Electrical Power Systems Technology

Electrical Power Distribution Systems 227

ver to produce a conductor with better solderability and, also, a conductor

that has better high-frequency characteristics. This is due to the high con-

ductivity of the silver and the “skin effect” present at higher frequencies.

Where little vibration and no flexing are required of a wire or cable,

single-strand conductors may be used. The advantage of a single-strand

conductor is its lower cost compared to that of equivalent types of strand-

ed wire. Wire and cable with solid conductors may be used as intercon-

nection wires for electrical instruments and similar equipment. Stranded

conductors are used to provide more flexibility. They also have a longer

usage life than do solid conductors. If a solid conductor were cut by wire

strippers during its installation, it would probably break after being bent

a few times. However, stranded wire would not break in this situation.

Wires having from 26 to 41 strands may be used where much flexibility is

needed, while wires with from 65 to 105 strands may be used for special

purposes.

Flat or ound braided conductors r are occasionally used for certain ap-

plications where they are better suited than solid or stranded cables. These

conductors are seldom insulated, since this would hinder their flexibility.

CONDUCTOR AREA

The unit of measurement for conductors is the circular mil (cmil),

since most conductors are round. One mil is equal to 0.001 inch (0.0254

mm); thus, one cmil is equal to a circle whose diameter is 0.001 inch. The

cross-sectional area of a conductor (in cmils) is equal to its diameter (D), in

mils squared, or cmil = D^2. For example, if a conductor is 1/4 inch (6.35

mm) in diameter, its circular mil area can be found as follows. The deci-

mal equivalent of 1/4 inch is 0.250 inch, which equals 250 mils. Inserting

this value into the formula for the cross-sectional area of a conductor gives

you:

Area = D^2 (in mils)

= (250)^2

= 62,500 cmils

If the conductor is not round, its area may still be found by applying

the following formula: