Transformer VA Rating

S = Vp × Ip = Vs × Is

Quick answer

This calculator solves S = V × I (volt-amperes) for Apparent Power. With Winding Voltage 12 V, Winding Current 2 A, the result is 24 W. It solves in 3 directions, so you can enter the values you have and get the one you need.

Calculator

Apparent Power (W)
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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.

Symbols and units

Symbol Meaning Default unit Accepted units
S Apparent Power W µW, mW, W, kW
V Winding Voltage V pV, nV, µV, mV, V, kV
I Winding Current A pA, nA, µA, mA, A, kA

Step by step

  1. 1. What you know: Winding Voltage 12 V, Winding Current 2 A
  2. 2. The relation: S = V × I (volt-amperes)
  3. 3. Substitute: V = 12 V, I = 2 A
  4. 4. Result: Apparent Power = 24 W

Related quantities from the same inputs

  • P = V × I = 24 W

Worked examples

Minimum range

Given: Winding Voltage 0.33 V, Winding Current 2 mA

Result: Apparent Power = 0.66 mW

Low range

Given: Winding Voltage 3.3 V, Winding Current 20 mA

Result: Apparent Power = 66 mW

Typical

Given: Winding Voltage 12 V, Winding Current 2 A

Result: Apparent Power = 24 W

High range

Given: Winding Voltage 0.23 kV, Winding Current 100 A

Result: Apparent Power = 23 kW

Maximum range

Given: Winding Voltage 2.3 kV, Winding Current 1 kA

Result: Apparent Power = 2300 kW

Solving for each variable

Solve for Apparent Power

Winding Voltage 0.33 V, Winding Current 2 mA → 0.66 mW

Solve for Winding Voltage

Apparent Power 10 mW, Winding Current 2 mA → 5 V

Solve for Winding Current

Apparent Power 10 mW, Winding Voltage 0.33 V → 30.3 mA

How Apparent Power changes with Winding Voltage

Winding Voltage (input) Apparent Power (result)
1.2 V 2.4 W
3 V 6 W
6 V 12 W
9 V 18 W
12 V 24 W
18 V 36 W
24 V 48 W
48 V 96 W
84 V 0.168 kW
120 V 0.24 kW

The same result in other units

Unit Value
mW 24000
W 24
kW 0.024

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Frequently asked questions

How do you calculate Transformer VA Rating?

Use S = V × I (volt-amperes). With Winding Voltage 12 V, Winding Current 2 A, Apparent Power works out to 24 W.

What is the formula for Transformer VA Rating?

The formula is 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.

How do you find Winding Voltage when you know the other values?

Rearrange for Winding Voltage: with Apparent Power 10 mW, Winding Current 2 mA, Winding Voltage is 5 V.

How do you find Winding Current when you know the other values?

Rearrange for Winding Current: with Apparent Power 10 mW, Winding Voltage 0.33 V, Winding Current is 30.3 mA.

What units does Transformer VA Rating use?

Apparent Power is shown in mW, W, kW. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

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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