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Branch-Circuit, Feeder and Service Calculations, Part XXXVII
by
Charles R. Miller
Published: March 2009
Article 220—Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
It is essential for the professional electrician to know where to find load-calculation requirements and understand how to perform the calculations in accordance with the National Electrical Code. Loads must be calculated before selecting conductors, overcurrent protective devices, switchboards or panelboards, and even raceways. Understanding and fol-lowing load-calculation requirements is necessary before sizing branch circuits, feeders and services.
Article 220 provides requirements for calculating branch-circuit, feeder and service loads. Article 220 is divided into five parts. Part I provides general requirements for calculation methods. Branch-circuit load-calculation methods are in Part II. Feeder and service calcu-lation methods are in Part III. Although not official, Part III is commonly referred to as the “standard load-calculation method.” Part IV of Article 220 also provides calculation methods for feeders and services. The calculation method covered in this part is an alternative calculation method to the method in Part III. Part IV provides optional calculation methods for a single dwelling unit, an existing dwell-ing unit, a multifamily dwelling, two dwelling units, a school, an existing installation and a new restaurant. Part V provides calculation methods for farms.
Last month’s column covered electric cooking equipment in 220.55. This month, the discussion continues with calculating loads for electric ranges and other cooking appliances in dwelling units.
While the demand factors in Table 220.55 cannot be applied to commercial kitchen equipment, they can be applied to household cooking appliances located in nondwelling occupancies under one condition. As specified in the last note (Note 5) under Table 220.55, this table also applies to household cooking appliances rated more than 1¾ kW and used in instructional programs. For example, a high school classroom will be converted into a culinary arts classroom. Five 12-kW, 240-volt household electric ranges will be installed. A separate panelboard will be installed to supply the ranges. What is the kilowatt-demand load for the feeder and panelboard supplying power to these ranges? From the information provided, we know the ranges will be used in instructional programs. We also know the ranges are household electric ranges.
Although the ranges will be installed in a nondwelling occupancy, Note 5 under Table 220.55 provides permission to apply the Table 220.55 demand factors. The feeder and panelboard could be sized on 100 percent of the equipment’s nameplate ratings, which is 60 kW, but this is not necessary. Look in the left column of Table 220.55 for five appliances.
Because of the range ratings, the maximum demand is in Column C. The maximum demand required for five 12-kW, 240-volt house-hold electric ranges is 20 kW. Therefore, the feeder and panelboard must be capable of carrying a load of at least 20 kW (see Figure 1). Household electric cooking equipment installed in other than dwellings not used in instructional programs must be calculated in accor-dance with 220.56 and Table 220.56.
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Table 220.55 shall not apply to commercial electric ranges used in instructional programs. For example, a high school classroom will be converted into a culinary arts classroom. Five 12-kW, 240-volt commercial electric ranges will be installed. A separate panelboard will be installed to supply the ranges. What is the kilowatt-demand load for the feeder and panelboard supplying power to these ranges? From the information provided, we know the ranges will be used in instructional programs. We also know that these ranges are not household electric ranges.
Therefore, it is not permissible to apply the demand factors from Table 220.55. Demand factors for commercial electric cooking equipment are in Table 220.56. Look in the left column of Table 220.56 for five units of equipment and follow across for the demand factor. The demand factor percent for five units is 70 percent. These five ranges have a total rating of 60 kW (5 × 12 = 60 kW). The load after applying the demand factor is 42 kW (60 × 70% = 42 kW). The feeder and panelboard supplying power to these five commercial ranges must be capable of carrying a load of at least 42 kW (see Figure 2).
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220.56 Kitchen Equipment—Other Than Dwelling Unit(s)
As previously discussed, Table 220.55 is not the only table containing demand factors for electric ranges and cooking appliances. In accordance with 220.56, it is permissible to calculate the load for commercial electric cooking equipment, dishwasher booster heaters, water heaters, and other kitchen equipment in accordance with Table 220.56. Unlike Table 220.55, this table is not just for ranges and cooking appliances. Although commercial electric cooking equipment, dishwasher booster heaters and water heaters are specifically mentioned, these demand factors also can be applied to all equipment that has either thermostatic control or intermittent use as kitchen equipment. For example, what is the demand load for a restaurant with two 14-kW stoves, a 6-kW oven, a 5-kW dishwasher, a 3-kW booster heater, and a 4-kW food waste disposer? The total number of units is six. The demand factor percent in Table 220.56 for six units is 65 percent. The total rating of the equipment is of 46 kW (14 + 14 + 6 + 5 + 3 + 4 = 46 kW). The load after applying the Table 220.56 demand factor is 29.9 kW (46 × 65% = 29.9 kW). The demand load for this restaurant is 29.9 kW or 29,900 watts (see Figure 3). The demand factors in Table 220.56 shall not be applied to space-heating, ventilating or air conditioning equipment.
After finding the demand load for commercial kitchen equipment, one additional step must be followed. The last paragraph of 220.56 states the calculated load for the feeder or service must not be less than the sum of the largest two kitchen equipment loads. Af-ter finding the demand load, the additional step is comparing the demand load to the sum of the largest two kitchen equipment loads. The absolute minimum rating is the total combined rating of the largest two kitchen equipment loads. In the last example, the demand load is 29.9 kW. The sum of the largest two kitchen equipment loads is 28 kW (14 + 14 = 28). Since the demand load of 29.9 kW is higher than the sum of the two highest rated units, it is permissible to use the demand load.
There is a possibility the demand load calculated by Table 220.56 will be less than the sum of the largest two kitchen equipment loads. For example, what is the demand load for the following commercial kitchen equipment: five 3-kW ovens, one 5-kW cooktop and two 20-kW ranges? The total number of units is eight. The demand factor percent in Table 220.56 for eight units is 65 percent. The total rating of the equipment is of 60 kW (3 + 3 + 3 + 3 + 3 + 5 + 20 + 20 = 60 kW). The load after applying the demand factor is 39 kW (60 × 65% = 39 kW). The sum of the largest two kitchen equipment loads is 40 kW (20 + 20 = 40). Since the demand load of 39 kW is lower than the sum of the two highest rated units, the demand load must not be used. The minimum load for these eight units of kitchen equipment is 40 kW (see Figure 4).
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Next month’s Code in Focus continues the discussion of feeder and service load calculations.
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.
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