Transformer Sizing Calculator

Enter the transformer kVA rating, primary voltage and secondary voltage to calculate the full-load amps (FLA) on both windings. Covers single-phase and three-phase transformers.

How It Works

The formula

The full-load amps on each winding are calculated from the kVA rating and the winding voltage:

  • Single-phase: FLA = kVA × 1000 ÷ V
  • Three-phase: FLA = kVA × 1000 ÷ (√3 × V)

The √3 factor (approximately 1.732) appears in three-phase calculations because the three windings share the line-to-line voltage in a balanced system. The formula applies to both primary and secondary windings; only the voltage changes.

Worked example

A 10 kVA single-phase transformer with a 240 V primary and 120 V secondary:

  • Primary FLA = 10,000 ÷ 240 = 41.67 A
  • Secondary FLA = 10,000 ÷ 120 = 83.33 A

A 75 kVA three-phase transformer with a 480 V primary and 208 V secondary:

  • Primary FLA = 75,000 ÷ (1.732 × 480) = 90.21 A
  • Secondary FLA = 75,000 ÷ (1.732 × 208) = 208.17 A

Standard Transformer kVA Ratings

Transformers are manufactured in standard kVA ratings. Choose the next standard size above your calculated requirement and add at least a 20% margin for future load growth.

Single-phase (common ratings)

1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 kVA

Three-phase (common ratings)

3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA

Frequently Asked Questions

How do I size a transformer for my load?

Add up the total connected load in kVA (or convert kW to kVA by dividing by the power factor). Then select the next standard transformer rating above that total. A common rule is to add 20-25% margin for load growth and to keep the transformer from running at full capacity continuously.

What is the difference between primary and secondary FLA?

The primary winding is the input side (connected to the supply) and the secondary winding is the output side (connected to the load). Because power is conserved, a step-down transformer draws less current on the primary than it delivers on the secondary - the lower the secondary voltage, the higher the secondary current. Both FLA values come from the same kVA figure divided by the respective winding voltage.

Why is kVA used for transformer ratings rather than kW?

Transformers are rated in kVA (apparent power) because their core and copper losses depend on voltage and current, not on the load power factor. A transformer rated at 10 kVA can supply 10 kVA at any power factor; the actual kW delivered depends on what the load draws. Using kVA avoids ambiguity and matches the way transformer heating is determined.

What changes when calculating three-phase transformer FLA?

For a three-phase transformer the FLA is divided by the square root of 3 (approximately 1.732) compared to a single-phase unit of the same kVA and voltage. This is because three-phase power is the product of three voltages and currents acting together. A 75 kVA three-phase transformer at 480 V primary draws only about 90 A, whereas a 75 kVA single-phase unit at the same voltage would draw about 156 A.

Should I add a safety margin to the calculated FLA?

The calculated FLA is the full-load value at 100% utilisation. In practice, NEC 450.3 and good engineering practice recommend sizing overcurrent protection based on the nameplate FLA. For conductor sizing, NEC 215.2 requires conductors rated at 125% of the continuous load. Always consult the applicable electrical code for the specific installation, and consider future load growth when selecting the transformer kVA rating.

Related Calculators

Continue with these related electrical tools

kVA Calculator

Apparent power (kVA) from volts and amps, single or three phase

Three-Phase Power Calculator

Three-phase power triangle: line current, kW, kVA, kVAR

Fault Current Calculator

Available short-circuit current from transformer kVA and impedance

Embed This Calculator

Add this free calculator to your website. Copy the code below and paste it into your page.

Electrical Calculators Electrical Calculators

Fast, free calculators for electricians and engineers

More Calculators

© 2026 Electrical Calculators. All rights reserved.

Privacy Policy Terms of Service Cookie Policy