EleCanada
Equipment electrical quantities

Transformer current calculator

Calculate nominal primary and secondary full-load line currents from a known nameplate, or solve kVA from one supplied rated current.

Calculation inputs

Known transformer rating

kVA

Use total transformer nameplate kVA, not kW or a per-winding rating. This tool does not select a transformer size.

V RMS

Use the full primary terminal voltage for single phase, or line-to-line voltage for three phase, at the applicable nameplate tap.

V RMS

Use the full secondary terminal voltage for single phase, or line-to-line voltage for three phase. Do not substitute line-to-neutral voltage.

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Nominal full-load line currents

Enter the phase, total nameplate kVA and both rated voltages.

Uses the same apparent power on both sides. Reverse mode requires a known rated line current at the selected rated voltage. An arbitrary measured current cannot establish a transformer's nameplate rating.

No losses, excitation current, tap/voltage regulation, harmonics or imbalance are modelled. Results do not determine conductor, fuse or circuit-breaker ratings.

Use one known nameplate

Enter the transformer's total apparent-power rating and matching primary/secondary rated voltages. For three phase, use line-to-line voltages and a balanced load. For single phase, use the full terminal voltage corresponding to the total nameplate kVA.

When rated line current is known instead, select its primary or secondary side and enter the matching rated current. The reverse mode calculates the corresponding apparent power and the other side's nominal current, using the same apparent power on both sides. It does not determine an unknown transformer rating from an operating-current measurement.

The voltage ratio is a ratio of the entered terminal voltages. It is not a winding turns ratio for every connection. This tool does not analyse centre-tap loading, separate winding ratings, delta/wye winding currents, open-delta banks or autotransformer internal winding power. Inputs and results stay in the page and are not saved or transmitted.

Method and examples

Single phase: I = 1000 × kVA ÷ V. Balanced three phase: line I = 1000 × total kVA ÷ (√3 × line-to-line V). Apply the same rating separately to each side's voltage. The reverse mode rearranges the same relationship as kVA = phase factor × V × I ÷ 1000 for a supplied primary or secondary line current. Voltage ratio = primary V ÷ secondary V.

Example: 15 kVA single-phase at 600 V primary and 240 V secondary gives nominal line currents of 25 A and 62.5 A, with a 2.5:1 terminal-voltage ratio. At 75 kVA, 600/208 V balanced three-phase, the calculated currents are about 72.169 A primary and 208.179 A secondary.

In reverse, a supplied primary rated line current of 25 A at 600 V single phase gives 15 kVA and 62.5 A at a 240 V secondary. Selecting the other side clears the current so it cannot be silently reused at a different rated voltage.

These are illustrative nameplate scenarios, not recommended transformer ratings. The arithmetic retains precision and uses exact √3; display rounds to three decimals or scientific notation for very small or large values.

What the currents do not establish

Rated-current arithmetic does not predict actual primary input with losses and excitation current, or a varying load's operating current. It does not calculate starting/inrush current, available fault current, voltage regulation or thermal capability.

Transformer selection, protection, conductor sizing, tap use, grounding/bonding and installation approval require applicable Canadian sources, manufacturer instructions and qualified review. No code allowance or protection multiplier is applied here.

Common questions

Do I need a power factor?

Not for this nameplate-kVA calculation. kVA is apparent power. If you only know a load's kW, the AC electrical calculator requires an explicit power factor to obtain kVA; that conversion does not select a suitable transformer rating.

Is the three-phase result a winding current?

No. It is terminal line current using total three-phase kVA and line-to-line voltage. Delta and wye winding currents depend on their connection and are not reported.

Can I use the secondary current for a breaker or conductor size?

Not from this calculation alone. It applies no installation, protection, continuous-load or conductor rules. Use the applicable reviewed requirements and manufacturer instructions.

Electrical relationshipsSolve DC quantities, convert AC power/current, compare PF current ratios, or relate balanced wye voltages.Motor currentConvert mechanical shaft power or estimate motor current from matching full-load data.Connected loadTotal loads sharing a voltage, phase and power factor.

Source

Schneider Electric Canada, What is the formula to determine the KVA on a Transformer?, FAQ FA101600, modified December 15, 2025; rechecked November 10, 2026. Its single/three-phase kVA relationships calculate apparent power from line voltage and line current, or are rearranged for current at each rated voltage. Exact √3 is used rather than the FAQ's rounded 1.732.

The primary/secondary workflow uses one supplied nameplate rating on both sides and does not model transformer losses or import the source's sizing advice. Report a problem or suggest a source; inputs are not attached automatically.