54 Fundamentals of Electricity and Electronics
transfer of electrical energy occurs at an amazing speed.
The speed has been accurately measured, and it
approaches the speed of light: 186,000 miles per second!
In metric terms, this speed is 300,000 kilometers (or
300,000,000 meters) per second.
The unit for measuring conductance is the siemens.
This unit was named after the German inventor Ernst von
Siemens, who did a great deal of work in the development
of telegraphy use. The abbreviation for the siemens is the
capital letter S. The unit formula for computing electrical
conductance is as follows:
where G is conductance (in siemens) and R is resistance
(in ohms).
The conductance of a material is the reciprocal of the
resistance of the material.
LESSON IN SAFETY:
Your body is a good conductor of electricity. Never
touch a circuit conductor or component unless you are
sure that it is not energized. An electric current flow-
ing in one hand, across your chest and heart, and out
your other hand, is dangerous and can be fatal. The
smart technician uses only one hand when working on
high-voltage circuits. The other hand is kept in a
pocket. Technicians who work on high voltage lines
work in insulated bucket trucks or on insulated plat-
forms. Standing on a rubber mat while working on
electrical systems is a standard safety (and a wise)
practice. While the technician is standing on an insu-
lated material, there is no direct path to ground in
case of accidental contact with conductors.
Conductor Sizes
Conductors are sized by their cross-sectional area.
The amount of cross-sectional area a conductor has deter-
mines how much current it can handle without overheat-
ing. Circular conductors are arranged according to size by
the American Wire Gauge System. The larger the gauge
number, the smaller the cross-sectional area a wire will
have. For example, a No. 14 wire is larger in diameter
than a No. 20 wire. See Figure 3-2.
This system of wire sizes was developed over a hun-
dred years ago. At that time, no one could foresee that the
demand for electrical products would make this system
obsolete. Some wire sizes are now larger than the No. 1
1
R
G =
conductor. A temporary solution to this problem created
the next set of larger sizes. The next four sizes that were
developed are 0 (pronounced “ought”), 00, 000, and
0000–or one through four ought (1/0 through 4/0).
When it became apparent that even larger sizes
would be needed, the final solution to the sizing problem,
a third set of sizes, was developed. The final system of
sizing is the circular mil system. The circular mil system
is based on the diameter of the conductor measured in
mils (1/1000 of an inch). Typical conductors larger than
four ought are from 250 kcm to 2000 kcm. The letters
kcm represent 1000 circular mils, so a 250 kcm repre-
sents 250,000 circular mils. Figure 3-3 contains all the
wire sizes you would normally encounter.
Circular Mils
Circular mil area is the common way to express the
cross-sectional area of a conductor. As previously noted,
one mil is equal to 1/1000th of an inch (0.001 inches).
The number of circular mils (cmil) in a conductor is
equal to the diameter of a round conductor squared (D2),
where D is in mils. Therefore, a wire with the diameter of
1 mil has an area of 1 circular mil (1 cmil). A wire with a
diameter of 2 mils has an area of 4 cmil (2 2 = 4). A
wire with a diameter of 15 mils gives us an area of 225
cmil (15 15 = 225). See Figure 3-4.
The circular mil is a more convenient method of
expressing the size of a conductor than πr2, which is used
to compute the area of a circle. If we must find the area
of a square or rectangular conductor, their equivalent in
cmils is not difficult to find. To find the circular mils
Figure 3-2. A gauge is used to find wire size.