How to Properly Size a Transformer

In the HVAC industry, we commonly use Class 2 control transformers to lower the voltage to 30 volts or less, with a power rating of 100VA or less. This allows us to run low voltage wiring outside of an enclosure without electrical conduit.

The primary voltage is the input voltage, normally 120, 208, 240 or 460 volts. The secondary voltage or the output voltage will normally be between 24 and 30 volts. The actual output voltage will depend on the actual primary voltage. The transformer power is rated by VA capability. The VA needs to be large enough to handle the total control power requirements, so a high enough VA should be selected.

To properly size a transformer, first the primary voltage must be selected. If the primary voltage is 208V, care must be taken to wire only to the 208V primary tap. If the primary voltage is 240V, care must be taken to wire only to the 240V primary tap. 208V wired to the 240V tap will result in low secondary voltage. The low secondary voltage will cause contactor, relay chatter or failures. This should be checked when installing 208/240 volt package equipment.

To determine the required transformer VA, multiply the secondary voltage by the required amperes: 

For example, VA=Volts x Amperes or 28V x 1.2 amperes = 33.6 VA.

To determine the maximum transformer current, divide the transformer VA by transformer secondary voltage.

Amperes = VA/Secondary Volts or 40VA/28V = 1.43 amperes.

Transformers are designed to fail if a short in the secondary or control side occurs. If the transformer fails, you most likely have a short or overload somewhere in the control circuit. This should be found and repaired before replacing the transformer, as a transformer rarely fails on its own. An inline fuse or a transformer with a circuit breaker can be installed to protect the new transformer while the problem is located.

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