46
MP500/4N-8-16 Installation
4.2.2 Sizing of the power supply cable
The section of the cables must be selected so that the voltage of the power supply to various devices is correct, for the purpose of
obtaining stability, efficiency, and immunity from disturbances.
The section must be calculated after having taken into account the most critical situation of the system power supply, which is equal to
a blackout and the back-up battery with only a minimal charge (10.5 V
⎓
). Under these conditions, when fully charged, all the devices in
the MP500/4N, MP500/8 and MP500/16 systems must be guaranteed at least 9 V
⎓
.
IMPORTANT!
Check that there is in any case the minimum voltage of the functioning of the other devices used, like the IR
detectors, actuators, and sirens, etc.. Some of these devices may require a power supply voltage of over 9 V
⎓
(the power
supply data and the tolerances allowed are reported in the technical specifications of the various devices).
As a result,
The maximum voltage reduction admitted on cables is: 1.5 V
⎓
or 0.75 V on the positive cable and 0.75 V on the negative cable.
The formula to calculate this is
V
CONTROL PANEL
=
2
x
length
x
R
CABLE
x
I
DEVICES
where:
V
CONTROL PANEL
is the reduction in power in Volts
length
is the length of the cable (single conductor), in metres
R
CABLE
is the resistance of the cable in ohm/m
I
DEVICES
is the maximum current absorbed by the devices in Amperes (a value that can be found on their technical
specifications)
The values of copper cable resistance are:
Cross section sq.mm
2
(*)
0.22
0.50
0.75
1.00
1.50
Resistance in Ohm/m
0.0795
0.0350
0.0233
0.0175
0.0117
(*) Standards require that the cross-section of the cable cannot be less than 0.1 mm
2
.
Table 5 - Copper wire resistance
In the case of a complex power supply network, with multiple branches, it is necessary to calculate each individual stretch.
For different wire sizes the maximum acceptable lengths according to the maximum current running in the conductor are shown by way
of example only.
Section 0.22 mm
2
Section 0.50 mm
2
Section 0.75 mm
2
Section 1 mm
2
Section 1.5 mm
2
Current
Length
Current
Length
Current
Length
Current
Length
Current
Length
0.300 A
31 m
0.300 A
71 m
0.300 A
107 m
0.300 A
143 m
0.300 A
214 m
0.500 A
19 m
0.500 A
43 m
0.500 A
64 m
0.500 A
86 m
0.500 A
129 m
0.750 A
13 m
0.750 A
29 m
0.750 A
43 m
0.750 A
57 m
0.750 A
86 m
1.000 A
9 m
1.000 A
21 m
1.000 A
32 m
1.000 A
43 m
1.000 A
64 m
1.500 A
6 m
1.500 A
14 m
1.500 A
21 m
1.500 A
29 m
1.500 A
43 m
2.000 A
5 m
2.000 A
11 m
2.000 A
16 m
2.000 A
21 m
2.000 A
32 m
2.500 A
4 m
2.500 A
9 m
2.500 A
13 m
2.500 A
17 m
2.500 A
26 m
3.000 A
3 m
3.000 A
7 m
3.000 A
11 m
3.000 A
14 m
3.000 A
21 m
