Transmission Techniques: Wire and Cable 399There are two basic forms of wire, solid and
stranded. A solid conductor is one continuous piece of
metal. A stranded conductor is made of multiple smaller
wires combined to make a single conductor. Solid wire
has slightly lower resistance, with less flexibility and
less flex-life (flexes to failure) than stranded wire.
14.2.4 Drawing and Annealing
Copper conductors start life as copper ore in the ground.
This ore is mined, refined, and made into bars or rod.
Five sixteenth^ inch copper rod is the most common form
used for the making of wire and cable. Copper can be
purchased at various purities. These commonly follow
the ASTM (American Society for Testing and Materials)
standards. Most of the high-purity copper is known as
ETP, electrolytic tough pitch. For example, many cable
products are manufactured with ASTM B115 ETP. This
copper is 99.95% pure. Copper of higher purity can be
purchased should the requirement arise. Many con-
sumer audiophiles consider these to be oxygen free,
when this term is really a discussion of copper purity
and is determined by the number of nines of purity. The
cost of the copper rises dramatically with each “9” that
is added.
To turn inch rod into usable wire, the copper rod
is drawn through a series of dies. Each time it makes the
rod slightly smaller. Eventually you can work the rod
down to a very long length of very small wire. To take
inch rod down to a 12 AWG wire requires drawing
the conductor through eleven different dies. Down to
20 AWG requires fifteen dies. To take that wire down to
36 AWG requires twenty-eight dies.
The act of drawing the copper work hardens the
material making it brittle. The wire is run through an
in-line annealing oven, at speeds up to 7000 feet per
minute, and a temperature of 900 to 1000°F (482 to
537°C). This temperature is not enough to melt the
wire, but it is enough to let the copper lose its brittleness
and become flexible again, to reverse the work hard-
ening. Annealing is commonly done at the end of the
drawing process. However, if the next step requires
more flexibility, it can be annealed partway through the
drawing process. Some manufacturers draw down the
wire and then put the entire roll in an annealing oven. In
order to reduce oxygen content, some annealing ovens
have inert atmospheres, such as nitrogen. This increases
the purity of the copper by reducing the oxygen content.
But in-line annealing is more consistent than a whole
roll in an oven.
Lack of annealing, or insufficient annealing time or
temperature, can produce a conductor which is stiff,
brittle, and prone to failure. With batch annealing, the
inner windings in a roll may not be heated as effectively
as the outer windings. Cables made in other countries
may not have sufficient purity for high-performance
applications. Poor--quality copper, or poor annealing,
are very hard to tell from initial visual inspection but
often shows up during or after installation.14.2.5 Plating and TinningMuch of the wire manufactured is plated with a layer of
tin. This can also be done in-line with the drawing and
annealing by electroplating a layer on the wire. Tinning
makes the wire especially resistant to pollutants, chemi-
cals, salt (as in marine applications). But such a plated
conductor is not appropriate for high-frequency applica-
tions where the signal travels on the skin of the conduc-
tor, called skin effect. In that case, bare copper
conductors are used. The surface of a conductor used for
high frequencies is a major factor in good performance
and should have a mirror finish on that surface. Wires
are occasionally plated with silver. While silver is
slightly more conductive, its real advantage is that silver
oxide is the same resistance as bare silver. This is not
true with copper, where copper oxide is a semiconduc-
tor. Therefore, where reactions with a copper wire are
predicted, silver plating may help preserve perfor-
mance. So silver plating is sometimes used for marine
cables, or cables used in similar outdoor environments.
Some plastics, when extruded (melted) onto wires,
can chemically affect the copper. This is common, for
instance, with an insulation of extruded TFE (tetrafluo-
roethylene), a form of Teflon™. Wires used inside these
cables are often silver plated or silver-clad. Any
oxidizing caused by the extrusion process therefore has
no effect on performance. Of course, just the cost of
silver alone makes any silver-plated conductor signifi-
cantly more expensive than bare copper.14.2.6 Conductor ParametersTable 14-2 shows various parameters for solid wire from
4 AWG to 40 AWG. Table 14-3 shows the same parame-
ters for stranded wire. Note that the resistance of a spe-
cific gage of solid wire is lower than stranded wire of the
same gage. This is because the stranded wire is not com-
pletely conductive; there are spaces (interstices) between(^5) / 16
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