398 Chapter 14
tions, such as coaxial cables, can be difficult to deter-
mine from just knowing the constituent parts. The
center conductor might be easy to determine but a braid
or braid + foil shield can be difficult. In those cases,
consult the manufacturer.
Table 14-1 show the resistance in ohms (ȍ) per foot
per circular mil area for various metals, and combina-
tions of metals (alloys). Of the common metals, silver is
the lowest resistance. But silver is expensive and hard to
work with. The next material, copper, is significantly
less expensive, readily available, and lends itself to
being annealed, which is discussed in Section 14.2.4.
Copper is therefore the most common material used in
the manufacture of wire and cable. However, where
price is paramount and performance not as critical,
aluminum is often used. The use of aluminum as the
conducting element in a cable should be an indication to
the user that this cable is intended to be lower cost and
possibly lower performance.
One exception to this rule might be the use of
aluminum foil which is often used in the foil shielding
of even expensive high-performance cables. Another
exception is emerging for automobile design, where the
weight of the cable is a major factor. Aluminum is
significantly less weight than copper, and the short
distances required in cars means that resistance is less
of a factor.
Table 14-1 may surprise many who believe, in error,
that gold is the best conductor. The advantage of gold is
its inability to oxidize. This makes it an ideal covering
for articles that are exposed to the atmosphere, pollu-
tion, or moisture such as the pins in connectors or the
connection points on insertable circuit boards. As a
conductor, gold does not require annealing, and is often
used in integrated circuits since it can be made into very
fine wire. But, in normal applications, gold would make
a poor conductive material, closer to aluminum in
performance than copper.
One other material on the list commonly found in
cable is steel. As can be seen, this material is almost ten
times the resistance of copper, so many are puzzled by
its use. In fact, in the cables that use steel wires, they are
coated with a layer of copper, called copper-clad steel
and signal passes only on the copper layer, an effect
called skin effect that will be discussed in Section
14.2.8. Therefore, the steel wire is used for strength and
is not intended to carry signals.
Copper-clad steel is also found in cables where cable
pulling strength (pulling tension) is paramount. Then a
stranded conductor can be made up of many
copper-clad steel strands to maximize strength. Such a
cable would compromise basic resistive performance.
As is often the case, one can trade a specific attribute
for another. In this case, better strength at the cost of
higher resistance.14.2.3 Resistance and Gage SizeIn the United States, wire is sized by the American
Wire Gage (AWG) method. AWG was based on the
previous Brown and Sharpe (B & S) system of wire
sizes which dates from 1856. AWG numbers are most
common in the United States, and will be referred to
throughout this book. The wire most often used in
audio ranges from approximately 10 AWG to
30 AWG, although larger and smaller gage sizes exist.
Wire with a small AWG number, such as 4 AWG, is
very heavy, physically strong but cumbersome, and
has very low resistance, while wire of larger numbers,
such as 30 AWG can be very light weight and fragile,
and has high resistance. Resistance is an important
factor in determining the appropriate wire size in any
circuit. For instance, if an 8: loudspeaker is being
connected to an amplifier 500 ft away through a #19
wire, 50% of the power would be dropped in the wire
in the form of heat. This is discussed in Section 14.25
regarding loudpeaker cable.
Each time the wire size changes three numbers, such
as from 16 AWG to 19 AWG the resistance doubles.
The reverse is also true. With a wire changed from
16 AWG to 13 AWG, the resistance halves. This also
means that combining two identical wires of any given
gage decreases the total gage of the combined wires by
three units, and reduces the resistance. Two 24 AWG
wires combined (twisted together) would be 21 AWG,
for instance. If wires are combined of different gages,
the resulting gage can be easily calculated by adding the
circular mil area (CMA) shown in Tables 14-2 and 14-3.
For instance, if three wires were combined, one
16 AWG (2583 CMA), one 20 AWG (1022 CMA) and
one 24 AWG (404 CMA), the total CMA would be
2583 + 1022 + 404 = 4009 CMA. Looking in Table
14-1, this numbers falls just under 14 AWG. While even
number gages are the most common, odd number gages
(e.g., 23 AWG) can sometimes be found. There are
many Category 6 (Cat 6) premise/data cables that are
23 AWG, for instance. When required, manufacturers
can even produce partial gages. There are coaxial cables
with 28.5 AWG center conductors. Such specialized
gage sizes might require equally special connectors.