What is the American wire gage?
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; hence the ratio of any diameter to the diameter of
the next greater number is given by this expression
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).
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