SELECTION OF WIRE SIZE
Several factors must be considered in selecting the size of wire to be used for transmitting and
distributing electric power. These factors will be discussed throughout this section. Military specifications
cover the installation of wiring in aircraft, ships, and electrical/electronic equipment. These specifications
describe the technical requirements for material purchased from manufacturers by the Department of
Defense. An important reason for having these specifications is to ensure uniformity of sizes to reduce the
danger of fires caused by the improper selection of wire sizes. Wires can carry only a limited amount of
current safely. If the current flowing through a wire exceeds the current-carrying capacity of the wire,
excess heat is generated. This heat may be great enough to burn off the insulation around the wire and
start a fire.
FACTORS GOVERNING THE CURRENT RATING
The current rating of a cable or wire indicates the current capacity that the wire or cable can safely
carry continuously. If this limit, or current rating, is exceeded for a length of time, the heat generated may
burn the insulation. The current rating of a wire is used to determine what size is needed for a given load,
or current drain.
The factors that determine the current rating of a wire are the conductor size, the location of the wire
in a circuit, the type of insulation, and the safe current rating. Another factor that will be discussed later in
this chapter is the material the wire is made of. As you have already seen, these factors also affect the
resistance in ohms of a wire-carrying current.
An increase in the diameter, or cross section, of a wire conductor decreases its resistance and
increases its capacity to carry current. An increase in the specific resistance of a conductor increases its
resistance and decreases its capacity to carry current.
The location of a wire in a circuit determines the temperature under which it operates. A wire may be
located in a conduit or laced with other wires in a cable. Because it is confined, the wire operates at a
higher temperature than if it were open to the free air. The higher the temperature under which a wire is
operating, the greater will be its resistance. Its capacity to carry current is also lowered. Note that, in each
case, the resistance of a wire determines its current-carrying capacity. The greater the resistance, the more
power it dissipates in the form of heat energy.
Conductors may also be installed in locations where the ambient (surrounding) temperature is
relatively high. When this is the case, the heat generated by external sources is an important part of the
total conductor heating. This heating factor will be explained further when we discuss temperature
coefficient. We must understand how external heating influences how much current a conductor can
carry. Each case has its own specific limitations. The maximum allowable operating temperature of
insulated conductors is specified in tables. It varies with the type of conductor insulation being used.
The insulation of a wire does not affect the resistance of the wire. Resistance does, however,
determine how much heat is needed to burn the insulation. As current flows through an insulated
conductor, the limit of current that the conductor can withstand depends on how hot the conductor can get
before it burns the insulation. Different types of insulation will burn at different temperatures. Therefore,
the type of insulation used is the third factor that determines the current rating of a conductor. For