86 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. Larger wires are designated by their actual cross-sectional area in terms of kcmils (thou- sands of circular mils). A mil is 0.001 inch (a thousandth of an inch) and one inch is 1000 mils. One circular mil (1 cmil) is equivalent to the area of a circle one mil in diameter. This is a very small area. Table 8 of Chapter 9 of the Code is shown in Figure 6-3. The fi rst column of the table contains fi wire size. From 18 AWG through 4/0 AWG, the sizes are expressed as AWG. The corresponding area in circular mils is shown in the second col- umn. This area is the actual area of the conduct- ing material for stranded conductors, the voids between strands are not included. The sixth column contains the overall area expressed in square inches. This area includes the voids in stranded conductors, so it is slightly larger than the circular mils area. See Figure 6-4. Conductors larger than 4/0 AWG are expressed in thousand circular mils (kcmil). Conductors that are 250 kcmil (250,000 cmils) are larger than 4/0 AWG conductors (211,600 cmils). The relation between inches and mils can be easily calculated: 1 mil = 0.001 inch 1 inch = 1000 mils 1 cmil = 0.785 mils2 1 mils2 = 0.000001 in2 1 in2 = 0.000000785 cmil. SAMPLE PROBLEM 6-1 Problem: Determine i the area of a 6 AWG co op - per conductor in sq qu are inches. Solution: From C Ch apter 9, 9 Table 8, the area a of a 6 AWG conductor o is 26240 cmils. To convert this t to square inches, multiply y cmils by 0.000000785. 5 26240 cmils s × 0.000000785 = 0.0206 6 in2 This value differs iff from the 0.027 in2 ff 2 shown in Table 8 because e the calculated valu ue is the actual area o of the strands. The area in n the table includes the e voids between strands. 6.1.2 Conductor Material There are three types of conducting material used for general wiring: copper, aluminum, and copper-clad aluminum. Copper has excellent electrical and physical properties. It has been used as a conductor material from the earliest efforts to conduct electricity. Aluminum is a good conductor, but must be installed with additional care and prepara- tion. Aluminum must be cleaned just prior to terminating due to the rapid oxidation of the freshly exposed surface. Exposed aluminum at terminals should be coated with an antioxi- dant. In addition, aluminum connections tend to deform and loosen more than copper connec- tions. If installed properly, aluminum conduc- tors are dependable and economical. Aluminum has a greater resistance than cop- per, requiring a larger size to equal the same ampacity as copper—generally, two sizes larger. For example, a 2/0 AWG copper conductor will carry 200 amperes, whereas a 4/0 AWG alumi- num conductor would be required to handle the same current. CODE ALERT Copper-Clad Aluminum According to Article 100, copper-clad aluminum conductors are conductors drawn from a cop- per-clad aluminum rod with the copper metal- lurgically 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. SAFETY NOTE Improperly conn c ne cted d aluminum u nu u condu o uc tors o can deteriora r at e w wi th time t m e and c ca use pro ob lems e in the electrical ct a sys st em. em Alw wa wa ys us u se devicesv d approved for f use with w t alumi m in um conduc n ct ors and follow w proper o installation n st l n n procedur e re s.