Clear-Com CS-702 - Manual (Page 19)

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CABLE CONSIDERATIONS
The Clear-Com intercom line is intended to run on a shielded twisted pair of
cable per channel of intercom. One conductor carries full duplex (two-way)
audio, the other conductor carries the DC power for remote stations. The
shield is used for ground return for audio and power. When choosing
interconnect cable, keep the following considerations in mind:
1. DC resistance of the ground or common conductor affects crosstalk. For
runs longer than 100 ft. (30.5 m) do not use wire smaller than 20 gauge.
The total resistance of the ground return (the combined parallel sum of all
shields to a location) to any point in the system should be under 1.5 Ω.
2. The capacitance of the interconnect cable affects system frequency
response and sidetone stability. Total capacitance should not be greater
than 0.25 uF (capacitance between conductor and shield) equivalent to an
intercom system containing 5,000 ft. (1525 m) of cable at 50 pF per ft.
(0.305 m).
Portable Installation Cable
Typical cable for portable system interconnections is flexible, two-conductor,
shielded microphone cable. For runs less than 500 ft. (152.5 m) a cable made
of 24-gauge wire is acceptable. For runs longer than 500 ft. (152.5 m) use a
20-gauge cable or larger.
Portable remote stations such as beltpacks have a pair of input and output
connectors; when installing a system that includes these, they can be
daisy-chained or loop-through connected along one interconnect path.
Daisy-chaining and line-splitting decreases the amount of cable required and
simplify the installation.