Electrical Power Distribution Systems 237
insulations almost unlimited. Most insulation is composed of compounds
made of synthetic rubber polymers (thermosetting) and from synthetic
materials (thermoplastics). These synthetic materials are combined to pro-
vide specific physical and electrical properties.
Thermosetting materials are characterized by their ability to be
stretched, compressed, or deformed within reasonable limits under me-
chanical strain, and then to return to their original shape when the stress
is removed.
Thermoplastic insulation materials are best known for their excellent
electrical characteristics and relatively low cost. These materials are popu-
lar, since they allow much thinner insulation thicknesses to be used to ob-
tain good electrical properties, particularly at the higher voltages.
There are many types of insulation used today for electrical conduc-
tors. Some new materials have been developed that will last for exception-
ally long periods of time and will withstand very high operating temper-
atures. The operating conditions where the conductors are used mainly
determine the type of insulation required. For instance, system voltage,
heat, and moisture affect the type of insulation required. Insulation must
be used that will withstand both the heat of the surrounding atmosphere
and the heat developed by the current flowing through the conductor.
Exceptionally large currents will cause excessive heat to be developed in
a conductor. Such heat could cause insulation to melt or burn. This is why
overcurrent protection is required as a safety factor to prevent fires. The
ampacity or current-carrying capacity of a conductor depends upon the
type of insulation used. The NEC has developed a system of abbreviations
for identifying various types of insulation. Some of the abbreviations are
shown in Table 8-7.Table 8-7. Common Abbreviations for Types of Electrical Insulation
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Abbreviation Type of Insulation
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R Rubber—140° F
RH Heat-Resistant Rubber—167° F
RHH Heat-Resistant Rubber—194° F
RHW Moisture and Heat-Resistant Rubber—167° F
T Thermoplastic-140° F
THW Moisture and Heat-Resistant Thermoplastic—167° F
THWN Moisture and Heat-Resistant Thermoplastic With Nylon—194° F
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