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Siemens Desigo Room automation - Engineering, mounting and installation CM111043en_14
Smart Infrastructure Wiring of power lines 2020-10-30
6.3.4 AC 24 V power line cable lengths
The permissible voltage drop of 0.48 V (2%) on the power wire between the
transformer and the most distant power point (room automation station, power
supply module, bus connection module) is the basis for calculations.
The table below provides cable lengths and cable cross sections based on load.
Cable cross-section Cable length
Power
5 VA 10 VA 20 VA 50 VA 100 VA 200 VA
2.50 mm
2
/ AWG14 135 m
440 ft
70 m
220 ft
35 m
110 ft
13 m
44 ft
7 m
22 ft
3 m
11 ft
1.50 mm
2
/ AWG16 85 m
280 ft
42 m
140 ft
21 m
70 ft
8 m
30 ft
4 m
15 ft
2 m
7 ft
1.00 mm
2
/ AWG18 55 m
180 ft
27 m
90 ft
13 m
45 ft
5 m
17 ft
3 m
9 ft
1 m
4 ft
· The supply (AC 24 V) and return lines (^) can each have the indicated lengths.
· Each supply point (room automation stations/power supply module/bus
connection module) is either connected separately to the transformer's terminal
block (star wiring) or looped via the room automation station.
· Power is added together for multiple back-to-back looped devices ("daisy chain")
which reduces the cable length accordingly.
· Cables may be wired in parallel to increase the cross section.
· In practice, the small level of permissible voltage drop-off means that the
transformer must always be installed in close proximity to the TRA devices
and that any cascading powering of room automation stations is only
possible over short distances or at small outputs.
Notes