Honeywell Heat/1Cool - T841A1563 Premier 2 Stage Thermostat User Manual page 16

HEAT PUMP THERMOSTAT CROSS REFERENCE/SELECTION GUIDE
NEMA Standard Transformers Give You Better Control
UL sets the safety standards all transformers must meet. But these are minimum standards. The Honeywell AT20A,
AT40A, AT72D and AT87A Transformers do even more; they meet tough voluntary standards of the National Electrical
Manufacturer's Association (NEMA).
Why is this so important? Because today's sophisticated control systems require reliable, accurate voltage output for
optimal system performance. It's especially important for electrical controls (OEM boards in high efficiency furnaces, for
example) or resistive heaters, such as thermostat heat anticipators.
For better performance, some heat pumps require a NEMA standard transformer. Conformance to NEMA standards
assures the voltage output of a transformer will remain within acceptable limits, regardless of load conditions. No
excessively high open-circuit voltage under light load conditions. No low, "brown-out" voltage levels under heavy load
conditions. At 100% load, the transformer secondary must supply between 23 and 25 volts to meet the NEMA standard.
How can you tell that the transformers you buy meet this tough, high standard? Look for the transformer to be marked:
NEMA Type B (20VA), NEMA Type D (40VA), NEMA Type E (48-50 VA). Only transformers conforming to the NEMA
standard can be marked in this way. The following are recommended Honeywell NEMA standard transformers:
AT72D1691, AT72D1683, AT87A1106
70-6627 • 14
27
26
25
24
23
22
21
20
19
18
17
16
15
14
0 50 100
% OF LOAD
Fig. 15. NEMA Class 2 transformer band.
200
150
M993