AUWiring

Transformer current formulas

Formula chart for transformer kVA, primary current and secondary current relationships.

  • Type: Formula chart
  • Scope: Australia
  • Reviewed: 2026-07-13
  • Sensitivity: Low

Transformer current relationships by winding side

Use these row summaries to read the calculation relationship before checking the value table.

  1. Single-phase winding current

    Calculate primary or secondary current from kVA and winding voltage.

    I = kVA x 1000 / V
    Input basis
    Transformer kVA, winding voltage and phase basis from the same transformer record.
    Output unit
    A
    Use when
    Use when the transformer side, voltage basis and phase basis are known.
    Boundary
    Use the correct project primary or secondary winding voltage.
    Not for
    Transformer product selection, thermal rating decision or protection setting.

  2. Three-phase line current

    Calculate line current from three-phase kVA.

    I = kVA x 1000 / (sqrt(3) x VLL)
    Input basis
    Transformer kVA, winding voltage and phase basis from the same transformer record.
    Output unit
    A
    Use when
    Use when the transformer side, voltage basis and phase basis are known.
    Boundary
    Use line-to-line voltage source and record the transformer side.
    Not for
    Transformer product selection, thermal rating decision or protection setting.

  3. kVA from single-phase current

    Estimate kVA from winding current and voltage.

    kVA = V x I / 1000
    Input basis
    Transformer kVA, winding voltage and phase basis from the same transformer record.
    Output unit
    kVA
    Use when
    Use when the transformer side, voltage basis and phase basis are known.
    Boundary
    Not an equipment thermal, protection or product selection check.
    Not for
    Transformer product selection, thermal rating decision or protection setting.

  4. kVA from three-phase current

    Estimate three-phase apparent power.

    kVA = sqrt(3) x VLL x I / 1000
    Input basis
    Transformer kVA, winding voltage and phase basis from the same transformer record.
    Output unit
    kVA
    Use when
    Use when the transformer side, voltage basis and phase basis are known.
    Boundary
    Confirm phase, winding voltage and project current margin basis.
    Not for
    Transformer product selection, thermal rating decision or protection setting.
  • Single-phase and three-phase transformer current relationships.
  • Show transformer current formulas while leaving product selection to calculator, manufacturer and project review.
  • Engineering formula context; product data and project requirements govern final use.
  • Primary and secondary current estimates must use the matching winding voltage; three-phase rows use VLL and sqrt(3).
  • Current margin, protection and product selection are project review items, not outcomes set by this chart.

Transformer current and kVA formula rows

Show transformer current formulas while leaving product selection to calculator, manufacturer and project review.

Transformer current formulas
RelationshipFormulaUseBoundary
Single-phase winding currentI = kVA x 1000 / VCalculate primary or secondary current from kVA and winding voltage.Use the correct project primary or secondary winding voltage.
Three-phase line currentI = kVA x 1000 / (sqrt(3) x VLL)Calculate line current from three-phase kVA.Use line-to-line voltage source and record the transformer side.
kVA from single-phase currentkVA = V x I / 1000Estimate kVA from winding current and voltage.Not an equipment thermal, protection or product selection check.
kVA from three-phase currentkVA = sqrt(3) x VLL x I / 1000Estimate three-phase apparent power.Confirm phase, winding voltage and project current margin basis.
  • This chart supports formula review; the transformer calculator owns entered values.

Formula variables

Use these symbols and units when reading the chart rows.

Formula variables
VariableMeaningUnitUse
kVATransformer apparent power.kVA
VWinding voltage.VUse the primary or secondary winding voltage that matches the side being calculated.
VprimaryPrimary winding voltage.V
VsecondarySecondary winding voltage.V
VLLLine-to-line voltage for three-phase formulas.V
sqrt(3)Three-phase line-current factor.Used only with the correct three-phase line-to-line voltage basis.
ILine or winding current.A
IprimaryPrimary-side current estimate.AUse the primary winding voltage basis.
IsecondarySecondary-side current estimate.AUse the secondary winding voltage basis.
marginProject-entered current margin.% or AA worksheet allowance, not a product selection rule.

Worked relationship example

This example shows how one set of entered values reads through the relationship.

400 V three-phase transformer current reading

A 500 kVA transformer secondary is reviewed using a 400 V line-to-line basis.

Transformer rating
500 kVA
Secondary voltage
400 Vline-to-line
Phase basis
three-phase
  1. Apply three-phase formulaI = kVA x 1000 / (sqrt(3) x VLL).
  2. Calculate currentI = 500 x 1000 / (1.732 x 400) = 722 A.
Secondary current estimateabout 722 A

Current margin, product rating and protection review remain separate project decisions.

Use the example as a transformer current review record before opening the transformer calculator.

  • 400 V line-to-line secondary basis.
  • Transformer product data and project requirements remain controlling.

Where the transformer chart belongs

Use the chart to understand the formula basis; calculators handle project-specific inputs, warnings and results.

Limits of formula-only transformer review
ItemValue
PurposeShow transformer current formulas while leaving product selection to calculator, manufacturer and project review.
How to use the sourceSingle-phase and three-phase transformer current relationships.
Standards and project contextEngineering formula context; product data and project requirements govern final use.
Where judgement is still neededUse this chart to understand the relationship behind a calculation; do not treat it as a substitute for entered project values.
Before relying on itConfirm current standards, local authority requirements, DNSP conditions, project documents and manufacturer data before relying on a decision.
Transformer current formulas standards and project context
ContextApplies toPublic boundary
Transformer product dataPrimary current, secondary current and winding voltage recordsUse current nameplate, datasheet or project data; the chart does not select transformer products or protection.
Phase and voltage basisSingle-phase and three-phase formulas using V, VLL and sqrt(3)Keep primary/secondary side and line-to-line basis visible before comparing current margin.
Project review boundaryTransformer current formulas chartConfirm current Australian standards, local authority requirements, DNSP conditions, project documents and manufacturer data before relying on a decision.

Formula basis for transformer current

Check the cited calculator source, winding-voltage basis, product-data boundary and review date before using these transformer-current relationships.

Transformer current formulas source basis
ItemValue
SourceAUWiring transformer current and kVA calculator formulas.
Source typeReleased calculator formula source
Derivation basisDerived from general transformer power relationships and AUWiring calculator logic.
Last checked2026-07-13
Review intervalAnnual or when transformer calculator formulas or product-data wording changes.
Review triggerCalculator formula change, transformer data practice change or project feedback.
Version usedR24-2026-07-13
Australian applicationAustralia; product data and project requirements can override general formula notes.

Use a calculator for project values

Use the chart to read the formula relationship, then open a calculator when you need entered project values, warnings and result context.

Transformer current and kVA

Calculate current or kVA from entered transformer values.

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