438 Chapter 14
capacitive because the twisting of the leads effectively
eliminated inductive coupling.
One application that is often ignored regarding
crosstalk is speaker wiring, especially 70V distributed
loudspeaker wiring. You will note in the first drawing
that the two wires are not a balanced line. One is hot the
other is ground. Therefore, that pair would radiate some
of the audio into the adjoining pair, also unbalanced.
Twisting the pairs in this application would do little to
reduce crosstalk.
The test was made on a 250 ft twisted pair run in the
same conduit with a similar twisted pair, the latter
carrying signals at 70.7 V. Measurements made for half
this length produced half the voltages, therefore the
results at 500 ft and 1000 ft were interpolated.
The disturbing line was driven from the 70 V termi-
nals of a 40 W amplifier and the line was loaded at the
far end with 125: , thus transmitting 40 W. The cross-
talk figures are for 1 kHz. The voltages at 100 Hz and
10 kHz are one-tenth and ten times these figures, respec-
tively.
There are two ways to effectively reduce crosstalk.
One is to run signals only on balanced-line twisted pairs.
Even shielding has a small added advantage compared
to the noise and crosstalk rejection of a balanced line.
The second way to reduce crosstalk is to move the two
cables apart. The inverse-square law tells us that
doubling the distance will produce four times less inter-
ference. Further, if cables cross at right angles, this is the
point where the magnetic fields have minimum interac-
tion. Of course, the latter solution is not an option in a
prebundled cable, or in cable trays or installations with
multiple cables run from point to point.14.28 National Electrical CodeThe National Electrical Code (NEC) is a set of guide-
lines written to govern the installation of wiring and
equipment in commercial buildings and residential
areas. These guidelines were developed to insure the
safety of humans as well as property against fires and
electrical hazards. Anyone involved in specifying cable
for installation should be aware of the basics of the code.
The NEC code book is made up of nine chapters,
with each chapter divided into separate articles
pertaining to specific subjects. Five articles pertain to
communication and power-limited cable. The NEC
book is written by and available from the NFPA
(National Fire Protection Association), 11 Tracy Drive,
Avon, MA 02322. They can be reached at
1-800-344-3555 or http://www.nfpa.org.Article 725—Class 1, Class 2, Class 3, Remote-
Control, Signaling, and Power-Limited Circuits.
Article 725 covers Class 1, Class 2, and Class 3 remote
control and signaling cables as well as power-limited
tray cable. Power-limited tray cable can be used as a
Class 2 or Class 3 cable. Cable listed multipurpose, com-
munications, or power-limited fire protective can be
used for Class 2 and Class 3 applications. A Class 3
listed cable can be used as a Class 2 cable.Article 760—Fire Protective Signaling Systems. Arti-
cle 760 covers power-limited fire-protective cable.
Cable listed as power-limited fire-protective cable can
also be used as Class 2 and Class 3 cable. Cable listed as
communications and Class 3 can be used as power-lim-
ited fire protective cable with restrictions to conductor
material and type gage size and number of conductors.Table 14-34. Loudspeaker Cable Selection Guide
Power (%) 11% 21% 50%
Loss (dB) 0.5 1.0 3.0
Wire Size Maximum Cable Length–ft4 : Loudspeaker12 AWG 140 305 1150
14 AWG 90 195 740
16 AWG 60 125 470
18 AWG 40 90 340
20 AWG 25 50 195
22 AWG 15 35 135
24 AWG 10 25 858 : Loudspeaker12 AWG 285 610 2285
14 AWG 185 395 1480
16 AWG 115 250 935
18 AWG 85 190 685
20 AWG 50 105 390
22 AWG 35 70 275
24 AWG 20 45 17070 V Loudspeaker12 AWG 6920 14,890 56,000
14 AWG 4490 9650 36,300
16 AWG 2840 6100 22,950
18 AWG 2070 4450 16,720
20 AWG 1170 2520 9500
22 AWG 820 1770 6650
Courtesy Belden.