24V 40VA Transformer

Understanding the Ratings

A transformer has three primary electrical ratings to check: input voltage, output voltage, and volt-ampere capacity.

The input voltage is the supply power coming into the transformer. In home systems, this is typically 120 V, 208 V, or 240 V. The output voltage must be 24 V to safely power low-voltage controls like your thermostat and control board. The VA rating indicates how much electrical load the unit can handle.

• Input volts must match your supply, such as 120 V or 208/240 V.
• Output volts must be 24 V for standard control circuits.
• The VA rating must be the same or higher than your old part, with 40 VA being standard.

How to Read Your Old Part Label

Before choosing a replacement, look at the metal frame or stamped label on your current transformer. The label lists the primary voltage, secondary voltage, and VA capacity.

Check whether your transformer uses a single primary voltage or multiple voltage taps. Some transformers accept only 120 V or only 208/240 V, while multi-voltage models include taps for 120/208/240 V. Also inspect how the transformer attaches to the cabinet, as parts are built with foot mounts or multi-mount configurations.

Differences Between Listed Models

Our listed 40 VA transformers differ by input voltage options, mounting styles, and wiring connections. Choosing the right one depends on your equipment wiring and cabinet layout.

• White Rodgers 90-T40F3: Foot mount, leadwire connections, 60 Hz, rated for 120/208/240 V primary, and replaces part number S84Z-401.
• White Rodgers 90-T40F1: Foot mount, leadwires, 60 Hz, dedicated 120 V input, and replaces S81-373.
• White Rodgers 90-T40M3: Multi-mount design, leadwire connections, 60 Hz, rated for 120/208/240 V primary.
• Packard PF12440: Foot mount, dedicated 120 V input, and replaces T2401, T5401, and T3401.
• Packard PF22440: Foot mount, dedicated 208/240 V input, and replaces T2403.
• Packard PF42440: Foot mount, rated for 120/208/240 V primary.
• Packard PM12440: Multi-mount, 120 V input, and replaces 24-VOLT.
• Packard PM32440: Multi-mount, rated for 208-240 V input.

Common Signs of Transformer Failure

When a transformer fails, low-voltage power stops flowing to your system controls. A thermostat display that goes completely blank or an indoor unit that fails to respond to any cooling or heating call can point to a dead transformer.

Transformers often fail because of an underlying issue on the low-voltage circuit. A shorted thermostat wire or a failing contactor coil can draw excessive current and burn out the transformer windings. Always find and fix the cause of the overload before installing a new part, or the replacement may fail as well.

How to confirm it fits

1. Turn off electrical power at the main breaker panel and verify there is no voltage present.
2. Locate the transformer inside the unit cabinet and read the printed label or stamped stamping.
3. Confirm the output rating reads 24 V and the capacity reads 40 VA.
4. Check the primary input voltage to see if your system supplies 120 V, 208 V, 240 V, or requires a multi-voltage unit.
5. Note the mounting configuration, comparing whether your frame requires a foot mount or a multi-mount bracket.
6. Check existing part numbers on the old casing to match listed replacement numbers such as S84Z-401, S81-373, or T2401.