In this chapter you learned that conductors are the means for tying the various components of an electrical or electronic system together. Many factors determine the type of conductor to be used in a specific application.
In order for you to compare the different types and sizes of conductors, we discussed the following factors:
Unit Size - The unit size of a conductor is the mil-foot. A mil-foot is a circular conductor 1 foot long with a diameter of 1 mil (0.001 inch, or one-thousandth of an inch).
Conductor Sizes - The square mil and the circular mil are the units of measure used to determine the cross-sectional area of electrical conductors. The square mil, as it relates to a square conductor, is the cross-sectional area of a square conductor that has a side of 1 mil.
The circular mil is the cross-sectional area of a circular conductor having a diameter of 1 mil. The circular mil area (CMA) of a conductor is computed by squaring the diameter of the circular conductor is mils. Thus, a wire having a diameter of 4 mils (0.004 inch) has a CMA of 42, or
16 circular mils. If the conductor is stranded, the CMA for a strand is computed, and the CMA for the conductor is computed by multiplying the CMA of the strand by the number of strands. The relationship of the square mil to the circular mil is determined by comparing the square mil area of a circular conductor having a diameter of 1 mil (A = πr2) to the circular mil area of the same conductor (D2). Therefore, there is 0.7854 square mil to 1 circular mil. There are more circular mils than square mils in a given area.
Specific Resistance - The specific resistance of a substance is the resistance in ohms offered by a unit volume (the circular-mil-foot) to the flow of electric current. The three factors that are used to calculate the specific resistance of a particular conductor are (1) its length, (2) its cross-sectional area, and (3) the specific resistance of a unit volume of the substance from which the conductor is made. The specific resistance for various sizes and lengths of standard solid copper wire can be determined by the use of tables.
Wire Gauge - A wire gauge is used to determine the American Standard Wire Gauge size of conductors. The measurement of a bare conductor is taken in the slot, not in the circular area at the bottom of the slot.
Selection of Wire Size - Four factors must be considered in selecting the proper wire size for a particular electrical circuit. These factors are (1) conductor size, (2) the material it's made of, (3) the location of the wire in the circuit, and (4) the type of insulation used. Some of the types of insulation used in a high-temperature environment are FEP, extruded polytetrafluoroethylene, and silicone rubber. The ambient (surrounding) temperature of a conductor is an important part of total conductor heating.
Copper-versus-Aluminum Conductors - The two most common metals used for electrical conductors are copper and aluminum.
Some advantages of copper over aluminum as a conductor are that copper has higher conductivity, is more ductile, has a higher tensile strength, and can be easily soldered. Two advantages of aluminum wire for carrying electricity over long distances are its lightness and it reduces corona (the discharge of electricity from a wire at high potential).
Temperature Coefficient of Resistance - The temperature coefficient of resistance is the amount of increase in the resistance of a 1-ohm sample of a conductor per degree of temperature rise above 0°C.
The resistance of copper and other pure metals increases with an increase in temperature.
Conductor Insulation - Insulators have a resistance that is so great that, for all practical purposes, they are nonconductors.
Two fundamental properties of insulating materials are (1) insulation resistance and (2) the resistance to current leakage through the insulation. Dielectric strength is the ability of the insulation material to withstand potential difference. The dielectric strength of an insulator is determined by raising the voltage on a test sample until it breaks down.
Insulating Materials - Some common insulating materials have properties and safety precautions that should be remembered. These are:
The purpose of coating a copper conductor with tin when rubber insulation is used is to prevent the insulation from deteriorating due to chemical action.
When extruded polytetrafluoroethylene insulation is heated, caution should be observed not to breathe the vapors.
The most commonly used insulating materials for extremely high-voltage conductors are varnished cambric and oil-impregnated paper.
The Navy is getting away from using asbestos insulation because asbestos fibers can cause lung disease and/or cancer.
Conductor Protection - There are several types of conductor protection in use. The type commonly used aboard Navy ships is wire-braid armor.