Branch-Circuit, Feeder and Service Calculations, Part XXX

by Charles R. Miller
Published: August 2008

Article 220 – Load Calculations

220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit

Knowing where to find load calculation requirements and understanding how to perform the calculations in accordance with the National Electrical Code (NEC) is essential for the professional electrician. Before selecting conductors, overcurrent protection, panelboards and even raceways, loads must be calculated. Decisions must be made about which loads will be on each branch circuit and how many branch circuits will be needed. Decisions must also be made about feeders and services. Branch-circuit, feeder and service load calculations are in Article 220 of the NEC. Article 220 is divided into five parts. General requirements pertaining to branch-circuit, feeder and service load calculations are in Part I.- Part II contains calculation provisions for branch circuits. Feeder and service calculation requirements are in Part III. Although not official, Part III is commonly referred to as the “standard load calculation method.” Optional feeder and service load calculation provisions are in Part IV of Article 220. This part 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. As the name implies, the optional calculation method is an alternative calculation method that is permitted in accordance with Part IV. Farm loads must be calculated according to the requirements in Part V.

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.

Household electric ranges, wall-mounted ovens, counter-mounted cooking units and other household cooking appliances can be added to a load calculation at the full kilowatt rating of the equipment or can be calculated in accordance with 220.55 and Table 220.55. When using Table 220.55 to find the cooking equipment’s rating for the service or feeder, the kilowatt rating from the table will be lower than the equipment’s nameplate rating. The lower or reduced kilowatt rating is referred to as the demand load. Column C provides the maximum demand load in kilowatts (kW) for cooking equipment over 8¾ kW through 12 kW. Either the first or second note under Table 220.55 applies when one or more ranges are individually rated more than 12 kW but not more than 27 kW. Use the first note when all the ranges are rated the same. Use the second note when the ranges are not rated the same. Where at least one range is rated over 12 kW through 27 kW and at least one range is rated more than 8¾ kW but not more than 27 kW, the second note applies. When using this note, any range with a nameplate rating that is less than 12 kW must be changed to 12 kW before finding the average value of rating. Find the average value of rating by adding together the ratings of all ranges to obtain the total connected load, and then divide by the number of ranges. The maximum demand in Column C shall be increased 5 percent for each kilowatt or major fraction thereof by which this average value exceeds 12 kW.

The process of using 12 kW for any range rated less than 12 kW is a critical step in calculating in accordance with Note 2 under Table 220.55. For example, what is the service demand load for eight 9-kW, six 13-kW and six 15-kW household electric ranges? Before finding the average value of rating, change the 9-kW ranges to 12-kW ranges. Next, find the total kilowatt rating of all the ranges [(8 × 12) + (6 × 13) + (6 × 15) = 96 + 78 + 90 = 264]. Now, find the average value by dividing the total connected load by the total number of ranges (264 ÷ 20 = 13.2 = 13 kW) (see Figure 1).

Now that the average value for the ranges of unequal rating is known, find the service demand load for 20 13-kW ranges. Because Column C is based on 12-kW ranges, subtract 12 from 13 (13 – 12 = 1). Since 13 kW exceeds 12 kW by 1, the maximum demand listed in Column C for 20 ranges must be increased by 5 percent. The increased amount is 1.75 kW (35 × 5% = 1.75 kW). This increased amount must be added to the Column C demand load for 20 ranges (35 + 1.75 = 36.75 kW). The service demand load for eight 9-kW, six 13-kW and six 15-kW household electric ranges is 36.75 kW (see Figure 2).

 
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When calculating the demand load for ranges of unequal rating in accordance with the second note under Table 220.55, all ranges rated more than 8¾ kW but less than 12 kW must be changed to 12 kW before finding the average value of rating. For example, what is the service demand load for five 9-kW, five 10-kW and five 17-kW household electric ranges? Before finding the average value of rating, change the 9-kW ranges and the 10-kW ranges to 12-kW ranges. Next, find the total kilowatt rating of all the ranges [(5 × 12) + (5 × 12) + (5 × 17) = 60 + 60 + 85 = 205]. Now, find the average value by dividing the total connected load by the total number of ranges (205 ÷ 15 = 13.67 kW). The average rating of all 15 ranges is 13.67 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. A “major fraction thereof” is .5 and larger. Since the .67 is a major fraction, round the average rating of 13.67 up to 14 kW (see Figure 3).

Now that the average value for the ranges of unequal rating is known, find the service demand load for 15 14-kW ranges. Because Column C is based on 12-kW ranges, subtract 12 from 14 (14 – 12 = 2). Since 14 kW exceeds 12 kW by 2, multiply 2 by 5 percent to find the amount Column C must be increased (2 × 5% = 10%). The increased amount is 3 kW (30 × 10% = 3 kW). This increased amount must be added to the Column C demand load for 15 ranges (30 + 3 = 33 kW). The service demand load for five 9-kW, five 10-kW and five 17-kW household electric ranges is 33 kW (see Figure 4).

 
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While forgetting to change ranges rated less than 12 kW up to 12 kW may not seem very important, it could be the difference between a normal condition and an overload. Also, if taking an electrician’s exam, leaving out this step on a range calculation that uses Note 2 under Table 220.55 will provide an incorrect answer. Missing this question could be the difference between pass and fail.

Next month, the discussion of feeder and service load calculations continues.

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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