Wire sizes have been for many years indicated in commercial practice almost entirely by gage numbers, especially in America and England. This practice is accompanied by some confusion because numerous gages are in common use.

The most commonly used gage for electrical wires, in America, is the American wire gage. The most commonly used gage for steel wires is the Birmingham wire gage.

There is no legal standard wire gage in this country, although a gage for sheets was adopted by Congress in 1893. In England, there is a legal standard known as the Standard wire gage.

In Germany, France, Austria, Italy, and other continental countries, practically no wire gage is used, but wire sizes are specified directly in millimeters. This system is sometimes called the millimeter wire gage. The wire sizes used in France, however, are based to some extent on the old Paris gage ( jauge de Paris de 1857) (for a history of wire gages, see NBS Handbook 100, Copper Wire Tables; also see Circ. 67, Wire Gages, 1918).

There is a tendency to abandon gage numbers entirely and specify wire sizes by the diameter in mils (thousandths of an inch). This practice holds particularly in writing specifications and has the great advantages of being both simple and explicit. A number of wire manufacturers also encourage this practice, and it was definitely adopted by the U.S. Navy Department in 1911.

Mil is a term universally employed in this country to measure wire diameters and is a unit of length equal to one-thousandth of an inch. Circular mil is a term universally used to define crosssectional areas, being a unit of area equal to the area of a circle 1 mil in diameter.

Such a circle, however, has an area of 0.7854 (or p/4) mil2. Thus a wire 10 mils in diameter has a cross-sectional area of 100 cmils or 78.54 mils2. Hence, a cmil equals 0.7854 mil2.

American wire gage, also known as the Brown & Sharpe gage, was devised in 1857 by J. R. Brown. It is usually abbreviated AWG. This gage has the property, in common with a number of other gages, that its sizes represent approximately the successive steps in the process of wire drawing.

Also, like many other gages, its numbers are retrogressive, a larger number denoting a smaller wire, corresponding to the operations of drawing. These gage numbers are not arbitrarily chosen, as in many gages, but follow the mathematical law upon which the gage is founded.

Basis of the AWG is a simple mathematical law. The gage is formed by the specification of two diameters and the law that a given number of intermediate diameters are formed by geometric progression.

Thus, the diameter of No. 0000 is defined as 0.4600 in and of No. 36 as 0.0050 in. There are 38 sizes between these two. The square of this ratio # 1.2610. The sixth power of the ratio, that is, the ratio of any diameter to the diameter of the sixth greater number, # 2.0050. The fact that this ratio is so nearly 2 is the basis of numerous useful relations or shortcuts in wire computations.

There are a number of approximate rules applicable to the AWG which are useful to remember:
1. An increase of three gage numbers (e.g., from No. 10 to 7) doubles the area and weight and consequently halves the dc resistance.
2. An increase of six gage numbers (e.g., from No. 10 to 4) doubles the diameter.
3. An increase of 10 gage numbers (e.g., from No. 10 to 1/0) multiplies the area and weight by 10 and divides the resistance by 10.
4. A No. 10 wire has a diameter of about 0.10 in, an area of about 10,000 cmils, and (for standard
annealed copper at 20°C) a resistance of approximately 1.0 #/1000 ft.
5. The weight of No. 2 copper wire is very close to 200 lb/1000 ft (90 kg/304.8 m).

Steel wire gage, also known originally as the Washburn & Moen gage and later as the American Steel & Wire Co.’s gage, was established by Ichabod Washburn in 1830. This gage, with a number of its sizes rounded off to thousandths of an inch, is also known as the Roebling gage. It is used exclusively for steel wire and is frequently employed in wire mills.

Birmingham wire gage, also known as Stubs’ wire gage and Stubs’ iron wire gage, is said to have been established early in the eighteenth century in England, where it was long in use. This gage was used to designate the Stubs soft-wire sizes and should not be confused with Stubs’ steel-wire gage.

The numbers of the Birmingham gage were based on the reductions of size made in practice by drawing wire from rolled rod. Thus, a wire rod was called “No. 0,” “first drawing No. 1,” and so on. The gradations of size in this gage are not regular, as will appear from its graph. This gage is generally in commercial use in the United States for iron and steel wires.

Standard wire gage, which more properly should be designated (British) Standard wire gage, is the legal standard of Great Britain for all wires adopted in 1883. It is also known as the New British Standard gage, the English legal standard gage, and the Imperial wire gage.

It was constructed by so modifying the Birmingham gage that the differences between consecutive sizes become more regular. This gage is largely used in England but never has been used extensively in America.

Old English wire gage, also known as the London wire gage, differs very little from the Birmingham gage. Formerly it was used to some extent for brass and copper wires but is now nearly obsolete.

Millimeter wire gage, also known as the metric wire gage, is based on giving progressive numbers to the progressive sizes, calling 0.1 mm diameter “No. 1,” 0.2 mm “No. 2,” etc.

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