Manualshelf

User Manual

ManualsBrandsSiemens ManualsBuilding automationBasic Documentation Desigo Control Point BACnet Systems
21222324252627282930
Planning (hardware)
Power requirements
7
A6V11666339_en--_b 23 | 35
7.4.2 Transformer sizing AC 24 V
The transformer power is the sum of the power consumption of the connected devices.
Operating voltage
The operating voltage is AC 24 V. It must meet the requirements for SELV or PELV per IEC 60364-4-41.
NEC: Class 2 transformers or class 2 circuits.
Permissible deviation to nominal voltage AC 24 V on the Transformer: +20%/-10%.
This guarantees a tolerance of +/- 20% on the devices after considering line and contact resistance.
Transformer specification AC 24 V
IEC: Use safety insulating transformers as per EN 61558 with double insulation designed for 100% duty to
supply SELV or PELV circuits.
NEC: Class 2 transformers.
Transformer nominal power should be at least 25 VA. For smaller transformers, the ratio of open circuit voltage
to full load is unfavorable (> + 20 %).
The nominal transformer power should not exceed 200% of maximum load for efficiency reasons
(effectiveness).
Wire lengths: Power supply AC 24 V
The basis for calculation is a permissible voltage drop off of 2.4 V (10%) on the power line from the transformer
to the farthest power point.
The following table outlines the wire lengths and diameters based on load.
Cross-section/power Cable length for AC 24 V
10 VA 20 VA 50 VA 100 VA 200 VA
2.50 mm
2
/ AWG14
350 m 175 m 75 m 35 m 18 m
1.50 mm
2
/ AWG16
210 m 105 m 42 m 20 m 10 m
1.00 mm
2
/ AWG18
135 m 68 m 27 m 14 m 7 m
The supply wire (AC 24 V) and return wire (⊥) can each have the indicated lengths.
Each power point is either connected separately to the terminal bar on the transformer (star wiring) or looped.
Power is added together for multiple back-to-back looped devices 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 devices and that any cascading powering of devices is only possible over short distances
or at small outputs.