Honeywell PUB4024S ユーザーズマニュアル

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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. 10 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. 10, 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. 10 that represents the 

worst case conditions.) When the I x R loss of four volts is 

subtracted, only 18.9 volts reaches the controller. 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 controller (see Fig. 10). 

Although acceptable, the four-volt line-loss in this exam-

ple is higher than recommended.

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 controller. 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. 10 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.

The Honeywell transformers listed in Table 5 meet the NEMA 

standard DC20-1986.

All wiring must comply with applicable electrical codes and 

ordinances, or as specified on installation wiring diagrams. 

Controller wiring is terminated to the screw terminal blocks 

located on the top and the bottom of the device.

Disconnect power supply before beginning wiring or 

making wiring connections, to prevent electrical shock 

or equipment damage.

NOTES:

—

For multiple controllers operating from a single 

transformer, the same side of the transformer 

secondary must be connected to the same power 

input terminal in each controller. Controller 

configurations will not necessarily be limited to 

three devices, but the total power draw, including 

accessories, cannot exceed 100 VA when 

powered by the same transformer (U.S. only). For 

power and wiring recommendations, See “Power” 

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