SELECTION OF WIRE SIZE FACTORS 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 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 nstance, rubber insulation will begin deteriorating at relatively low temperatures, whereas varnished cloth insulation retains its insulating properties at higher temperatures. Other types of insulation are fluorinated ethylene propylene (FEP), silicone rubber, or extruded polytetrafluoroethylene. They are effective at still higher temperatures.