Transformer Full-Load Current Calculator

Primary and secondary full-load amps for a single-phase or three-phase transformer from its kVA rating and voltages, plus the turns ratio.

kVA
Nameplate apparent power. Common sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500 kVA.
V
Three-phase: line-to-line voltage.
V
Three-phase: line-to-line voltage (e.g. 208 for a 208Y/120 V secondary).
%
100% gives full-load current. Enter your actual load to see its current.
Secondary current
208.2 A
Primary current
90.2 A
Voltage (turns) ratio
2.308 : 1 (step-down)
Real power at pf 0.8
60.0
Typical planning power factor; actual kW = kVA × pf.

Estimate generated with SortedSums: https://sortedsums.com/transformer-current-calculator/. For informational purposes only; verify before relying on it.

Estimate only. This tool is for informational and educational purposes. Results depend on your inputs and simplifying assumptions, and are not a substitute for professional engineering, design or financial advice. Always verify with a qualified professional and applicable codes before purchasing, building or making decisions.

How it’s calculated

A transformer is rated in kVA (apparent power). Ignoring the small losses, the same kVA flows in the primary and secondary, so current is inversely proportional to voltage: Is/Ip = Vp/Vs = Np/Ns. For three-phase, apparent power is √3 × line voltage × line current.

Single-phase: I = kVA × 1000 / V Three-phase: I = kVA × 1000 / (√3 × V_LL)

Example: a 75 kVA three-phase transformer, 480 V primary, 208Y/120 V secondary. Ip = 75,000 ÷ (1.732 × 480) = 90.2 A; Is = 75,000 ÷ (1.732 × 208) = 208.2 A. A 25 kVA single-phase 240 V secondary carries 25,000 ÷ 240 = 104.2 A.

Full-load current is the starting point for conductor and overcurrent device selection; the electrical code sets the actual percentages, conductor ampacity and protection rules.

Frequently asked questions

Why is the secondary current higher?

Power in equals power out (less small losses), so stepping voltage down by a factor steps current up by the same factor.

Does power factor change the current?

No, for a given kVA. Current depends on apparent power (kVA). Power factor only determines how much of that kVA is real power (kW).

What voltage should I use for 208Y/120 V?

Use the line-to-line voltage, 208 V, with the three-phase setting.

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Sources

Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.

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