Transformer Calculator - kVA, Primary Current and Secondary Current

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Transformers are sized in kVA because they carry volts and amps, whether the load power factor is nice or ugly. Calculate the kVA, then check primary and secondary current separately.

Formula at a glance

  • kVA = kW / PF
  • single-phase current: A = kVA x 1000 / V
  • three-phase current: A = kVA x 1000 / (1.732 x V)

Field note: Do not buy the transformer at exactly the calculated kVA unless the load is steady and well known. Field loads grow. They always do.

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Transformer Sizing Calculator

Calculate transformer kVA and currents

kW
V
V
Result
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Formulas

kVAkVA = kW / PF
I (1φ)I = kVA×1000 / V
I (3φ)I = kVA×1000 / (√3×V)

Standard Sizes (kVA)

Single-Phase: 5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500

Three-Phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500

Related

kVA to AmpskW to kVAMotor Current

How to use the Transformer Calculator

Use this for a first sizing pass, then check the actual code table, installation method, conductor material and temperature rating. A calculator can point you in the right direction. It cannot inspect the job.

Worked example

Example: a 40 kW load at PF 0.8 needs 50 kVA before margin. On 415 V three-phase, that is about 69.6 A.

Practical checks before you trust the number

  • Add design margin for future load and heating.
  • Motor loads need starting current checks.
  • Primary and secondary currents are different when voltage changes.

Common mistake

Do not buy the transformer at exactly the calculated kVA unless the load is steady and well known. Field loads grow. They always do.

Sources and references

Related calculators

Frequently Asked Questions

kVA = (V × A) ÷ 1000 for single-phase, or kVA = (√3 × V × A) ÷ 1000 for three-phase. Or from connected load: kVA = kW ÷ PF + design margin. Example: 50 kW load at PF 0.85 + 25% margin → 50 ÷ 0.85 × 1.25 = 73.5 kVA → choose 100 kVA standard transformer.

kVA = (V_line × A) ÷ 1000 for single-phase, or kVA = (√3 × V_line × A) ÷ 1000 for three-phase. Use line voltage. Always size to the next standard transformer kVA (25, 63, 100, 160, 250, 400, 630, 1000 kVA, etc.) with at least 20 to 30% spare capacity.

Sum the kW loads, divide by worst-case PF for kVA, add 25 to 30% margin and growth allowance, pick the next standard transformer size. Example: 80 kW at PF 0.85 → 94 kVA + 30% margin = 122 kVA → pick 160 kVA. For diversified loads, apply diversity factor (0.7 to 0.8) on the connected total.

Three-phase: A = (kVA × 1000) ÷ (√3 × V_line). For 25 kVA at 415 V: A = 25,000 ÷ (1.732 × 415) = 34.8 A per phase. The transformer secondary cable, breaker, and bus bar must all be sized for at least this current with continuous-duty factor applied.

Allow at least 20 to 30% spare capacity at initial commissioning. Some clients ask for 50% to allow for future expansion. Don't load transformers above 80% continuously — they run hot, lose efficiency, and age faster. Also leave room for unbalanced loading and harmonics if non-linear loads are involved.

Primary current = kVA ÷ (√3 × V_primary), and secondary current = kVA ÷ (√3 × V_secondary). For a 100 kVA, 11 kV / 415 V step-down: primary current = 100,000 ÷ (1.732 × 11000) = 5.25 A; secondary current = 100,000 ÷ (1.732 × 415) = 139 A. Primary cables are thin, secondary cables are thick.

Yes, when used with care. Enter load kW or kVA and PF; calculator suggests transformer size. Always add margin for future growth and unbalanced loading, then pick the next standard size up. For specialized applications (welding, harmonic-rich loads), consult the transformer manufacturer for K-factor or harmonic-rated units.