Honeywell Heat/1Cool User Guide - 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 SECONDARY VOLTAGE 27 26 25 24 23 22 21 20 19 18 17 16 15 14 0 50 100 % OF LOAD 150 200 M993 Fig. 15. NEMA Class 2 transformer band. 70-6627 • 14