lower edge of the shaded zone in Fig. 12 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 lineloss,
supplies 20.4V to the controller (see Fig. 12).
Although acceptable, the four-volt line-loss in this example is
higher than recommended.
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 lineloss 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
To meet the National Electrical Manufacturers Association
(NEMA) standards, a transformer must stay within the NEMA
limits. The chart in Fig. 12 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
Fig. 12. NEMA Class 2 transformer voltage output limits
% OF LOAD
The Honeywell transformers listed in Table 9 meet the NEMA
Table 9. Honeywell transformers that meet NEMA stan-
NOTE: The AT88A and AT92A transformers do not meet the
voluntary 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.
Electrical Shock Hazard.
Can cause severe injury, death or property
Disconnect power supply before beginning wiring or making
wiring connections, to prevent electrical shock or equipment
Guidelines for Power Wiring
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 device.
The earth ground terminal must be connected to a verified
earth ground for each controller in the group (see Fig. 15 on
page 16). Controller configurations are not necessarily limited
to two devices, but the total power draw, including
accessories, cannot exceed 100 VA when powered by the
same transformer (U.S. only).
• See Fig. 14 on page 16 for controller power wiring used in
UL 1995 equipment (U.S. only).
• Many controllers require all loads to be powered by the
same transformer that powers the controller.
• Keep the earth ground connection wire run as short as
possible (refer to Fig. 13-Fig. 15 beginning on page 16.
• Do not connect earth ground to the controller's digital or
analog ground terminals (refer to Fig. 13 and Fig. 15).
• Unswitched 24 Vac power wiring can be run in the same
conduit as the LONWORKS® Bus cable.
• Maintain at least a 3 in. (76 mm) separation between Triac
outputs and LONWORKS® Bus wiring throughout the