Branch-Circuit, Feeder and Service Calculations, Part XXXII
by Charles R. Miller
Published: October 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
The National Electrical Code (NEC) has one main reason or purpose for its existence: safety. The purpose of the Code is the “practical safeguarding of persons and property from the hazards arising from the use of electricity” [90.1(A)]. The NEC has evolved into the code we have today because of individuals who care about the well-being of workers and the protection of property. The NEC contains rules that are considered indispensable when it comes to safety. A Code-compliant installation actually provides peace of mind to designers and installers because they can feel confident that the installation is free from electrical hazards. While some may feel the Code is too restrictive, rest assured it is not. Where would we be without these requirements? Electrocutions and fires would probably be part of our everyday lives.
Code rules are like barriers that protect against electrical hazards. Imagine a bridge located high above a body of water. Because of guardrails or barriers, we can drive across the bridge safely and feel safe while crossing. Now imagine that bridge without guardrails or barriers. Without barriers, we could still cross but not with the same degree of safety. We would probably slow down to a crawl and try to hold fast to the absolute center while we cross. With barriers in place, we have peace of mind and are confident that we are safe. With an installation that is Code-compliant, we are able to feel safe from electrical hazards.
Without provisions for calculating loads, we might design and install wiring systems that are overloaded and electrically dangerous from the beginning. Or, we might design electrical systems that are much larger than required. When we need provisions for calculating branch circuit, feeder or service loads, we turn to Article 220. Other articles may be required when calculating loads in specialized applications. See Table 220.3 for requirements that are in addition to, or modifications of, requirements within Article 220.
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.
Up to this point in the series, all load calculations for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances have been for feeders and services. I have not discussed branch-circuit load calculations for household cooking appliances. Because Section 220.55 is in Part III, titled Feeder and Service Load Calculations, it might seem that branch-circuit load calculations are not covered here. But, the fourth note under Table 220.55 provides requirements for calculating branch-circuit loads for household cooking appliances. Results from the calculations can then be used to size the branch-circuit conductors and overcurrent protection.
Note 4 (under Table 220.55) can be divided into three parts. The first part states that calculating the branch-circuit load for one range in accordance with Table 220.55 is permissible. All of the rules that are applicable for calculating feeder and service loads for one range also are applicable for calculating branch-circuit loads. For example, what is the branch-circuit demand load for a 12-kW range? As with feeders and services, a conductor capable of carrying a load of 12 kW can be installed to provide power to this range. Or, instead of the nameplate rating, Table 220.55 can be used to find the demand load. Column A is used where the rating of the appliance is less than 3½ kW. Column B is used where the rating of the appliance is 3½ kW through 8¾ kW. Column C is the only column applicable for a 12-kW range. The maximum demand in Column C for one range (appliance) rated 12 kW is 8 kW. The branch circuit feeding a 12-kW range must be capable of carrying a load of at least 8 kW (see Figure 1).
When the rating is 3½ kW through 8¾ kW, the demand factor in Column B can be used to size the branch circuit for one range. For example, what is the minimum branch circuit size in watts for an 8-kW household electric range? The demand factor percent from Column B for one unit is 80 percent. Multiply the rating of the range by 80 percent (8 × 80% = 6.4 kW). Convert kilowatts to watts by multiplying 6.4 by 1,000 (6.4 × 1,000 = 6,400 watts). The branch circuit supplying power to an 8-kW range must be capable of carrying a load of at least 6,400 watts (see Figure 2).
When the rating of the range is 12.5 through 27 kW, use the first note under Table 220.55 to find the minimum size branch circuit. For example, what minimum branch-circuit size in kilowatts is required for a 16-kW household electric range? 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 four, multiply four by 5 percent to find the amount Column C must be increased (4 × 5% = 20%). The maximum demand listed in Column C for one range must be increased by 20 percent. The increased amount is 1.6 kW (8 × 20% = 1.6 kW). This increased amount must be added to the Column C demand load for one range (8 + 1.6 = 9.6 kW). The branch-circuit demand load for one 16-kW electric range, after applying Note 1 to Table 220.55, is 9.6 kW (see Figure 3).
Before sizing the branch-circuit conductors and overcurrent protection, the kilowatt-demand load must be converted to amperes. This is an easy calculation once the demand load and the voltage rating of the equipment are known. Divide volt-amperes by voltage to find amperes (volt-amperes ÷ volts = amperes). For example, what is the branch-circuit ampere rating for a range rated 14 kW at 240 volts? Subtract 12 from 14 (14 – 12 = 2). Since 14 kW exceeds 12 kW by two, multiply two by 5 percent to find the amount Column C must be increased (2 × 5% = 10%). The maximum demand in Column C for one range must be increased by 10 percent. The increased amount is 0.8 kW (8 × 10% = 0.8 kW). This increased amount must be added to the Column C demand load for one range (8 + 0.8 = 8.8 kW). The branch-circuit demand load is 8.8 kW or 8.8 kVA. Although the rating of this range is given in kilowatts, kilovolt-amperes (kVA) shall be considered equivalent to kilowatts when calculating loads in 220.55. Before finding amperes, the kilovolt-amperes load must be changed to volt-amperes (VA) by multiplying the load by 1,000 (8.8 kVA × 1,000 = 8,800 VA). Find the branch-circuit load in amperes by dividing 8,800 by 240 volts (8,800 ÷ 240 = 36.7 = 37 amperes). The minimum branch-circuit ampere rating for a range rated 14 kW at 240 volts is 37 amperes (see Figure 4).
Note that the minimum branch circuit for ranges rated 8¾ kW and more shall be 40 amperes [210.19(A)(3)].
Next month’s column 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 “Electrician's Exam Prep.” He can be reached at 615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.