Transformer kVA Calculator

Enter a transformer rating and voltage to get the full-load current — or work backwards from amps to the kVA you need to order. Works for three-phase and single-phase systems; no sign-up, instant results.

Enter a rating and a voltage, then press Calculate.

Quick reference — full-load current by kVA rating (three-phase)

Transformer ratingAmps at 480 VAmps at 4,160 V
75 kVA90.21 A10.41 A
100 kVA120.3 A13.88 A
150 kVA180.4 A20.82 A
300 kVA360.8 A41.64 A
500 kVA601.4 A69.39 A
1,000 kVA1,203 A138.8 A
2,000 kVA2,406 A277.6 A

Table values from I = kVA × 1000 / (V × √3). Single-phase current at the same kVA and voltage: I = kVA × 1000 / V. Always confirm against the transformer nameplate.

How the Calculator Works

The calculator applies the standard transformer current relationship. For a three-phase transformer, full-load line current is:

I (A) = kVA × 1000 / (V × √3)

For a single-phase unit the square-root-of-three factor drops out: I (A) = kVA × 1000 / V. Working in the reverse direction, the required rating is kVA = √3 × V × I / 1000 (three-phase) or kVA = V × I / 1000 (single-phase).

Three assumptions sit behind these formulas: the load is balanced (three-phase), the voltage is the nameplate line-to-line voltage, and the transformer is delivering its full rated kVA. The result is the current you size conductors, breakers and fuses around. Magnetizing current and impedance-driven voltage drop are not included — they typically shift the nameplate full-load figure by only a percent or two, which is why the calculated value is used for planning and the nameplate remains the reference for commissioning.

When Engineers Use This Calculator

Sizing a feeder: once the transformer kVA is fixed, the secondary full-load current determines conductor cross-section, breaker frame and busbar rating in the switchboard or MCC it feeds — the exact calculation this page does in one step.

Checking an existing installation: adding a new load to a transformer already in service starts with knowing how many amps of headroom remain at that kVA rating and voltage.

Specifying a replacement: when a site drawing shows voltage and current but no rating, converting amps back to kVA tells you which standard size to order — 75, 100, 150, 300, 500, 1000 or 2000 kVA are the common steps for distribution-class units, and the quick-reference table above shows the current each one delivers at 480 V and 4,160 V.

よくある質問

How do I convert kVA to amps?

Multiply kVA by 1000 and divide by the voltage — and by √3 additionally if the system is three-phase. Example: 100 kVA at 480 V three-phase gives 100 × 1000 / (480 × 1.732) ≈ 120.3 A. The calculator above runs this for any values you enter.

Why are transformers rated in kVA instead of kW?

A transformer carries apparent power. The actual real power it delivers depends on the load’s power factor, which the manufacturer cannot predict. Heating in the windings is driven by current, so kVA (voltage × current) is the honest thermal limit — and the reason conductor sizing also starts from kVA, not kW.

What size transformer do I need for 200 A at 480 V three-phase?

kVA = 1.732 × 480 × 200 / 1000 ≈ 166 kVA. In practice you select the next standard rating up — commonly 225 kVA — which leaves margin for load growth and keeps the unit from running at its thermal ceiling.

Does the calculator account for transformer losses?

No. It converts nameplate apparent power to current. Core and winding losses (typically 1–3% for distribution units) become heat inside the transformer rather than delivered load, and they have a negligible effect on the full-load current figure used for feeder sizing.

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