Article 220 contains provisions for calculating branch-circuit, feeder and service loads. Following the requirements set forth in this article will provide the minimum size branch circuit, feeder or service. This information is then employed to help determine the size of conductors and overcurrent protection. The minimum size protects against overloads and provides practical safeguarding of persons and property, but no consideration is given to future expansion. Article 220 is divided into four parts. Part I contains general requirements and some branch-circuit calculations. Part II covers feeder and service calculations. Optional calculations for computing feeder and service loads are in Part III. Since optional calculation methods are in Part III, calculations in Parts I and II are sometimes referred to as standard calculation methods. For example, if a one-family dwelling is said to have been calculated by the standard method, it was computed in accordance with the first two parts of Article 220. Part IV contains specifications for computing farm loads.
Load calculations can sometimes seem overwhelming, but by breaking down a load calculation into a series of steps, the process can be made easier. One step in most load calculations is cooking equipment. Requirements for computing cooking equipment loads (by the standard methods) are in 220.19 and 20. Demand loads for electric ranges and other cooking appliances in dwelling units are in 220.19 and Table 220.19. The demand loads are for household electric ranges, wall-mounted ovens, counter-mounted cooking appliances, and other household cooking appliances individually rated in excess of 13/4kW (See Figure 1).
Kitchen equipment demand loads for other than dwelling units are in 220.20 and Table 220.20. It is permissible to apply the demand factors in Table 220.20 to commercial electric cooking equipment, dishwasher booster heaters, water heaters and other kitchen equipment. These demand factors can be applied to all equipment that has either thermostatic control or intermittent use as kitchen equipment. Space-heating, ventilating and air-conditioning equipment must not be included.
Although household cooking equipment can be added to a load calculation at 100 percent of the rating, it is permissible to reduce the rating by applying the demand loads in Table 220.19. This table (and the five notes under the table) can be applied to household electric ranges, wall-mounted ovens, counter-mounted cooking appliances and other household cooking appliances individually rated in excess of 13/4kW or 1,750W. For example, an electric range having a rating of 12kW (12,000W) can be added to a dwelling’s load calculation at 12kW. Or instead of the full rating, the same range could be added at 8kW. By applying Table 220.19, a 12kW range has a maximum demand of 8kW (See Figure 2). The reason for this reduced rating is because a range is rarely used at full potential. On occasions, such as Thanksgiving and Christmas, the oven and all four top elements (eyes) could be in use at the same time. But even if everything is on at the same time, everything will probably not be on the highest setting. The thermostat could also cycle the elements so that everything is not energized at the same time.
The number of appliances and the kilowatt rating of each piece of cooking equipment must be known before using Table 220.19. Since the computation for branch circuits is not the same as for feeders and services, the application must also be known. A good starting point in understanding this table is to find the service or feeder demand load for one range. The demand load for one range with a rating higher than 8 3/4kW, but not higher than 12kW, is 8kW. For example, what is the service demand load for a 9kW range? Since Column A is for equipment rated less than 3 1/2kW and Column B for equipment rated between 3 1/2 and 8 3/4kW, the demand load can only be found in Column C. The demand loads shown in Column C are for appliances with ratings 12kW, or less. The maximum load required for one 9kW range, when computing a service (or feeder), is 8kW. Likewise, an 8kW load is all that is required if the range is 10, 11 or 12kW (See Figure 3).
The procedure is the same when there is more than one range. Look in the left column for the number of appliances and follow the row across to the appropriate column. For example, what is the service demand load for a 20-unit apartment building that has a 12kW range in each apartment? Since there are 20 apartments and there is one range in each, the number of appliances is 20 and each one has a rating of 12kW. Because of the ratings of the ranges, the answer is again found in Column C. The maximum demand load required is only 35kW (See Figure 4). Although the total rated load is 240kW (20 x 12), per Table 220.19, the maximum load at any one time is only 35kW. As the number of units in a multifamily dwelling increase, the percentage of ranges likely to be in use simultaneously decreases.
When there are more than 25 appliances with ratings over 8 3/4kW each, the demand load must be calculated. If the number of appliances is from 26 to 40, the calculation is a follows: 15kW + 1kW for each range. Simply add 15 to the number of appliances and the sum is the kilowatt demand load. For example, what is the service demand load for 35 10kW ranges? Because of the rating of the ranges, the demand load can only be found in Column C. Fifteen added to 35 appliances is 50 (15 + 35), therefore the demand load for 35 10kW ranges is 50kW. The demand load would be exactly the same if each range was rated 9, 11, or 12kW. (See Figure 5.) The procedure of adding 15 to the number of appliances actually starts with 5 appliances. For example, 15 plus 5 appliances equals 20. The kilowatt demand load in Column C for five appliances is 20kW.
Once the number of appliances (rated more than 8 3/4kW) reaches 41, the calculation method changes. The calculation for more than 40 ranges is as follows: 25kW + 3/4kW for each range. Instead of each appliance counting as 1kW, each appliance will only count as 3/4kW. Multiply the number of appliances by 0.75 and add to 25. For example, what is the service demand load for sixty 12kW ranges? First multiply 60 by 0.75 (60 x 0.75 = 45). Next, add 45 to 25 (45 + 25 = 70). The demand load for 60 12kW ranges is 70kW.
Next month’s In Focus, resuming with 220.19, will continue discussion of cooking equipment calculations. EC
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.