Rane MA 4, Инструкция по эксплуатации

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Constant-Voltage-
Wire Size – How Big Is Big Enough?
Since the whole point of using constant-voltage dis-
tribution techniques is to optimize installation costs,
proper wire sizing becomes a major factor. Due to
wire resistance (usually expressed as ohms per foot, or
meter) there can be a great deal of engineering involved
to calculate the correct wire size. The major factors
considered are the maximum current flowing through
the wire, the distance covered by the wire, and the
resistance of the wire. The type of wire also must be
selected. Generally, constant-voltage wiring consists of
a twisted pair of solid or stranded conductors with or
without a jacket.
For those who like to keep it simple, the job is rela-
tively easy. For example, say the installation requires
delivering 1000 watts to 100 loudspeakers. Calculating
that 1000 watts at 70.7 volts is 14.14 amps, you then
select a wire gauge that will carry 14.14 amps (plus
some headroom for I
2
R wire losses) and wire up all 100
loudspeakers. This works, but it may be unnecessarily
expensive and wasteful.
Really meticulous calculators make the job of select-
ing wire size a lot more interesting. For the above ex-
ample, looked at another way, the task is not to deliver
1000 watts to 100 loudspeakers, but rather to distribute
10 watts each to 100 loudspeakers. These are different
things. Wire size now becomes a function of the geom-
etry involved. For example, if all 100 loudspeakers are
connected up daisy-chain fashion in a continuous line,
then 14.14 amps flows to the first speaker where only
0.1414 amps are used to create the necessary 10 watts;
from here 14.00 amps flows on to the next speaker
where another 0.1414 amps are used; then 13.86 amps
continues on to the next loudspeaker, and so on, until
the final 0.1414 amps is delivered to the last speaker.
Well, obviously the wire size necessary to connect the
last speaker doesn’t need to be rated for 14.14 amps! For
this example, the fanatical installer would use a differ-
ent wire size for each speaker, narrowing the gauge as
he went. And the problem gets ever more complicated
if the speakers are arranged in an array of, say, 10 x 10,
for instance.
Luckily tables exist to make our lives easier. Some of
the most useful appear in Giddings 3
as Tables 14-1 and
Table 14-2 on pp. 332-333. These provide cable lengths
and gauges for 0.5 dB and 1.5 dB power loss, along with
power, ohms, and current info. Great book. Table 1
above reproduces much of Gidding’s Table 14-2
4
.
Table 1: 70.7V Loudspeaker Cable Lengths and Gauges for 1.5 dB Power Loss
Wire Gauge > 22 20 18 16 14 12 10 8
Max Current (A) > 5 7.5 10 13 15 20 30 45
Max Power (W) > 350 530 700 920 1060 1400 2100 3100
Load
Power
Load
Ohms Maximum Distance in Feet
1000 5 0 0 0 0 185 295 471 725
500 10 0 93 147 236 370 589 943 1450
400 12.5 0 116 184 295 462 736 1178 1813
250 20 117 186 295 471 739 1178 1885 2900
200 25 146 232 368 589 924 1473 2356 3625
150 33.3 194 309 490 785 1231 1962 3139 4829
100 50 292 464 736 1178 1848 2945 4713 7250
75 66.6 389 618 981 1569 2462 3923 6277 9657
60 83.3 486 774 1227 1963 3079 4907 7851 12079
50 100 584 929 1473 2356 3696 5891 9425 14500
40 125 729 1161 1841 2945 4620 7363 11781 18125
25 200 1167 1857 2945 4713 7392 11781 18850 29000