BACnet MS/TP Communication Data Bus Fundamentals
Network Guide 43
Peak Current
Particular attention should be paid to the peak current absorbed by devices that are powered
by an AC circuit. Most Distech Controls 24VAC devices use half-wave rectifiers to supply their
onboard electronics (this is common with most controls manufacturers). With this configura-
tion, the peak AC current is approximately 2.5 times the average RMS AC current. The ECB/
ECL-VAV controllers are isolated full-wave rectified with power factor correction. The peak AC
current is approximately 1.4 times the average RMS AC current.
Transformer Selection and Determining the Maximum Power
Run Length
It is recommended to wire only one controller per 24VAC transformer.
When calculating a controller’s power consumption to size the 24VAC transformer, you must
also add the external loads the controller is going to supply, including the power consumption
of any connected subnet module (for example, Allure™ Series Communicating Sensors).
If only one 24VAC transformer is available, determine the maximum number of daisy-chained
VAV controllers that can be supplied on a single power cable supplied by a 100 VA trans-
former, according to the controller’s expected power consumption including external loads, the
cable’s wire gauge, and the total cable length from the following figures (Figure 26 and Figure
27). Any installation condition that is outside of the parameters of the following graph should
be avoided.
To maximize daisy-chaining performance, the transformer should be installed as close as pos-
sible to the first VAV. If this is not possible, then use 14 AWG wire to power the first VAV which
can help reduce a voltage drop at the end of the daisy-chain.
Distech Controls' 24V-powered devices are Class 2 Products. To conform to Class
2 installation requirements, only use transformers of 100VA or less to power the
device(s).
The recommended minimum peak input voltage is 27.2V
p
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