The National Electrical Code (NEC) is divided into an introduction and nine chapters with 10 informational annexes. As specified in 90.5(D), the informative annexes are not part of the NEC’s enforceable requirements but are included for informational purposes only. Like annexes, informational notes are not enforceable. 


Chapter 1 contains two articles: Article 100—Definitions and Article 110­—Requirements for Electrical Installations. Article 110 contains some foundational rules, such as those pertaining to the approval of conductors and equipment, mechanical execution of work, electrical connections, and enclosure types.


Another considerable portion of this article is dedicated to spaces about, or around, electrical equipment.


There are thousands of conductor references in the Code. The first requirements pertaining to conductors are in Article 110. As stated in the first sentence of 110.5, conductors normally used to carry current shall be copper unless otherwise provided in the NEC. Although this section specifically mentions copper, aluminum and copper-clad aluminum also are permitted. The informational note under this section instructs readers to see 310.15 for aluminum and copper-clad aluminum conductors. Most of the time, when a section references an aluminum conductor, the same section will also reference a copper-clad aluminum conductor. For example, as stated in 230.23(A), the minimum overhead service conductor size shall be 8 AWG copper or 6 AWG aluminum or copper-clad aluminum. The ampacities listed in Table 310.15(B)(16) are the same for copper-clad aluminum as they are for aluminum. As defined in Article 100, copper-clad aluminum conductors are drawn from a copper-clad aluminum rod, with the copper metallurgically bonded to an aluminum core, where the copper forms a minimum of 10 percent of the cross-sectional area of a solid conductor or each strand of a stranded conductor.


Where the conductor material is not specified, the material and the sizes given in the NEC shall apply to copper conductors [110.5]. Throughout the Code, the conductor type usually is shown, but sometimes it is not. For example, Exception No. 2 to 210.19(A)(3) shows a minimum size neutral conductor for a certain size range. The minimum size neutral conductor is 10 AWG, but it does not specify copper or aluminum. The neutral conductor of a 3-wire branch circuit supplying a household electric range, a wall-mounted oven or a counter-mounted cooking unit shall be permitted to be smaller than the ungrounded conductors where the maximum demand of a range of 83/4 kilowatts (kW) or more rating has been calculated according to Column C of Table 220.55. However, such a conductor shall have an ampacity of not less than 70 percent of the branch-circuit rating and shall not be smaller than 10 AWG [210.19(A)(3) Exception No. 2]. Because the conductor material is not specified in this exception, the minimum size neutral conductor is 10 AWG copper (see Figure 1).


The last sentence in 110.5 states, where other materials are used, the size shall be changed accordingly. Most of the time in the Code, when a copper conductor size is shown, an aluminum and copper-clad aluminum conductor size is also shown. As long as it would be permissible to install an aluminum conductor and the aluminum conductor size is not shown, find the equivalent size aluminum from the copper conductor that is shown. For example, in Figure 1, the minimum size neutral conductor was 10 AWG copper. If copper conductors will be installed to supply a 9-kW electric range, the minimum size neutral conductor is 10 AWG. It does not automatically mean an aluminum conductor is not permitted if an aluminum conductor size is not shown. If aluminum conductors will be installed to supply a 9-kW range, it will be necessary to find the equivalent minimum size aluminum conductor from the minimum size copper conductor shown. The ampacities shown in Table 310.15(B)(16) for 10 AWG copper in the 60°C, 75°C and 90°C columns are 30, 35 and 40 amperes (A) respectively. The ampacities shown in the same table for 10 AWG aluminum in the 60°C, 75°C and 90°C columns are 25, 30 and 35A respectively. Since the ampacities for 10 AWG aluminum are not equal to or higher than the ampacities for 10 AWG copper, it is necessary to look at the next larger size aluminum conductor. The ampacities shown for 8 AWG aluminum in the 60°C, 75°C and 90°C columns are 35, 40 and 45A respectively. Because of the ampacities, an 8 AWG aluminum conductor is equivalent to a 10 AWG copper conductor. Therefore, the minimum aluminum conductor that can be installed in accordance with 210.19(A)(3) Exception No. 2 is 8 AWG aluminum (see Figure 2).


If a provision shows a copper conductor but does not show an aluminum conductor, there may be a good reason. For example, 250.52(A)(3)(2) states that a bare copper conductor not smaller than 4 AWG and having a length of at least 20 feet shall be permitted as a concrete-encased electrode. In this section, an aluminum conductor is not mentioned. Normally, a 2 AWG aluminum conductor would be an acceptable equivalent to 4 AWG copper, but installing an aluminum grounding electrode conductor is not permitted. In accordance with 250.52(B), aluminum shall not be used as a grounding electrode. Another section in Article 250 also has something to say about aluminum in or near the earth. Bare aluminum or copper-clad aluminum grounding electrode conductors shall not be used where in direct contact with masonry or the earth or where subject to corrosive conditions [250.64(A)]. Where used outside, aluminum or copper-clad aluminum grounding electrode conductors shall not be terminated within 18 inches of the earth.


There are times when finding the aluminum equivalent to copper is not necessary. For example, where raceways contain 4 AWG or larger insulated circuit conductors, and these conductors enter a cabinet, a box, an enclosure or a raceway, the conductors shall be protected by an identified fitting that provides a smoothly rounded insulating surface, unless the conductors are separated from the fitting or raceway by identified insulating material that is securely fastened in place [300.4(G)]. With this requirement for conductor protection, it does not matter if the conductor is copper or aluminum. Whether the raceway contains 4 AWG (or larger) insulated copper or 4 AWG (or larger) insulated aluminum, the conductors shall be protected by an identified fitting that provides a smoothly rounded insulating surface, unless the conductors are separated from the fitting or raceway by identified insulating material that is securely fastened in place (see Figure 3).


As stated in 110.6, conductor sizes are expressed in AWG or in circular mils. Because “AWG” is in the Code more than 600 times, it is necessary to explain the abbreviation. Section 110.6 explains that AWG is the abbreviation for American Wire Gage. It is interesting to note that this section is the only place the full term shows up in the Code. The most common AWG sizes in the Code are 18 AWG through 4/0 AWG. This section states that conductor sizes are also expressed in circular mils. Because circular-mil sizes are needed in some calculations, the circular-mil area of various conductors are shown in Chapter 9, Table 8. The circular-mil area is shown for all of the conductors in Table 8, even for the conductors expressed as AWG conductors. For example, what is the circular-mil area for the following conductors: 12 AWG, 6 AWG and 4/0 AWG? In accordance with Table 8 in Chapter 9, the circular-mil area for 12 AWG is 6,530 circular mils; the circular-mil area for 6 AWG is 26,240 circular mils; and the circular-mil area for 4/0 AWG is 211,600 circular mils. Although it is not mentioned in 110.6, a common abbreviation used throughout the Code is “kcmil,” which stands for one-thousand circular mils. In the circular mils column in Table 8, no circular mils are shown for conductors larger than 4/0 AWG. This is because the number or name of the conductor is the circular-mil area of the conductor. For example, the circular-mil area for a 500 kcmil conductor is 500,000 circular mils. Out in the field, conductors are usually referred to as “MCM” instead of “kcmil.” MCM also stands for one-thousand circular mils. M is the Roman numeral for 1,000. The letters CM is an abbreviation for circular mil. Therefore, the circular-mil area for a 500 MCM conductor is 500,000 circular mils. In the 1990 edition of the NEC, MCM was replaced with kcmil. Because the Code contains International System of Units (SI), Roman numerals were replaced with metric symbols. The metric prefix for 1,000 is kilo, and the symbol is k. The letters cmil are an abbreviation for circular mil. Therefore, the circular-mil area for a 500 kcmil conductor is 500,000 circular mils. Metric units were first introduced in the 1981 edition of the NEC.


Next month’s column continues the discussion of Article 110.