Branch-Circuit, Feeder and Service Calculations, Part XX

by Charles R. Miller
Published: October 2007

Article 220 – Load Calculations

220.54 Electric Clothes Dryers—Dwelling Unit

Understanding how to perform load calculations in accordance with the National Electrical Code (NEC) is an important part of an electrician’s professional career. Loads must be calculated before installing branch circuits, feeders and services. Branch-circuit, feeder and service load calculation requirements are in Article 220 of Chapter 2. The NEC contains an introduction, nine chapters and annexes. Chapters 1 through 4 apply generally to all electrical installations. Chapters 5 through 7 apply to special occupancies, special equipment or other special conditions. These three chapters supplement or modify the general rules specified in the first four chapters. Chapter 8 covers communications systems and is not subject to the requirements of Chapters 1 through 7 unless the requirements are specifically referenced therein. Chapter 9 consists of tables that are applicable as referenced. Annexes follow the tables in Chapter 9. Annexes are not part of the requirements of the NEC. They but are included for informational purposes only. While the 2005 edition contains seven annexes (A through G), the 2008 edition contains eight (A through H).

Last month’s Code in Focus concluded by covering fastened-in-place appliance calculations that are specified in 220.53. This month, the discussion continues with calculating loads for electric clothes dryers in dwelling units.

The load for household electric clothes dryers in a dwelling unit(s) shall be either 5,000 watts (volt-amperes) or the nameplate rating, whichever is larger, for each dryer served [220.54]. Unlike the required minimum load of 1,500 volt-amperes for a laundry branch circuit, there is no required minimum load if an electric clothes dryer will not be installed. Also, a gas dryer could be installed and, therefore, not draw the type of load specified in 220.54. When calculating the load on a feeder or service, if an electric clothes dryer will be installed, the calculation must include a load of at least 5,000 watts (volt-amperes). For example, what is the minimum service load for a clothes dryer that has a nameplate rating of 4.5 kilowatts (kW)? Assume dryer kilowatt ratings are equivalent to kilovolt-ampere (kVa) ratings for these load calculations. First, convert kilowatts to watts (or volt-amperes) by multiplying the kilowatts by 1,000 (4.5 × 1,000 = 4,500). Since this dryer is only 4,500 volt-amperes, it does not meet the minimum load specified for a clothes dryer. The minimum service load for this clothes dryer is 5,000 volt-amperes (see Figure 1).

 
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If the nameplate rating exceeds the required minimum rating of 5,000 volt-amperes, use the nameplate rating. For example, what is the minimum feeder load for a clothes dryer that has a nameplate rating of 5.6 kW? First, convert kilowatts to watts (or volt-amperes) by multiplying kilowatts by 1,000 (5.6 × 1,000 = 5,600). Since the load of this clothes dryer is more than the required minimum, use the nameplate rating. The minimum feeder load for this clothes dryer is 5,600 volt-amperes (see ­Figure 2).

 
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Where there are five or more electric clothes dryers on a feeder or service, the load can be derated or reduced. The demand factor for one, two, three or four dryers is 100 percent; therefore, no derating is permissible. In accordance with 220.54, it is permissible to apply the demand factors in Table 220.54 to household electric clothes dryers.

For example, what is the calculated dryer load for an eight-unit multifamily dwelling with each unit having a 5.2 kW clothes dryer? First, multiply the nameplate rating (because it is more than the required minimum) by 1,000 and then by the number of dryers (5.2 × 1,000 × 8 = 41,600). Next, find the demand factor percent across from eight dryers in Table 220.54 (60 percent). Finally, multiply the total dryer load by the demand factor (41,600 × 60 percent = 24,960). The minimum clothes dryer load for this multifamily dwelling is 24,960 volt-amperes (see Figure 3). Do not apply Table 220.54 demand factors to the laundry facility (laundromat) of a multifamily dwelling. It is possible that all dryers will be operating at one time in a laundry facility.

 
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Where the number of dryers is 12 or more, a calculation may be necessary to find the demand factor percent. For example, what is the calculated dryer load for a 20-unit multifamily dwelling with each unit having a 4.8-kW clothes dryer? Since each dryer is only 4,800 volt-amperes (4.8 × 1,000) and does not meet the required minimum, change each to 5,000 volt-amperes and multiply by the number of dryers (5,000 × 20 = 100,000). Now, find the demand factor percentage from the formula across from 12 to 22 dryers [% = 47 – (number of dryers – 11)]. Perform the part of the formula in the parentheses first [(number of dryers – 11) = (20 – 11) = (9)]. Now subtract 9 from 47 to find the demand factor percentage (% = 47 – 9 = 38%). Finally, apply the demand factor to the total dryer load (100,000 × 38% = 38,000). The minimum clothes dryer load for this multifamily dwelling is 38,000 volt-amperes (see Figure 4). In the 2008 NEC, the formula is stated as 47 minus 1% for each dryer exceeding 11. Both formulas will yield the same demand factor percentages when calculating 12 to 23 dryers. The number of dryers exceeding 11 is 9 (20 – 11 = 9). The demand factor is 38 percent after subtracting 9 from 47 (47 – 9 = 38%).

 
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The demand factor for 23 dryers is simply 35 percent. The formula is a little different when the number of dryers is 24 to 42. For example, what is the calculated dryer load for a 40-unit multifamily dwelling with each unit having a 5,500-watt electric clothes dryer? First, multiply 5,500 volt-amperes by the number of dryers (5,500 × 40 = 220,000). Now, find the demand factor percentage from the formula across from 24 to 42 dryers {% = 35 – [0.5 × (number of dryers – 23)]}. Start by solving the equation inside the parentheses (number of dryers – 23) = (40 – 23) = (17). Next, solve the equation within the brackets [0.5 × 17] = [8.5]. Now subtract 8.5 from 35 (35 – 8.5 = 26.5). Finally, apply the demand factor to the total dryer load (220,000 × 26.5% = 58,300). The minimum clothes dryer load for this multifamily dwelling is 58,300 volt-amperes (see Figure 5). In the 2008 NEC, the formula is stated as 35% minus 0.5% for each dryer exceeding 23. Since the number of dryers exceeding 23 is 17 (40 – 23 = 17), multiply 17 by 0.5% (17 × 0.5% = 8.5). The demand factor is 26.5 percent after subtracting 8.5 from 35 (35 – 8.5 = 26.5%).

 
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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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