Installation manual

Community S-Series - Operation and Installation Manual - Page 28
covered with a variety of jacket types. Jackets may be made of PVC, rubber, neoprene,
and other materials, depending on the intended conditions of use.
Generally speaking, the wires and cables that power loudspeakers do not need to be
twisted into pairs, though there is some benefit to doing so. A twisted pair of conductors
has the effect of cancelling electro-magnetic radiation, thereby reducing mutual induction
among circuits that share the same physical space (such as a cable tray or conduit), along
with the crosstalk that might otherwise result.
Twisted pairs are commonly used for balanced line signal and microphone cables, in which
the nominal voltages are very low and the input impedance of the load is typically quite
high (>10K ohms). Under such conditions, the use of a twisted pair is essential to reduce
crosstalk among adjacent cables. The twisting insures that the differential amplifier in a
balanced line receiver will see identical phase and amplitude of any extraneous Electro
Magnetic Interference (EMI) induced in the cable on both polarities, thereby allowing the
EMI to be differentially cancelled.
In contrast, however, loudspeakers have input impedances that are quite low and operate
on much higher voltages. The potential of inducing an audible signal from adjacent wiring
is close to zero. The installer may, however, choose to use twisted pair loudspeaker cable
for other reasons. Certain amplifiers may exhibit instability when driving long lengths of
wire installed in conduit. A twisted pair will insure that the reactance of the loudspeaker
cable is identical on both the plus and minus wires, thereby presenting a more stable load
to the amplifier.
Note that when specifying multiple twisted pairs of speaker cables intended to share the
same conduit, the conduit will need to be sized much larger than with loose or bundled
conductors.
Conductors and Insulation
Solid conductor wire is slightly less expensive than stranded wire, but much more difficult
to pull through conduit. Also, it does not terminate to most speaker connectors as easily as
stranded wire. Therefore, we recommend using stranded THHN type wire for installations
that involve conduit.
Densely stranded cables, typically used for portable cordage, will coil up easily and lay flat
on the stage, making them a good choice for applications requiring portability such as floor
monitors. Typical examples are 14/2 and 14/4 SJO. Such cable is normally stocked in
many hardware stores.
Wire and cable insulation is always rated for a working voltage and a maximum
temperature. In power distribution systems, wire and cables can get very hot, making the
temperature rating extremely important. When used with loudspeakers, the temperature of
the wire or cable will hardly ever rise more than 10º C above ambient, and voltages will
never exceed 300V (which is the minimum rating of most industrial wire and cable).
Special cables are manufactured for installation in air plenums, while others are made for
direct burial. Use of such products can save a lot of time and expense compared to
installing conduit. However, local, state, or federal building codes may require that
loudspeaker cables be installed in conduit or in cable trays. It’s a good idea to check
applicable regulations carefully, before beginning the installation.
Conductors are sized according to a numbering system know as the American Wire Gauge,
or AWG. Larger numbers, such as #22 or #24 indicate smaller diameter wire, while smaller
numbers such as #10 and #12 indicate larger diameter wire.
The larger the diameter, the lower the resistance will be for a given conductor length.
Resistance is normally stated per foot, or per hundred feet of wire. For example, #10
stranded copper THHN has a resistance of .204 ohms per hundred feet, though this can
vary slightly among manufacturers.
The resistance of the wire, the impedance of the load, and the output voltage of the
amplifier will determine how much loss occurs in the wire. These parameters also govern
the damping factor of the amplifier/speaker combination (more on this later).