Branch-Circuit, Feeder and Service Calculations, Part XIX

by Charles R. Miller
Published: September 2007

Article 220 – Load Calculations

220.53 Appliance Load—Dwelling Unit

While knowing how to perform load calculations from Article 220 of the National Electrical Code (NEC) could be the difference between pass and fail on a journeyman or master electrician’s exam, it actually is important in an electrician’s daily life. Before sizing conductors and overcurrent protective devices, the loads must be calculated. It could be as simple as looking on a nameplate and finding the minimum circuit amperes or the maximum overcurrent protection. Then again, it may be necessary to calculate the load in accordance with Article 220 before selecting conductors and overcurrent protective devices.

Requirements in Article 220 are divided into five parts. Part I covers general requirements for load calculation procedures. Branch-circuit load calculation procedures are in Part II. Feeder and service load calculation requirements (sometimes referred to as the standard method) are in Part III. Part IV contains optional feeder and service load calculation provisions. Farm loads must be calculated in accordance with Part V of Article 220.

Last month’s column concluded by covering small appliance and laundry load calculations in 220.52. This month, the discussion continues with appliance loads in dwelling units.

If there are four or more fastened-in-place appliances on the same feeder or service in a dwelling unit, it is permissible to derate or reduce the load. It shall be permissible to apply a demand factor of 75 percent to the nameplate rating load of four or more appliances fastened in place—other than electric ranges, clothes dryers, space-heating equipment or air conditioning equipment—that are served by the same feeder or service in a one-family, two-family or multifamily dwelling [220.53]. Appliance is defined as utilization equipment (generally not industrial) that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions such as clothes washing, air conditioning, food mixing, deep frying, etc. [Article 100]. Some examples of appliances that are fastened in place include dishwashers, kitchen-waste disposers, trash compactors, attic fans and water heaters (see Figure 1).

 
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Certain appliances and equipment must not be included when determining the number of fastened-in-place appliances. In accordance with 220.53, electric ranges, clothes dryers, space-heating equipment or air conditioning equipment must not be included with the number of appliances that are fastened in place (see Figure 2). Provisions for these appliances are in other sections of Article 220’s Part III. Electric clothes dryers are covered in 220.54 and Table 220.54. Electric ranges are covered in 220.55 and Table 220.55. Space heating load calculation provisions are in 220.51. As specified in 220.50, loads for air conditioning equipment must be calculated in accordance with 440.6. 

 
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Applying the demand factor to less than four fastened-in-place appliances in a dwelling is not permitted. For example, the following appliances will be installed in a one-family dwelling: a dishwasher rated 10 amperes at 120 volts; a ½ hp, 120-volt kitchen-waste disposer; and a 4.5 kW, 240-volt water heater. Kilovolt-amperes (kVA) shall be considered equivalent to kilowatts (kW) for loads calculated in this article. The load for these three appliances must be calculated at 100 percent. The dishwasher has a load of 1,200 watts (10 × 120 = 1,200). Since the kitchen-waste (or garbage) disposer is a motor, the load must be determined by multiplying the full-load current in Table 430.248 by the system voltage. In accordance with Table 430.248, the full-load current of a ½-hp, 120-volt motor is 9.8 amperes. Although the current value of 9.8 amperes is in the column titled “115 volts,” the currents listed in this column are permitted for system voltage ranges of 110 to 120 volts (see the last sentence of the text in Table 430.248). The disposer has a load of 1,176 watts (9.8 × 120 = 1,176). The load of the water heater is 4,500 watts (4.5 × 1,000 = 4,500). Since derating is not permissible for only three appliances, the calculated load at 100 percent is 6,876 watts (1,200 + 1,176 + 4,500 = 6,876) (see Figure 3).

 
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With the addition of one more fastened-in-place appliance, applying a demand factor of 75 percent to the nameplate-rating load is permissible. For example, what is the service load calculation for a one-family dwelling with the following appliances: a dishwasher rated 10 amperes at 120 volts; a ½-hp, 120-volt kitchen-waste disposer; a trash compactor rated 7.5 amperes at 120 volts; and a 4.5-kW, 240-volt water heater? The dishwasher has a load of 1,200 watts (10 × 120 = 1,200). The load of the kitchen-waste disposer is 1,176 watts (9.8 × 120 = 1,176). The compactor has a load of 900 watts (7.5 × 120 = 900). The load of the water heater is 4,500 watts (4.5 × 1,000 = 4,500). The total before applying the demand factor is 7,776 watts (1,200 + 1,176 + 900 + 4,500 = 7,776). Next, because there are four appliances, multiply the load by the 75 percent demand factor. The calculated service load for these appliances is 5,832 watts (7,776 × 0.75 = 5,832). Note that three of the appliances in this example were the same as the three appliances in the previous example. By adding one more appliance, the load calculation is now less than the load calculation in Figure 3 (see Figure 4).

 
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The fastened-in-place appliance-demand factor also can be applied when calculating a feeder. It is permissible to apply the 75 percent demand factor to each feeder with four or more
fastened-in-place appliances. There may be an occasion when there will be at least four appliances on the service but less than four appliances on each feeder. For example, a one-family dwelling will have two feeders supplied by the service. One feeder will provide power to a water heater and two attic ventilation fans. The other feeder will provide power to a dishwasher, a kitchen-waste disposer and a trash compactor. Since only three appliances are on each feeder, it is not permissible to apply the 75 percent demand factor to either feeder load calculation. The demand factor can be applied to the service load calculation because it will supply power to six fastened-in-place appliances (see Figure 5).

 
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Next month’s column 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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