voltage ampere (VA)

Volt ampere( VA) is the power rating for apparent power (S) where the alternating current is not in phase with the alternating voltage. The volt-ampere parameter is specified wherever alternating current or three-phase current together with an alternating voltage form an energetic power quantity.

The apparent power is calculated from the geometric addition of active power and reactive power. The ratio of active power to apparent power is expressed in the power factor, which is a measure of the phase shift between the alternating current and the alternating voltage. The unit volt-ampere corresponds to the unit watt (W) as long as it is a resistive load and there is no phase shift between current and voltage. Only when the circuit is loaded by inductive or capacitive load, a phase shift occurs and reactive power is added to the active power. A voltampere is the electromotive force that causes an alternating current of one ampere (A) at an alternating voltage of one volt (V).

The volt ampere is critical to the design and loading of high-voltage, medium-voltage, and low-voltage power systems; it is used to determine conductor cross-sections, as well as to design transformers, switchgear, UPS systems, high-voltage power lines, and other equipment. Inpower electronics, the volt ampere is expressed with the appropriate prefixes in kilo volt-amperes (kVA), which is equivalent to 1,000 volt-amperes (VA), and mega volt-amperes( MVA), which is equivalent to 1,000,000 VA.

In computer technology, VA ratings are often on the nameplates. However, the VA specifications can differ considerably from the watt specification, whereby the latter must always be lower than the VA specification. Both specifications are directly related to each other via the power factor.