1-13
Table 1-4.—Comparative Characteristics of Copper and Aluminum
CHARACTERISTICS
COPPER
ALUMINUM
Tensile strength (lb/in2).
55,000
25,000
Tensile strength for same
conductivity (lb).
55,000
40,000
Weight for same conductivity (lb).
100
48
Cross section for same conductivity
(C.M.).
100
160
Specific resistance ("/mil ft).
Copper has a higher conductivity than aluminum. It is more ductile (can be drawn out). Copper has
relatively high tensile strength (the greatest stress a substance can bear along its length without tearing
apart). It can also be easily soldered. However, copper is more expensive and heavier than aluminum.
Although aluminum has only about 60 percent of the conductivity of copper, its lightness makes long
spans possible. Its relatively large diameter for a given conductivity reduces corona. Corona is the
discharge of electricity from the wire when it has a high potential. The discharge is greater when smaller
diameter wire is used than when larger diameter wire is used. However, the relatively large size of
aluminum for a given conductance does not permit the economical use of an insulation covering.
Q14. State two advantages of using aluminum wire for carrying electricity over long distances.
Q15. State four advantages of copper over aluminum as a conductor.
TEMPERATURE COEFFICIENT
The resistance of pure metals, such as silver, copper, and aluminum, increases as the temperature
increases. However, the resistance of some alloys, such as constantan and manganin, changes very little
as the temperature changes. Measuring instruments use these alloys because the resistance of the circuits
must remain constant to get accurate measurements.
In table 1-1, the resistance of a circular-mil-foot of wire (the specific resistance) is given at a specific
temperature, 20º C in this case. It is necessary to establish a standard temperature. As we stated earlier,
the resistance of pure metals increases with an increase in temperature. Therefore, a true basis of
comparison cannot be made unless the resistances of all the substances being compared are measured at
the same temperature. The amount of increase in the resistance of a 1-ohm sample of the conductor per
degree rise in temperature above 0º C is called the temperature coefficient of resistance. For copper, the
value is approximately 0.00427 ohm.
A length of copper wire having a resistance of 50 ohms at an initial temperature of 0º C will have an
increase in resistance of 50 0.00427, or 0.214 ohms. This applies to the entire length of wire and for
each degree of temperature rise above 0º C. A 20º C increase in resistance is approximately 20 0.214, or
4.28 ohms. The total resistance at 20º C is 50 + 4.28, or 54.28 ohms.
Q16.
Define the temperature coefficient of resistance.
Q17.
What happens to the resistance of copper when it is heated?
CONDUCTOR INSULATION
To be useful and safe, electric current must be forced to flow only where it is needed. It must be
"channeled" from the power source to a useful load. In general, current-carrying conductors must not be
allowed to come in contact with one another, their supporting hardware, or personnel working near them.
