Branch-Circuit, Feeder and Service Calculations, Part XXXIV

by Charles R. Miller
Published: December 2008

Article 220 – Load Calculations

220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit

The National Electrical Code (NEC) contains a large number of calculations. Understanding how to perform NEC calculations is essential to electricians, engineers, inspectors and others involved in the electrical industry. Knowing how to perform calculations could even be the difference between pass and fail on a journeyman or master electrician’s exam.

Article 220 contains a number of calculations. Requirements for calculating branch-circuit, feeder and service loads are in Article 220. It may be necessary to calculate the loads by using the provisions in Article 220 before selecting conductors and overcurrent de-vices. For example, results from calculations in Article 220 are used with provisions in 210.19(A) to find the minimum branch-circuit conductor size.

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.

Examples in last month’s column covered demand-load calculations for ranges and also the selection of the branch-circuit conductors supplying power to those ranges. Although there are many conductor types to choose from, last month’s examples specifically asked for copper conductors with a 60°C temperature rating. In sizing conductors, another factor that must be considered is the temperature rat-ings of the conductor terminations. Table 310.16 provides allowable ampacities for conductors with temperature ratings of 60°C (140°F), 75°C (167°F) and 90°C (194°F). The temperature ratings at the top of Table 310.16 are the maximum operating temperatures for the con-ductor types listed in the column. See Table 310.13(A) for conductor applications and insulations. Different conductor types have different allowable ampacities. In most cases, the allowable ampacity increases as the temperature rating of the conductor increases. For example, look at the ampacities for a 6 AWG copper conductor. A 6 AWG copper conductor in the 60°C column has an allowable ampacity of 55 amperes; the same conductor in the 75°C column has an allowable ampacity of 65 amperes; and the same 6 AWG copper conductor in the 90°C column has an allowable ampacity of 75 amperes (see Figure 1).

There is an asterisk adjacent to some of the conductors in Table 310.16. A footnote at the bottom of the table references the small-conductor rule in 240.4(D). The small-conductor rule stipulates that unless specifically permitted in 240.4(E) or (G), overcurrent protec-tion shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG and 30 amperes for 10 AWG copper conductors. See 240.4(D) for limitations on other conductors.

Although a conductor is listed in the 90°C column, the ampacity will be limited by the temperature rating of the termination. As specified in 110.14(C), the temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor or device. Equipment termination provisions are divided into two categories. The first, in 110.14(C)(1)(a), covers equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors. The second, in 110.14(C)(1)(b), covers equipment for circuits rated over 100 amperes or marked for conduc-tors larger than 1 AWG. Branch circuits calculated in accordance with Table 220.55 will fall under equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors. Except for certain motors, there are only three stipulations in 110.14(C)(1)(a). Compliance with one of the three provisions is required. The first provision states conductors from the 60°C must be installed. The second provision states conductors with higher temperature ratings can be installed provided the ampacity of those con-ductors are not greater than the ampacity listed in the 60°C column. The third provision states that conductors with higher temperature ratings can be installed, and the ampacity from the conductor’s column can be used if the equipment is listed and identified for use with such conductors.

When selecting branch-circuit conductors from the 75°C and 90°C columns, the temperature ratings of the conductor terminations must be considered. For example, what minimum size THHN copper conductors are required for a household range rated 17 kW at 120/240 volts? In this example, the temperature ratings of the conductor terminations are unknown. 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. Subtract 12 from 17 (17 – 12 = 5). Since 17 kW exceeds 12 kW by 5, mul-tiply 5 by 5 percent to find the amount Column C must be increased (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 (10 × 1,000 = 10,000 watts). To find amperes, divide the load by the rated voltage (10,000 ÷ 240 = 41.7 = 42 amperes). The minimum rating of the branch circuit sup-plying power to this 17-kW range is 42 amperes. The conductor must have an ampacity of at least 42 amperes (see Figure 2).

THHN conductors are in the 90°C column. In that column, 8 AWG conductors have an allowable ampacity of 55 amperes. But, be-cause of 110.14(C), an 8 AWG conductor is not adequate for this 42-ampere load. Section 110.14(C)(1)(a)(2) states that conductors with higher temperature ratings (such as this THHN conductor) can be installed, but the conductor’s ampacity must be determined based on the 60°C ampacity of the conductor size used. The ampacity out of the 60°C column for an 8 AWG copper conductor is only 40 amperes. The smallest size copper conductor in the 60°C column with an allowable ampacity of at least 42 amperes is 6 AWG copper. The mini-mum size THHN copper conductor required for the household electric range rated 17 kW at 120/240 volts (in this example) is 6 AWG (see Figure 3).

In the last example, the temperature ratings of the conductor terminations were unknown.

If the terminations are listed and identified for use with conductors with higher temperature ratings, it may be permissible to use a higher ampacity. Do not exceed the rating of the lowest rated termination. For example, what minimum size THHN copper conductors are required for a household range rated 19 kW at 120/240 volts? In this example, the temperature ratings of the conductor termina-tions are rated 75°C. Subtract 12 from 19 (19 – 12 = 7). Since 19 kW exceeds 12 kW by 7, multiply 7 by 5 to find the percentage Column C must be increased (7 × 5 = 35%). The maximum demand listed in Column C for one range must be increased 35 percent. The increased amount is 2.8 kW (8 × 35% = 2.8 kW). This increased amount must be added to the Column C demand load for one range (8 + 2.8 = 10.8 kW). Convert kilowatts to watts by multiplying 10.8 by 1,000 (10.8 × 1,000 = 10,800 watts). To find amperes, divide the load by the rated voltage (10,800 ÷ 240 = 45 amperes). The branch-circuit conductor for this range must have an ampacity of at least 45 amperes (see Figure 4).

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.