The lower limit is specified for rubber covered wires to prevent gradual deterioration of the high insulations by the heat of the wires, but not from fear of igniting the insulation. The question of drop is not taken into consideration in the table on [page 731].
The carrying capacity of Nos. 16 and 18 B. & S. gauge wire is given, but no smaller than No. 14 is to be used, except as allowed under rules for fixture wiring.—Underwriters' Rules.
Circular Mils.—The unit of measurement in measuring the cross sectional area of wires is the circular mil; it is the area of a circle one mil (.001 in.) in diameter.
The area of a wire in circular mils is equal to the square of the diameter in mils.
Fig. 816.—Diagram illustrating circular mils. The circular mil is used as a unit of cross sectional area in measuring wires. It is equal to the area of a circle .001 in. diameter; its value is .0000007854 square inch. In the figure the sum of the areas of the nine small circles equals the area of the large circle. This is evident from the following: Take the diameter of the small circles as unity, then the diameter of the large circle is three. Hence, the sum of the area of the small circles × (¼ π × 12) × 9 = .7854 × 9 = 7.0686; area of the large circle = ¼ π × 32 = .7854 × 9 = 7.0686. Therefore since the area of the large circle equals the sum of the areas of the small circles, the area of a wire in circular mils is equal to the square of its diameter expressed in mils.
Thus a wire 2 mils in diameter (.002 in.) has a cross sectional area of 2 × 2 = circular mils. Accordingly to obtain the area of a wire in circular mils, measure its diameter with a micrometer which reads directly in mils or thousandths of an inch, and square the reading.
The circular mil (abbreviated C.M.) applies to all round conductors, and has a value of .7854 times that of the square mil, that is, 1 circular mil = .7854 square mil. If the diameter be expressed as a fraction of an inch, as for instance 1/3 in., the circular mil area may be found as follows: Reduce the fraction 1/3 to the decimal of an inch, multiply the result by 1,000 to express the diameter in mils, and square the diameter so expressed, thus: 1/3 = 1,000 ÷ 3 = .333. .333 × 1,000 = 333 mils; 333 × 333 = 110,889 circular mils.
The diameter of any wire may be found when its circular mil area is known by extracting the square root of the circular mil area.
Square Mils.—For measuring conductors of square or rectangular cross section, such as bus bars, copper ribbon, etc., the square mil is used. A square mil is the area of a square whose sides are one mil (.001 in. long) and is equal to .001 × .001 = .000001 square inch.
