SYLK™ I/O DEVICES
5
31-00028—02
Line-Loss
Devices 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 device 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 device. To assure the device 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. 3 shows this voltage load
dependence.
In the preceding I x R loss example, even though the
device load is only 37 VA, a standard 40 VA transformer is
not sufficient due to the line-loss. Looking at Fig. 3, 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 lower edge of the shaded zone in Fig. 3 that
represents the worst case conditions.) When the I x R loss
of four volts is subtracted, only 18.9 volts reaches the
device. This is not enough voltage for proper operation.
In this situation, the engineer has three alternatives:
1.
Use a larger transformer. For example, if an 80 VA
model is used, an output of 24.4 volts, minus the
four volt line-loss, supplies 20.4V to the device (see
Fig. 3). Although acceptable, the four-volt line-loss
in this example is higher than recommended.
IMPORTANT
No installation should be designed where the
line-loss is greater than two volts. This allows for
nominal operation if the primary voltage drops to
102 Vac (120 Vac minus 15 percent).
2.
Use heavier gauge wire for the power run. 14 AWG
(2.0 sq mm) wire has a resistance of 2.57 ohms per
1,000 ft. Using the preceding formula results in a
line-loss of only 1.58 volts (compared with 4.02
volts). This would allow a 40 VA transformer to be
used. 14 AWG (2.0 sq mm) wire is the recommended
wire size for 24 Vac wiring.
3.
Locate the transformer closer to the device. This
reduces the length of the wire run, and the line-loss.
The issue of line-loss is also important in the case of the
output wiring connected to the Triac digital outputs. The
same formula and method are used. Keep all power and
output wire runs as short as practical. When necessary,
use heavier gauge wire, a bigger transformer, or install the
transformer closer to the controller.
To meet the National Electrical Manufacturers Association
(NEMA) standards, a transformer must stay within the
NEMA limits. The chart in Fig. 3 shows the required limits
at various loads.
With 100 percent load, the transformer secondary must
supply between 23 and 25 volts to meet the NEMA
standard. When a purchased transformer meets the
NEMA standard DC20-1986, the transformer voltage
regulating ability can be considered reliable. Compliance
with the NEMA standard is voluntary.
Fig. 3. NEMA Class 2 transformer voltage output limits.
The Honeywell transformers listed in Table 5 meet the
NEMA standard DC20-1986.
Table 5. Honeywell transformers that meet
NEMA standard DC20-1986.
NOTE:
The AT88A and AT92A transformers do not meet
the voluntary NEMA standard DC20-1986.
Wiring
All wiring must comply with applicable electrical codes
and ordinances, or as specified on installation wiring
diagrams. Device wiring is terminated to the screw
terminal blocks located on the top and the bottom of the
device.
Transformer Type
VA Rating
AT40A
40
AT72D
40
AT87A
50
AK3310 Assembly
100
27
26
25
24
23
22
21
20
19
18
17
16
15
14
0
50
100
150
% OF LOAD
SECONDAR
Y
VOL
TAGE
200
M993
