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You will often find 40VA transformers in residential HVAC equipment; multi-tap primary transformers are common, and they allow you to connect the primary to several different voltages via pins or wires of varying colors. The label on the transformer will give you the volt-amp rating (VA), cycles per second (hertz/Hz), and primary and secondary voltage(s).
In the case of the multi-tap primary transformer in this video, you can only use white and one other color (we can’t jumper the other two together, either). The unused wires need to be cut or capped off under SEPARATE wire nuts. Connecting the other colors together will cause you to shunt the primary.
A transformer can step voltage up or down from the primary to the secondary; a step-up transformer boosts the voltage from the lower primary to the higher secondary, and a step-down transformer takes higher voltage from the primary and reduces it to a lower voltage on the secondary. The transformer in this video is a step-down transformer. A typical gas furnace generally requires a step-down transformer that takes 120v and reduces it to 24v. On the other hand, a residential air handler would often use a 240v step-down transformer that reduces the voltage to 24v.
Each transformer has two sets of windings (the primary and the secondary). The number of wraps of wire on each winding dictates the amount of voltage that the transformer steps up or down.
Volt-amps can let us know how many amps the secondary of the transformer can handle given the VA and the voltage. (For example, 40 VA / 24v = 1.66 amps.) There may or may not be enough amperage to power zoning panels, IAQ accessories, etc., so there may be cases where we have to upsize the transformer or get another transformer.
Adding a transformer also adds more of a load to the power coming in, so the transformer(s) may have an effect on wire sizing. It’s worth noting that VA and wattage are only the same if there is a power factor of 1 (unity). However, when the power factor is NOT 1, the VA will be higher than the wattage. To account for volts x amps and make sure it doesn’t exceed the rated VA, you will need to measure voltage with a voltmeter and amperage with an amp clamp; multiply the two and compare it to the VA rating.
You might want to try using your amp clamp on the primary and the secondary to see the difference. If we were to do that to the transformer in the video and read 1 amp on the secondary (24v), we would read 0.1 amps on the primary (240v) due to the difference between the number of wraps.