WattMaster VAVBOX - Manual (Page 7)

VAVBOX Controller
Technical Guide
7
24VAC Power - Transformer & Wire Sizing Considerations For VAVBOX Controllers
120 / 24VAC
120 / 24VAC
Distance A to B cannot exceed 61.73 Ft.
Distance from A to B cannot exceed 123.46 Ft.
Distance from A to C cannot exceed 123.46 Ft.
Distance from A to B cannot exceed 230.40 Ft.
Distance from A to C cannot exceed 308.64 Ft.
Distance from A to D cannot exceed 308.64 Ft.
Distance from A to E cannot exceed 308.64 Ft.
Distance from A to F cannot exceed 308.64 Ft.
120 / 24VAC
Some installers like to use one large 24VAC transformer to power several devices. This is allowable as long as polarity is maintained to each device on
the transformer circuit.
Using
separate transformers also allows redundancy in case of a transformer failure. Instead of having 8 controllers inoperative because of a malfunctioning
transformer you have only 1 controller off line. If the installer does decide to use a large transformer to supply power to several devices, the following
transformer and wire sizing information is presented to help the installer correctly supply 24VAC power to the devices.
Following is a typical example to help the installer to correctly evaluate transformer and wiring designs.
Each VAVBOX Controller requires 6 VA @ 24VAC power. In the examples below we have a total of 10 VAV Controllers.
10 VAVBOX Controllers @ 6VA each................ 10 x 6VA =60VA.
The above calculation determines that our transformer will need to be sized for a minimum of 60VA if we are to use one transformer to power all the
controllers. We will use a 75 VA transformer, as this is a readily available size that meets our VA load requirements.
Next we must determine the maximum length of run allowable for the wire gauge we wish to use in the installation. Each wire gauge below has a
voltage drop per foot value we use to calculate total voltage drop.
18ga wire.................................0.00054 = voltage drop per 1’ length of wire
16ga wire.................................0.00034 = voltage drop per 1’ length of wire
14ga wire.................................0.00021 = voltage drop per 1’ length of wire
For our example we will use 18 gauge wire. WattMaster recommends 18 gauge as a minimum wire size for all power wiring.
Next use the voltage drop per foot value for 18 gauge wire from the list above and multiply by the total VA load of the 10 controllers to be installed.
0.00054 (Voltage drop per foot for 18 gauge wire) x 60VA controller load = Volts/Ft.
WattMaster VAVBOX controllers will operate efficiently with a voltage drop no greater than 2 Volts. Divide the total allowable voltage drop of 2 Volts by
the number you arrived at above and you have the maximum number of feet you can run the 18 gauge wire with an 75 VA transformer with no more
than a 2 Volt drop at the farthest controller from the transformer..
2 (Volts total allowable voltage drop)
= 61.73
0.0324 (Voltage drop per 1 ft. @ 60VA load)
Parallel circuiting of the wiring instead of wiring all 10 controllers in series allows for longer wire runs to be used with the same size wire (as shown in
our examples below).
Warning: If polarity is not maintained, severe damage to the devices may result. WattMaster Controls recommends
using a separate transformer for each device in order to eliminate the potential for damaging controllers due to incorrect polarity.
0.0324
feet
It is often necessary for the installer to calculate and weigh the cost and installation advantages and disadvantages of wire size,
transformer size, multiple transformers, circuiting, etc., when laying out an installation. No matter what layout scheme is decided upon, it is mandatory
that the farthest controller on the circuit is supplied with a minimum of 22 Volts and that the polarity is maintained to all controllers connected to the
transformer.
A
A
A
B
C
D
E
F
B
B C
Transformer Sizing & Wiring
Each VAVBOX controller requires 6 VA minimum at 24 VAC in order
to function properly. Wiring and transformer information shown above
is provided to help correctly size and layout wiring schemes for the
VAVBOX controllers. As stated above, all wiring should be 18 Ga.
minimum. Long runs may require heavier gauge wire to keep the volt-
age drop within the 2 volt maximum range. Please follow these guide-
lines.
Warning: If polarity between controllers is not maintained,
severe damage to the controllers may result.
WattMaster recommends using a separate transformer
for each controller in order to eliminate the potential
for damaging controllers due to incorrect polarity.
Figure 5: WMVAV Controller Transformer Sizing