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4.2.1
M-Bus modules
Each M-Bus module loads the M-Bus network too. According to the standard, an M-Bus module should load the network
with 1 unit load (UL) corresponding to 1.5 mA. Some modules, however, load with up to 4 UL.
Capacitively the load of an M-Bus module is 0.5 – 1 nF.
The number of connected M-Bus slaves is shown on the display of M-Bus Master 250L. Please note that it is not possible to
show the exact number of connected slaves. This is due to tolerances in the slaves.
4.3 Installation parameters
The following parameters are essential to the possible cable length of an M-Bus network.
4.3.1
Cable
The cable resistance and capacity must be as low as possible. The thicker the cable, the lower the resistance. The thicker the
cable, the higher the capacity.
An M-Bus cable must minimum be able to handle 50 V and 500 mA.
Diameter (mm ø)
0.5
0.65
0.8
1.0
1.13
1.26
1.39
1.6
Examples of resistance in copper cable
Please note that the resistance in copper depends on its purity. The purer the copper, the lower its resistance.
LiYY
Current load
Cable resistance
Capacity
J-Y(St)YY
Current load
Cable resistance
Capacity
Examples of cable types
16
Cross section (mm
0.20
0.33
0.50
0.79
1.00
1.25
1.52
2.0
2x0.34 mm
2x0.50 mm
2
Max. 4.5 A
Max. 6 A
56 Ω/km
39 Ω/km
110 nF/km
120 nF/km
2x0.60 mm
65 Ω/km
120 nF/km
55121391_B1_GB_12.2013
)
Resistance in Ohm per
2
1,000 metres
90
53
35
23
18
14
12
8.7
2x0.75 mm
2
Max. 10 A
26 Ω/km
120 nF/km
2x0.80 mm
2
-
-
37 Ω/km
100 nF/km
Length in metres per Ohm
2x1.0 mm
2
2
Max. 12 A
20 Ω/km
120 nF/km
2
11
19
29
45
57
71
87
115
2x1.5 mm
2
Max. 18 A
12 Ω/km
120 nF/km
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