Transmission Techniques: Wire and Cable 403and environment conditions such as ambient temperature
and air flow. The four lines marked with temperatures
apply to different insulation plastics and their melting
point. Consult the manufacturer’s Web site or catalog for
the maximum insulation or jacket temperature.
The current ratings of Fig. 14-1 are intended as
general guidelines for low-power electronic communi-
cations and control applications. Current ratings for
high-power applications generally are set by regulatory
agencies such as Underwriters Laboratories (UL), Cana-
dian Standards Association (CSA), National Electrical
Code (NEC), and others and should be used before final
installation.
Table 310-15(b)(2)(a) of the NEC contains amperage
adjustment factors for whenever more than three current
carrying conductors are in a conduit or raceway.
Section 240-3 of the NEC provides requirements for
overload protection for conductors other than flexible
cords and fixture wires. Section 240-3(d), Small
Conductors, states that #14 to #10 conductors require a
maximum protective overcurrent device with a rating no
higher than the current rating listed in the 60°C column.
These currents are 15 A for #14 copper wire, 20 A for
#12 copper wire, and 30 A for #10 copper wire. These
values are familiar as the breaker ratings for commercial
installations.
When connecting wire to a terminal strip or another
wire etc., the temperature rise in the connections must
also be taken into account. Often the circuit is not
limited by the current carrying capacity of the wire but
of the termination point.14.2.9.1 Wire Current RatingsCurrent carrying capacity of wire is controlled by the
NEC, particularly in Table 310-16, Table 310-
15(b)(2)(a), and Section 240-3.
Table 310-16 of the NEC shows the maximum
current carrying capacity for insulated conductors rated
from 0 to 2000 V, including copper and aluminum
conductors. Each conductor amperage is given for three
temperatures: 60°C, 75°C, and 90°C. Copper doesn’t
melt until almost 2000q so the current limit on a copper
wire is not the melting point of the wire but the melting
point of the insulation. This number is listed by most
manufacturers in their catalog or on their Web site. For
instance, PVC (polyvinyl chloride) can be formulated to
withstand temperatures from 60qC to as high as 105qC.
The materials won’t melt right at the specified tempera-
ture, but may begin to fail certain tests, such as cracking
when bent.14.3 InsulationWire can be bare, often called bus bar or bus wire, but is
most often insulated. It is covered with a non-conducting
material. Early insulations included cotton or silk woven
around the conductor, or even paper. Cotton-covered
house wiring can still be found in perfect operating con-
dition in old houses. Today, most insulation materials are
either some kind of rubber or some kind of plastic. The
material chosen should be listed in the manufacturer’s
catalog with each cable type. Table 14-7 lists some of
the rubber-based materials with their properties. Table
14-8 lists the properties of various plastics. The ratings
in both tables are based on average performance of gen-
eral-purpose compounds. Any given property can usu-
ally be improved by the use of selective compounding.14.3.1 Plastics and Dielectric ConstantTable 14-9 is a list of various insulation materials with
details on performance, requirements, and special
advantages. Insulation, when used on a cable intended to
carry a signal, is often referred to as a dielectric. The
performance of any material, its ability to insulate with
minimal effect to the signal running on the cable is
called the dielectric constant and can be measured in aFigure 14-1. Current ratings for electronic cable. Courtesy
Belden.
26 24 22 20 18 16 14 12 10 8100
80
60
40
30
2010
8 6 4 3 2 1
Current—AConductor size—AWG28# Conductors* Factor # Conductors* Factor
1 1.6 6—15 0.7
2—3 1.0 16—30 0.5
4—5 0.8
* do not count shields unless used as a conductor200 °C rating150 °C rating105 °C rating60 & 80 °C rating