Electrical Power Distribution Systems 229
RESISTANCE OF CONDUCTORS
The resistance of a conductor expresses the amount of opposition it will
offer to the flow of electrical current. The unit of measurement for resistance
is the ohm (Ω. The ) resistivity (p) of a conductor is the resistance for a speci-
fied cross-sectional area and length. This measurement is given in circular
mil-feet (cmil-ft). The resistivity of a conductor changes with the tempera-
ture, so resistivity is usually specified at a temperature of 20° Celsius. The
resistivity for some common types of conductors is listed in Table 8-2.Table 8-2. Resistivity of Common Conductors
———————————————————————
Conductor Resistivity in ohms per cmil-ft
———————————————————————
Silver 9.8
Copper 10.4
Aluminum 17.0
Tungsten 33.0
Nickel 50.0
Iron 60.0
———————————————————————We can use Table 8-2 to calculate the resistance of any size conductor.
We know that resistance increases as the length increases and decreases
as the cross-sectional area increases. The following method can be used to
find th0e resistance of 500 feet (152.4 meters) of aluminum conductor that
is V. inch (6.35 mm) in diameter. According to Table 8-2, aluminum has a
resistivity of 17 ohms. The diameter (D) equals 1/4 inch, which equals 0.250
inch, which is the equivalent of 250 mils. Using the formula and substitut-
ing, we have:
Resistivity × Length (in feet)
Resistance = ———————————————
Diameter^2 (in mils)17 × 500
= ————
(250)^28,500
= ————
62,500= 0.136 ohms