Branch-Circuit, Feeder and Service Calculations, Part XXII

by Charles R. Miller
Published: December 2007

Article 220 – Load Calculations

220.54 Electric Clothes Dryers—Dwelling Unit

The National Electrical Code (NEC) contains an introduction and nine chapters; Annex A through Annex G (A through H in the 2008 edition) are located in the back of the Code, just before the Index. Annexes are not part of the requirements of the NEC but are included for informational purposes. The introduction (Article 90) covers material that is essential to all the chapters and sections in the Code. For example, 90.5(A) through (C) covers mandatory rules, permissive rules and explanatory material. Mandatory rules use the terms “shall” or “shall not.” These terms identify actions that are specifically required or prohibited. Permissive rules use the terms “shall be permitted” or “shall not be required.” These terms identify actions that are allowed. Permissive rules also describe options or alternative methods.

The Code includes explanatory material in the form of fine print notes (FPNs). FPNs are informational only and are not en-forceable. Examples of FPNs include references to other standards, references to related sections of the Code or information related to a Code rule. A description detailing the arrangement of the Code is in 90.3.

Last month’s Code in Focus covered electric clothes dryers in 220.54. This month, the discussion continues with calculat-ing 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 name-plate rating, whichever is larger, for each dryer served. Using the demand factors in Table 220.54 shall be permitted [220.54]. Until now, calculations for clothes dryers have been based on single-phase feeders or services. Occasionally, single-phase clothes dryers are installed on three-phase feeders and services and, therefore, require a different calculation procedure. Where two or more single-phase dryers are supplied by a three-phase, four-wire feeder or service, the total load shall be calculated on the basis of twice the maximum number connected between any two phases [220.54]. For example, a multi-family dwelling will have a 208-volt, single-phase, 4,400-watt clothes dryer in each of 10 units. The electrical service will be supplied by a 208Y/120-volts, three-phase, four-wire system. What is the minimum service load for these clothes dryers? The first step is to find the maximum number of dryers connected between phases. Four dryers will be connected between phases A and B, three dryers will be connected between phases B and C, and three dryers will be connected between phases A and C (see Figure 1). The maximum number of single-phase clothes dryers connected between any two phases is four.

 
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The second step is to multiply the maximum number connected between phases by two and then apply Table 220.54’s de-mand factor. After multiplying the maximum number by two, this load calculation will be based on eight clothes dryers (4 × 2 = 8). Since each dryer is only 4,400 volt-amperes and does not meet the required minimum, change each to 5,000 volt-amperes. Table 220.54’s demand factor for eight clothes dryers is 60 percent. Now apply the demand factor to eight 5,000 volt-amperes clothes dryers (8 × 5,000 × 60% = 24,000). After applying the 60 percent demand factor, the calculated load is 24,000 volt-amperes (see Figure 2). At this point it looks like the calculation is finished, but it is not.

 
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Step three is an additional step not specifically detailed in 220.54. Example D5(A) in Annex D provides a calculation with single-phase ranges supplied by a three-phase, four-wire service. Although the example is calculating ranges, the concept is the same. The 24,000 volt-amperes (in this dryer example) represents only two of the three phases. Since the calcu-lation at this point represents two phases, divide 24,000 by two to find the demand per phase (24,000 ÷ 2 = 12,000). To find the equivalent three-phase load, multiply the per-phase demand by the total number of phases. The total number of phases in a three-phase system is three (12,000 × 3 = 36,000). The calculated load for 10 4,400-watt clothes dryers on a 208Y/120-volts, three-phase, four-wire service is 36,000 volt-amperes (see Figure 3).

 
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Requirements for calculating cooking equipment loads (by the standard methods) are in 220.55 and 220.56. Both sec-tions provide tables containing demand factors. Cooking equipment calculation requirements are few in number compared to the total requirements in Article 220, but don’t underestimate their importance. Understanding requirements pertaining to household cooking equipment is necessary before calculating the load on a feeder or service that supplies electric cooking appliances. Most electricians who have taken a journeyman or master electrician’s exam know how important it is to have a thorough understanding of cooking equipment calculations.

This section contains feeder and service load calculations, and it includes branch circuit load calculations. Branch circuits supplying household electric cooking equipment must be calculated and sized in accordance with 220.55 [220.14(B)]. Demand factors and demand loads for household electric ranges, wall-mounted ovens, ¬counter-mounted cooking units and other household cooking appliances individually rated in excess of 13⁄4 kW are in Table 220.55 (see Figure 4).

 
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Kitchen equipment demand factors for occupancies other than dwelling units are in 220.56 and Table 220.56. It shall be permissible to calculate the load for commercial electric cooking equipment, dishwasher booster heaters, water heaters and other kitchen equipment in Table 220.56. These demand factors can be applied to all equipment that has either ther-mostatic control or intermittent use as kitchen equipment. These demand factors shall not apply to space-heating, ventilat-ing or air-conditioning equipment [220.56].

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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