Branch-Circuit, Feeder and Service Calculations, Part XXI

by Charles R. Miller
Published: November 2007

Article 220 – Load Calculations

220.54 Electric Clothes Dryers—Dwelling Unit

Load calculation requirements are in Article 220 of the National Electrical Code (NEC). It provides requirements for calculating branch-circuit, feeder and service loads. The Code divides requirements in Article 220 into five parts. Part I covers general requirements for calculation procedures. Part II provides calculation provisions for branch circuits. Part III contains feeder and service calculation requirements (sometimes referred to as the standard method). Part IV specifies optional feeder and service load calculations are specified. Part V regulates the calculation of farm loads. Results from calculations in Article 220 are used in conjunction with requirements from other articles to find conductor sizes and ampere rating for overcurrent protection. While Article 220 does not contain provisions for determining the minimum number of branch circuits, results from this article’s calculations together with rules from other articles can be used to find the minimum number of branch circuits. As stipulated in 210.11, the minimum number of branch circuits shall be determined from the total calculated load and the size or rating of the circuits used. In all installations, the number of circuits shall be sufficient to supply the load served. In no case shall the load on any circuit exceed the maximum specified by 220.18.

Last month’s Code in Focus covered electric clothes dryers in 220.54. 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]. As discussed last month, an electric clothes dryer is not a required item in a dwelling unit load calculation, but if an electric clothes dryer will be installed, the load added to the calculation must be at least 5,000 volt-amperes. For example, what is the minimum one-family dwelling service load for a 4.7-kW combination clothes washer and dryer? While there is no mention of a combination clothes washer and dryer in Article 220, an example in Annex D shows a load calculation with a washer/dryer. In the load calculation of Example D2(b), a combination clothes washer and dryer is calculated exactly the same as a clothes dryer (see Example D2(b) in Annex D). Since the washer/dryer is only 4,700 volt-amperes (4.7 × 1,000) and does not meet the required minimum, change this load to 5,000 volt-amperes. The minimum service load for this combination clothes washer and dryer is 5,000 volt-amperes (see Figure 1).

 
CIF_Nov_07_01.jpg
 
   

This first example illustrates a washer/dryer load calculation in a dwelling unit. Having a combination washer/dryer load in the calculation does not mean it is permissible to omit the laundry circuit load required by 220.52(B). A load of not less than 1,500 volt-amperes shall be included for each two-wire laundry branch circuit installed as required by 210.11(C)(2) [220.52(B)]. Likewise, a 125-volt receptacle outlet is still required for the laundry area even if one is not required for the washer/dryer unit [210.11(C)(2), 210.52 and 210.52(F)] (see Figure 2).

 
CIF_Nov_07_02.jpg
 
   

It is permissible to apply an additional demand factor of 70 percent to the calculated feeder or service load of household electric clothes dryers [220.61(B)]. For example, a clothes dryer will be installed in a single-family dwelling. What is the minimum neutral load on the service for a 5,000-watt clothes dryer? When calculating feeder or service neutral loads for clothes dryers, start by finding the feeder or service load. Because there is only one dryer, the minimum service load (at 100 percent) is 5,000 volt-amperes. Now, multiply the service load by 70 percent (5,000 × 70% = 3,500). The minimum neutral load on the service for a 5,000 watt clothes dryer is 3,500 volt-amperes (see Figure 3).

 
CIF_Nov_07_03.jpg
 
   

Where there are five or more electric clothes dryers on a feeder or service, the load can be derated or reduced. In accordance with 220.54, it is permissible to apply the demand factors in Table 220.54 to household electric clothes dryers. Where the number of dryers is one through four, no derating is permissible. The demand factors change with every unit where the number of clothes dryers is five through 42. Regardless of the number of dryers over 42, the demand factor remains the same. Where there are 43 or more electric clothes dryers on a feeder or service, the total dryer load can be multiplied by 25 percent. For example, a multifamily dwelling will have a 5,500-watt electric clothes dryer in each of 100 units. What is the minimum service load for these clothes dryers? If the nameplate is more than 5,000 volt-amperes, the load calculation must be based on the nameplate. First, multiply the nameplate rating (because it is more than the required minimum) by the number of dryers (5,500 × 100 = 550,000). Next, because the number of dryers is more than 42, multiply the total dryer load by 25 percent (550,000 × 25% = 137,500). The minimum clothes dryer load for this multifamily dwelling is 137,500 volt-amperes, or 135.5 kVA (see Figure 4).

 
CIF_Nov_07_04.jpg
 
   

The demand factor of 70 percent for dryer neutral loads can be applied to the calculated load even after demand factors in Table 220.54 have been applied. For example, what is the minimum neutral load on the service for a 10-unit multifamily dwelling with each unit having a 5.5-kW clothes dryer? First, find the total load before the application of demand factors (5.5 × 1,000 × 10 = 55,000). Now, find the demand factor percent across from 10 dryers in Table 220.54 (50 percent). Next, multiply the total dryer load by the demand factor (55,000 × 50% = 27,500). Finally, apply the demand factor permitted for household electric clothes dryers (27,500 × 70% = 19,250). The minimum neutral load on the service for 10 5.5-kW clothes dryers in this multifamily dwelling is 19,250 volt-amperes (see Figure 5).

 
CIF_Nov_07_05.jpg
 
   

Although this section provides requirements for feeder and service loads, these calculations also apply to branch-circuit loads. Load calculations shall be permitted as specified in 220.54 for electric dryers and in 220.55 for electric ranges and other cooking appliances [220.14(B)].

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.

   
Featured Books November-26-2008
forumbutton_.gif

bannertabs4.png

Featured Books:



NFPAs_Electrical_References.png

NFPAs_pocket_electrical_references.png
Read more...