Specifications

21
The circular forms of wire gauge measurement devices are the most popular, and are
generally 3 3/4 inch. (95 mm) in diameter, with thirty-six notches; may have the decimal
equivalents of the sizes stamped on the back. Oblong plates are similarly notched. Many gauges
are made with a wedge-like slot into which the wire is thrust; one edge being graduated, the
point at which the movement of the wire is arrested gives its size. The graduations are those of
standard wire, or in thousandths of an inch. In some cases both edges are graduated differently
to serve for comparison between two systems of measurement. A few gauges are made with
holes into which the wire has to be thrust. All gauges are hardened and ground to dimensions.
Fig. 2.1 - Gauge plate
In some applications wire sizes are specified as the cross sectional area of the wire, usually
in mm². Advantages of this system include the ability to readily calculate the physical dimensions
or weight of wire, ability to take account of non-circular wire, and ease of calculation of electrical
properties. This determines the amount of electric current a wire can safely carry, as well as its
electrical resistance and weight per unit of length. Wire gauge is applicable to both electrical
and non-electrical wires, mostly used in electrical wiring.
The basis being the mil. No. 7/0, the largest size, is 0.50 in. (500 mils or 12.7 mm) in
diameter, and the smallest, No. 50, is 0.001 in. (1 mil or about 25 µm) in diameter. Between
these, the diameter, or thickness, decreases by 10.557%, and the weight reduces by 20%. The
‘mil’ denotes one thousandth part of an inch (more commonly denoted as ‘thou’). The circular
mil is the equivalent area of a circle whose diameter is 0.001" (10
-3
) inch.
This can also be expressed as the diameter in thousandths of an inch raised to the power
2 and may or may not be rounded to the nearest ten. The circular mil is an old unit of cross
sectional area, used for denoting the cross-sectional size of an electrical conductor or cable.