[creative_mind]

I have a transformer which it's labelled 24 VAC and 40 VOLT-AMP. How do I figure out the current rating mathematically?

Thanks.

[Stevenpats]

40VA/24Volts=1.6 amps or so,VA is just another term for power similiar to watts. Larger transformer are usually rated in KVA.
Steven

[creative_mind]

Looks like power in watts is about 65% of VA.

Am I correct?

[Al_The_Man]

The reason the wattage in a transformer is not the direct product of the voltage x current as it is in a DC or AC resistive circuit is that a transformer is an inductive device, so the voltage wave form leads the current wave form so they do not exactly peak at the same time. So in order to calculate the actual wattage rating this amount of lead has to be known and is known as the power factor, and varies on the device and the operating conditions etc.
An extreme example would be a perfect inductance where the phase shift was 90deg. Voltage would be applied and current would flow but wattage or power would be zero.
In circuits with a poor power factor, a capacitor can be added to compensate for the phase shift because in a capacitor the voltage LAGS the current.
Al

[limbo]

Stevenpats is right, its not 65%. Its 24V, 40 Watts. Maybe you confused the 24Vac with watts?

John