What is the formula for Transformer VA Rating?

The Transformer VA Rating formula is S = V × I (volt-amperes), where S—Apparent power (VA); V—Winding RMS voltage (V); I—Winding RMS current (A).

Transformer VA Rating

Q: How do you calculate Transformer VA Rating?

Use the formula S = V × I (volt-amperes). A transformer is rated in volt-amperes (VA) rather than watts because its core and copper losses depend on voltage and current, not on the load power factor. Enter the values you know in the calculator above and it solves for the remaining variable.

S = Vp × Ip = Vs × Is

Calculator

Apparent Power

Winding Voltage

Winding Current

Result

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Formula

S = V × I (volt-amperes)

Description

A transformer is rated in volt-amperes (VA) rather than watts because its core and copper losses depend on voltage and current, not on the load power factor. The apparent power equals the product of winding voltage and current, and is the same on the primary and secondary sides (minus small losses). Sizing a transformer means ensuring its VA rating exceeds the apparent power the load draws.

Variables

S — Apparent power (VA)
V — Winding RMS voltage (V)
I — Winding RMS current (A)

Practical Notes

For a reactive load, real power P = S × PF is less than the VA rating, but the transformer must still be sized for the full VA. Add 20–30% headroom for inrush and ambient derating.

Related Concepts

All Magnetics & Transformers formulas →

Transformer EMF Equation Transformer VA Rating Transformer Voltage Regulation Transformer Current Ratio Transformer Copper Loss Transformer Iron (Core) Loss Transformer Turns-per-Volt Transformer Impedance Matching

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