If the kilowatt rating is within the parameters of Column B, the demand load can be calculated from either Column B or C. Permission to use either column is stated in the first sentence of the third note under Table 220.19. In previous editions of the Code the columns were arranged in a different order. The column for equipment not over 12kW rating is now located on the right side of the table.
If the equipment is rated from 31/2 to 83/4kW, calculate the demand load from Column B, and then compare that demand with the demand found in Column C. It is permissible to use the lowest demand load. For example, what is the service demand load for one 8kW range? First, calculate the demand load from Column B by multiplying 8kW by 80 percent (8 x .80 = 6.4kW). Next, find the demand load in Column C, compare the two, and select the lowest. The demand load for one unit in Column C is 8kW. The lowest demand load permitted for one 8kW range is 6.4kW (See Figure 1). Do not drop numbers to the right of the decimal point because each tenth of a kilowatt is equal to 100W. For example, .4kW is 400W.
Do not make the mistake of thinking that the lowest demand can always be found in one particular column; it depends on the number of units as well as the kilowatt ratings. For example, what is the lowest demand load for 12 7kW ranges? Since there is more than one appliance, find the total load before multiplying by the demand factor percent in Column B. If all appliances are the same rating, multiply the number of units by the kilowatt rating of each (12 x 7 = 84kW). Next, find the demand factor percent in Column B for 12 appliances (12 appliances = 32 percent). Now compute the demand load by multiplying the total load by the demand factor percent (84 x .32 = 26.88kW). Finally, find the demand load in Column C, compare the two and select the lowest. The demand in Column C for six appliances is 27kW. The lowest demand load for 12 7kW ranges is 26.88kW or 26,880W (See Figure 2).
By adding one more 7kW range to the last example, the column with the lowest demand load changes. For example, find the lowest demand load for 13 7kW ranges. Compute the demand load in Column B by multiplying the total load by 32 percent (13 x 7 x .32 = 29.12kW). Now, find the demand load in Column C, compare the two and select the lowest. The demand in Column C for 13 appliances is 28kW. The lowest demand load for 13 7kW ranges is 28kW or 28,000W (See Figure 3).
Where the ratings of the cooking appliances fall under both Columns A and B, apply the demand factors for each column separately and add the results together. This specification is stated in the last sentence of the third note under Table 220.19. For example, what is the service demand load for 10 3kW ovens and 10 5kW counter-mounted cooking appliances (cooktops)? The demand for the ovens, under Column A, is 14.7kW (10 x 3 x .49 = 14.7). The demand load for the cooktops, under Column B, is 17kW (10 x 5 x .34 = 17). Add the results together to find the total demand load. The demand for these household cooking appliances is 31.7kW or 31,700W (See Figure 4).
When applying the demand factors in Columns A and B, remember to compare the total demand to the demand found in Column C and select the lowest. For example, what is the lowest demand load for 20 3kW ovens and 20 6.5kW cooktops? The demand for the ovens, under Column A, is 21kW (20 x 3 x .35 = 21). The demand load for the cooktops, under Column B, is 36.4kW (20 x 6.5 x .28 = 36.4). The total demand from Columns A and B combined is 57.4kW (21 + 36.4 = 57.4). Since these appliances are rated less than 12kW each, find the demand load from Column C to see if it is lower than Columns A and B. The total number of appliances is 40 (20 ovens and 20 cooktops). Where the number of appliances is from 26 to 40, add 15 to the number of appliances and the sum is the kilowatt demand load. Fifteen added to 40 appliances is 55. Now compare the two columns and select the lowest demand. The combination of Columns A and B is 57.4kW, but the demand load from Column C is lower. The demand load for 20 3kW ovens and 20 6.5kW cooktops is 55kW or 55,000W (See Figure 5).
Next month’s In Focus, resuming with 220.19, will continue discussion of cooking equipment 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.