Branch-Circuit, Feeder and Service Calculations, Part XXXI

by Charles R. Miller
Published: September 2008

Article 220 – Load Calculations

220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit

Article 220 of the National Electrical Code (NEC) contains requirements for calculating branch-circuit, feeder and service loads. Article 220 is divided into five parts. General requirements for load-calculation procedures are in Part I. Part II covers requirements for branch-circuit load calculations. Feeder and service load calculations are in Part III. Part III is sometimes referred to as the standard method for feeder and service load calculations. Part IV is titled “Optional Feeder and Service Load Calculations,” and it contains optional load calculation procedures for a single dwelling unit, an existing dwelling unit, a multifamily dwelling, two dwelling units, a school, an existing installation and a new restaurant. Feeder and service loads can be calculated in accordance with the requirements in either Part III or Part IV. Whether calculating by the standard method or optional method, the neutral load for feeders and services must be calculated in accordance with the requirements in 220.61. Part V provides calculation specifications for farm loads. Figure 220.1 (in the NEC) shows a diagram on the organization of Article 220. Loads calculated in accordance with the provisions in Article 220 are used with requirements from other articles to find conductor sizes and ampere ratings for overcurrent protection. For example, results from Part II in Article 220 are used with the provisions in 210.19 to size branch-circuit conductors and 210.20 to determine ratings for overcurrent protective devices (fuses and breakers).

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.

Calculating the neutral load for feeders and services supplying household electric ranges, wall-mounted ovens and counter-mounted cooking units is accomplished by using Table 220.55 and 220.61(B).

First, calculate the feeder or service load by applying the appropriate Table 220.55 demand factors or obtaining the maximum demand in kilowatts. After the maximum unbalanced load has been determined, apply an additional demand factor of 70 percent to determine the neutral load [220.61(B)]. For example, what is the neutral demand load for a feeder supplying one 12-kW range? The maximum demand in Column C of Table 220.55 for one 12-kW range is 8 kW. Therefore, the feeder load for this range is 8 kW. Now apply the additional demand factor of 70 percent (8 × 70% = 5.6 kW). Instead of 12,000 watts or even 8,000 watts, the neutral load for a feeder supplying one 12-kW range is 5.6 kW or 5,600 watts (see Figure 1).

Regardless of the mixture of ranges, wall-mounted ovens and counter-mounted cooking units, after finding the Table 220.55 demand load for the feeder or service, multiply the demand load by 70 percent to obtain the neutral demand load. For example, what is the service neutral demand load for five 3½-kW wall-mounted ovens, five 5-kW counter-mounted cooking units and 10 8-kW ranges? Because none of the appliances are rated less than 3½ kW or more than 8¾ kW, the demand factor will come from Column B.

First, find the total number of units (5 + 5 + 10 = 20). Next, find the demand factor percent in Column B across from 20 units (28 percent). Now find the total kilowatts of all the appliances. The ovens have a total rating of 17.5 kW (5 × 3.5 = 17.5). The cooktops have a total rating of 25 kW (5 × 5 = 25). The ranges have a total rating of 80 kW (10 × 8 = 80). The combined rating of all the appliances is 122.5 kW (17.5 + 25 + 80 = 122.5).

Next, multiply the total kilowatt load by the demand factor percent (122.5 × 28% = 34.3). The service demand load for these 20 units is 34.3 kW. Any time Column A or B can be used to calculate household cooking appliances, Column C can be used because it is for household cooking appliances “not over 12 kW rating.”

With this example, the calculated demand in Column B will be used because it is lower than the maximum demand in Column C. Finally, multiply the service demand load of 34.3 kW by 70 percent (34.3 × 70% = 24.01 kW). The service neutral demand load for five 3½-kW wall-mounted ovens, five 5-kW counter-mounted cooking units and 10 8-kW ranges is 24.01 kW (see Figure 2).

 
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When finding the service or feeder neutral demand load for ranges individually rated more than 12 kW (or more than 8¾ kW for ranges of unequal ratings) but not more than 27 kW, start by finding the service or feeder demand load by using either Note 1 or Note 2 under Table 220.55. Then, apply the additional demand factor of 70 percent, as permitted by 220.61(B), to determine the neutral load. For example, what is the service neutral demand load for five 14-kW, five 16-kW and five 17-kW household electric ranges? Because these ranges are rated more than 12 kW and are of unequal ratings, use Note 2 under Table 220.55 to calculate the service demand load. Find an average value of rating by adding together the ratings of all ranges to obtain the total connected load. The total connected load is 235 kW [(5 × 14) + (5 × 16) + (5 × 17) = 70 + 80 + 85 = 235]. Now divide the total connected load by the number of ranges for the average value of rating (235 ÷ 15 = 15.67 kW). The average rating of all 15 ranges is 15.67 kW. Notes 1 and 2 specify that the range rating must be increased for each kilowatt of rating or major fraction thereof by which the rating of the individual ranges exceeds 12 kW. Since the .67 is a major fraction, round the average rating of 15.67 up to 16 kW (see Figure 3).

Because the average range rating is 16 kW, find the service demand load for 15 16-kW ranges. Since Column C is based on 12-kW ranges, the maximum demand in Column C must be increased 5 percent for each additional kilowatt or rating by which the rating of individual ranges exceeds 12 kW. Subtract 12 from 16 (16 – 12 = 4). Since 16 kW exceeds 12 kW by 4, multiply 4 by 5 percent to find the amount Column C must be increased (4 × 5 = 20%). The maximum demand listed in Column C for 15 ranges must be increased by 20 percent. The increased amount is 6 kW (30 × 20% = 6 kW). This increased amount must be added to the Column C demand load for 15 ranges (30 + 6 = 36 kW). The service demand load for these 15 ranges after applying Note 2 to Table 220.55 is 36 kW. Finally, multiply the service demand load of 36 kW by 70 percent (36 × 70% = 25.2 kW). The service neutral demand load for five 14-kW, five 16-kW and five 17-kW household electric ranges is 25.2 kW (see Figure 4).

 

 
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The permitted reduction for a feeder or service neutral load calculation also applies to household electric clothes dryers that are calculated in accordance with Table 220.54.

Next month’s Code in Focus will continue 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 “Electrician's Exam Prep.” He can be reached at 615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.

 
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