31-00083—02
14
Power Budget
A power budget must be calculated for each device to
determine the required transformer size for proper operation.
A power budget is simply the summing of the maximum power
draw ratings (in VA) of all the devices to be controlled. This
includes the controller itself and any devices powered from the
controller, such as equipment actuators (ML6161 or other
motors) and various contactors and transducers.
IMPORTANT:
• If a controller is used on Heating and Cooling Equipment
(UL 1995, U.S. only) and transformer primary power is
more than 150 volts, connect the transformer secondary
common to earth ground (see Fig. 14 on page 16).
• When multiple controllers operate from a single
transformer, connect the same side of the transformer
secondary to the same power input terminal in each
device. The earth ground terminal (terminal 3) must be
connected to a verified earth ground for each controller in
the group (see Fig. 15 on page 16).
Power Budget Calculation Example
Table 7 is an example of a power budget calculation for a
typical CVL controller.
Table 7. Power budget calculation example.
The system example above requires 30.7 VA of peak power.
Therefore, a 100 VA AT92A transformer could be used to
power one controller of this type. Because the total peak
power is less than 50 VA, this same transformer could be on
page 11 for illustrations of controller power wiring. See Table 8
for VA ratings of various devices.
Table 7. Power budget calculation example
Table 8. VA ratings for transformer sizing.
For contactors and similar devices, the in-rush power ratings
should be used as the worst case values when performing
power budget calculations. Also, the application engineer
must consider the possible combinations of simultaneously
energized outputs and calculate the VA ratings accordingly.
The worst case, which uses the largest possible VA load,
should be determined when sizing the transformer. Each
controller requires 24 Vac power from an energy-limited Class
II power source. To conform to Class II restrictions (U.S. only),
transformers must not be larger than 100 VA. A single
transformer can power more than one controller.
Line-Loss
Controllers must receive a minimum supply voltage of 20 Vac.
If long power or output wire runs are required, a voltage drop
due to Ohms Law (I x R) line-loss must be considered. This
line-loss can result in a significant increase in total power
required and thereby affect transformer sizing. The following
example is an I x R line-loss calculation for a 200 ft. (61m) run
from the transformer to a controller drawing 37 VA and using
two 18 AWG (1.0 sq mm) wires.
The formula is:
Loss = [length of round-trip wire run (ft.)] x [resistance in wire
(ohms per ft.)] x [current in wire (amperes)]
From specification data:
18 AWG twisted pair wire has a resistance of 6.52 ohms per
1000 feet.
Loss = [(400 ft.) x (6.52/1000 ohms per ft.)] x [(37 VA)/(24V)] =
4.02 volts
This means that four volts are going to be lost between the
transformer and the controller. To assure the controller
receives at least 20 volts, the transformer must output more
than 24 volts. Because all transformer output voltage levels
depend on the size of the connected load, a larger transformer
outputs a higher voltage than a smaller one for a given load.
Fig. 12 shows this voltage load dependence.
In the preceding I x R loss example, even though the controller
load is only 37 VA, a standard 40 VA transformer is not
sufficient due to the line-loss. Looking at Fig. 12, a 40 VA
transformer is just under 100 percent loaded (for the 37 VA
controller) and has a secondary voltage of 22.9 volts. (Use the
Device
VA
Information
Obtained From
CVL4022AS-VAV1
controller (include
Series 60 Floating
Damper Actuator)
9.0
R8242A Contactor
fan rating
21.0
TRADELINE
®
Catalog
inrush rating
D/X Stages
0.0
For example, assume
cooling stage outputs are
wired into a compressor
control circuit and have
no impact on the budget.
M6410A Steam
Heating Coil Valve
0.7
TRADELINE
®
Catalog,
0.32A 24 Vac
TOTAL
30.7
Device
Description
VA
CVL4022AS-VAV1
controllers and
Series 60 Floating
Damper Actuator
Controller and Actuator
9.0
CVL4024NS-VAV1
Controller
5.0
ML684
Versadrive Valve Actuator
12.0
ML6161
Damper Actuator, 35 lb-in.
2.2
ML6185
Damper Actuator SR 50 lb-in
12.0
ML6464
Damper Actuator, 66 lb-in.
3.0
ML6474
Damper Actuator, 132 lb-in.
3.0
R6410A
Valve Actuator
0.7
R8242A
Contactor
21.0
