Branch-Circuit, Feeder and Service Calculations, Part XXXIII

by Charles R. Miller
Published: November 2008

Article 220 – Load Calculations

220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit

The National Electrical Code (NEC) is divided into an introduction and nine chapters. Some items covered in the introduction include purpose and scope of the Code; arrangement of the Code; an explanation of mandatory rules, permissive rules and explanatory material; formal interpretations; and units of measurement. Chapters 1 through 4 apply generally to all electrical installations. Chapter 1, titled General, contains definitions and requirements for electrical installations. Chapter 2 contains requirements pertaining to wiring and protection. Chapter 3 covers wiring methods and materials.

The last of the chapters that apply generally is Chapter 4. This chapter contains requirements for general use equipment. Chapters 5 through 7 supplement or modify the general rules found in the first four chapters. Chapter 5 applies to special occupancies, such as hazardous (classified) locations, healthcare facilities, assembly occupancies, manufactured homes and buildings, marinas and boatyards, and temporary installations. Chapter 6 applies to special equipment, such as electric signs and outline lighting, cranes, elevators, welders, information technology equipment, industrial machinery, swimming pools, and fire pumps. Chapter 7 applies to special conditions, such as emergency systems; standby systems (legally required and optional); Class 1, 2 and 3 circuits; fire alarm systems; and the installation of optical fiber cables and raceways. See the Table of Contents in the NEC for a complete listing of each chapter. Chapter 8 covers communications systems and is not subject to the requirements of Chapters 1 through 7 unless the requirements are specifically referenced in Chapter 8. Chapter 9 contains tables that are applicable as referenced. The NEC also contains eight annexes (A through H) that are not part of NEC requirements. They are included for informational purposes only.

Article 220, one of the articles that applies generally to all electrical installations, provides requirements for calculating branch-circuit, feeder and service loads. This article has been the topic of a detailed study in this column.

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.

Requirements for calculating branch-circuit loads for household cooking appliances are in the fourth note under Table 220.55. Results from the calculations are used to size the branch-circuit conductors and overcurrent protection. As discussed last month, Note 4 (under Table 220.55) can be divided into three parts. The first part covers a branch-circuit load calculation for one range. Branch circuits are similar to feeders and services in that the size of the branch circuit can be based on the nameplate rating of the range, or the branch circuit can be sized based on the demand factors in Table 220.55. Determining the load, whether full load or demand load, is the first step (see last month’s column). Determining the conductor size and the overcurrent protective device are the next steps. Requirements for sizing branch-circuit conductors and overcurrent devices are in Article 210.

As specified in 210.19(A)(3), branch-circuit conductors supplying household ranges, wall-mounted ovens, counter--mounted cooking units and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. The minimum rating of the branch circuit for one range can be determined by applying the demand factors in Table 220.55 [220.18(C)]. For example, what minimum size 60°C copper conductor is required for a household electric range rated 8 kW at 120/240 volts? 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 or volt-amperes (VA) by multiplying 6.4 by 1,000 (6.4 × 1,000 = 6,400 VA). To find amperes, divide the load by the rated voltage (6,400 ÷ 240 = 26.7 = 27 amperes). The minimum rating of the branch circuit supplying power to this 8-kW range is 27 amperes, and therefore, the conductor must have an ampacity of at least 27 amperes (see Figure 1).

Now that the minimum ampacity is known, the conductor can be selected. Table 310.16 provides the allowable ampacities of insulated conductors rated 0 through 2,000 volts. Because this example is asking for a 60°C copper conductor, the conductor must be selected from the 60°C column, under copper, in Table 310.16. The ampacities in Table 310.16 are based on three or fewer current-carrying conductors and an ambient temperature of 30°C or 86°F. Copper conductors are on the left side, and aluminum or copper-clad aluminum conductors are on the right side. Look on the left side of Table 310.16, and follow the 60°C column down until a conductor has an ampacity of at least 27 amperes. The smallest size conductor with an allowable ampacity of at least 27 amperes is 10 AWG copper. The minimum size 60°C copper conductor required for a household electric range rated 8 kW at 120/240 volts is 10 AWG (see Figure 2).

When calculating branch circuits for ranges over 12 kW, use the first note under Table 220.55. For example, what minimum size 60°C copper conductor is required for a household range rated 16.5 kW at 120/240 volts? 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 or major fraction thereof by which the rating of individual ranges exceeds 12 kW. Since the .5 is a major fraction, round the rating of 16.5 up to 17 kW. Subtract 12 from 17 (17 – 12 = 5). Since 17 kW exceeds 12 kW by 5, multiply 5 by 5 percent to find the amount Column C must be increased by (5 × 5% = 25%). The maximum demand listed in Column C for one range must be increased by 25 percent. The increased amount is 2 kW (8 × 25% = 2 kW). This increased amount must be added to the Column C demand load for one range (8 + 2 = 10 kW). Convert kilowatts to watts by multiplying 10 by 1,000. To find amperes, divide the load by the rated voltage (10,000 ÷ 240 = 41.7 = 42 amperes). The minimum rating of the branch circuit supplying power to this 16.5 kW range is 42 amperes. The conductor must have an ampacity of at least 42 amperes (see Figure 3).

Because this example also is asking for a 60°C copper conductor, the conductor must be selected from the 60°C column, under copper, in Table 310.16. Look on the left side of Table 310.16 and follow the 60°C column down until a conductor has an ampacity of at least 42 amperes. The smallest size conductor with an allowable ampacity of at least 42 amperes is 6 AWG copper. The minimum size 60°C copper conductor required for a household electric range rated 16.5 kW at 120/240 volts is 6 AWG (see Figure 4).

While cooktops and ovens are quite often permanently connected (hard-wired), ranges are usually cord-and-plug connected. In accordance with 210.21(B)(4), the ampere rating of a range receptacle can be based on a single range demand load as specified in Table 220.55.

Discussion of feeder and service load calculations continues next month.

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.