AUWiring

Star-delta starting current calculator

Estimate star-delta starter line current from entered DOL current, star current ratio and transition current basis for Australian motor-start records.

  • Calculator
  • Motors
  • Australia
Use the motor schedule, starter record or asset label so the current basis can be checked later.
V
Use line-to-line voltage for the balanced three-phase starting kVA estimate.
A
Enter the direct-on-line current from motor, starter, schedule or project data.
%
Enter the expected star line current as a percent of DOL line current.
%
Optional. Enter transition current as a percent of DOL current when a transition basis is documented.
A
Optional. Enter a project, equipment or supply discussion current for comparison.
Istar = IDOL x star_% / 100; Idelta = IDOL; Itransition = IDOL x transition_% / 100; Sstar_3ph = sqrt(3) x VLL x Istar / 1000
  • DOL current must come from motor, starter, schedule or project data.
  • Star current percent is an entered starter record value.
  • Transition current is optional and should be entered only when a transition basis is documented.
  • Starting kVA is a worksheet relationship, not a supply or generator decision.
Formula variables
VariableMeaningUnitUse
IDOLDirect-on-line currentAEntered line current basis for the motor.
star_%Star line current ratio%Entered star current ratio as a percent of DOL line current.
IstarEstimated star currentALine current estimate during the star starting portion.
IdeltaDelta line currentADOL current carried as the delta reference current.
transition_%Transition current ratio%Optional transition current ratio as a percent of DOL current.
ItransitionTransition currentAOptional current estimate for the transition record.
VLLLine voltageVEntered three-phase line-to-line voltage.
Sstar_3phStar starting apparent powerkVAThree-phase apparent-power estimate using star current.
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Star-delta starting current calculator technical guide

Estimate star-delta starter line current from entered DOL current, star current ratio and transition current basis for Australian motor-start records.

Use this page when a motor-start record needs a transparent star-delta current estimate from an entered direct-on-line current. The calculator multiplies the DOL current by the entered star current ratio, keeps the DOL current as the delta reference and optionally records transition current as a percent of DOL current.

The output is a current worksheet for later project review. It does not replace motor data, starter data, load-torque review, transition timing, voltage-dip calculation or manufacturer instructions.

Field use cases

Practical star-delta current use cases
Work settingReal questionUseful action from this page
Motor schedule reviewWhat star current follows from the DOL current already recorded?Enter DOL current and star current ratio, then export the current basis.
Tender estimateWhat reduced current should be carried before starter data is complete?Use an editable star ratio and label the source of the assumption.
Switchboard discussionHow much lower is the star line current than the DOL current?Record the star current and current-reduction percent beside the entered DOL current.
Transition noteIs there a documented transition current basis?Enter transition percent only when the basis exists.
Voltage-dip preparationWhich starting current should feed a later dip worksheet?Carry the star current forward with the DOL and starter basis attached.

Confirm that the DOL current, star current ratio and transition basis belong to the same motor and starting arrangement before using the record.

Starter boundary

What the worksheet includes and excludes
AreaIncluded hereKept outside this page
Star currentCurrent from entered DOL current and star ratio.Product-specific current curves.
Delta referenceEntered DOL current kept visible for comparison.Running current and overload setting selection.
Transition currentOptional percent-of-DOL record.Open-transition behaviour, torque dip and transient study.
Starting kVAThree-phase kVA relationship using star current and voltage.Generator choice, supply arrangement and DNSP assessment.
Review currentOptional entered comparison current.A project threshold created by the calculator.

The calculator is deliberately narrow. It records the current relationships needed for review, while starter selection, load acceleration and equipment coordination stay with product data and project engineering.

Result interpretation

Reading the star-delta result
Result stateWhat it meansPractical response
Star-delta recordNo review current was entered.Treat the result as a current estimate record and add a real comparison value only when one exists.
Within entered review currentThe estimated star current is not above the entered comparison current.Keep the DOL current source and starter ratio visible before downstream use.
Above entered review currentThe estimated star current is above the entered comparison current.Recheck the DOL current, starter ratio, transition basis and review-current source.
Transition not enteredThe transition current field is blank.Do not infer a transition value without starter or project data.
High star ratioThe star current is closer to the DOL current.Check whether the entered ratio matches the starter and load basis.

Formula basis

The star line current is:

text Istar = IDOL x star_% / 100

The delta reference and optional transition current are:

text Idelta = IDOL Itransition = IDOL x transition_% / 100

The three-phase star starting apparent power is:

text Sstar_3ph = sqrt(3) x VLL x Istar / 1000

These relationships use entered current and voltage values. They do not model acceleration, torque, transition timing or supply impedance.

Worked records

Star-delta current examples
Recorded situationCalculation basisExample resultFollow-up supported
Fan star-delta current record180 A DOL current, 33.3 percent star ratio and 400 V three phase.Star current about 59.94 A and star starting apparent power about 41.53 kVA.Carry the current into voltage-dip or supply review with the starter basis attached.
Pump record without transition value120 A DOL current and 33.3 percent star ratio.Star current about 39.96 A with no transition current entered.Add transition data only when starter or project data provides it.
Motor current review240 A DOL current and 40 percent star ratio compared with 90 A.Star current about 96 A, above the entered comparison current.Recheck DOL current, review current and starter assumptions before handoff.

Australian context

The page uses Australian 230/400 V, 50 Hz motor-start context, metric units and local project terminology. The formula is an engineering worksheet relationship, not a reproduced controlled standards table.

Motor-starting work can be affected by current standards, state or territory obligations, DNSP requirements, local authority requirements, project specifications, equipment ratings and manufacturer instructions. For large motors, weak supplies, generator-backed supplies, high-inertia loads, frequent starts or sensitive equipment, treat this estimate as one entry in a wider review.

Stop points

  • The DOL current source is unknown or may not match the actual motor.
  • The star current ratio is a placeholder but is being treated as product data.
  • Transition timing or torque dip matters to the driven load.
  • The result is being used as a voltage-dip, generator or starter-setting answer.
  • Manufacturer starter or motor data conflicts with the entered values.
  • The entered review current has no documented project source.

Fan star-delta current record

A three-phase fan has an entered 180 A direct-on-line current and a starter record using 33.3 percent star line current.

Motor reference
FAN-SD-1
Line voltage
400 V
DOL current
180 A
Star current ratio
33.3%
Transition current ratio
100%
Review current
70 A
  1. Estimated star current59.94 A
  2. Delta line current180 A
  3. Current reduction66.7%
  4. Starting apparent power41.53 kVA
  5. Transition current180 A
  6. Current margin10.06 A
Estimated star current59.94 A

Carry the current record forward with the starter and DOL basis.

The star current is within the entered review current. Keep the DOL current source and transition basis with the record.

  • 400 V line-to-line three-phase supply context.
  • DOL current is entered by the user from motor or project data.
  • Transition current is represented by an entered percent of the DOL current.

Pump record without transition value

A pump worksheet records a star-current estimate before the transition current basis has been documented.

Motor reference
PMP-SD-2
Line voltage
400 V
DOL current
120 A
Star current ratio
33.3%
Transition current ratio
Not entered
Review current
Not entered
  1. Estimated star current39.96 A
  2. Delta line current120 A
  3. Current reduction66.7%
  4. Starting apparent power27.69 kVA
Estimated star current39.96 A

Carry the current record forward with the starter and DOL basis.

The result is a star-delta current record only. Add transition and review values when real project data is available.

  • Star current ratio is entered as a percent of DOL line current.
  • No transition current was entered.
  • No review current was entered.

Motor current review

A maintenance replacement record compares a higher star-current ratio against an entered review current.

Motor reference
MTR-SD-9
Line voltage
400 V
DOL current
240 A
Star current ratio
40%
Transition current ratio
100%
Review current
90 A
  1. Estimated star current96 A
  2. Delta line current240 A
  3. Current reduction60%
  4. Starting apparent power66.51 kVA
  5. Transition current240 A
  6. Current margin-6 A
Estimated star current96 A

The estimate is above the entered review current.

The estimate is above the entered review current, so the starter basis, motor data and load assumptions need review.

  • The review current is an entered comparison value.
  • Transition current is set equal to the entered DOL current.
  • The worksheet does not model acceleration torque.

Questions

Why does the default star ratio use 33.3 percent?

It is a common worksheet starting point for star-delta line current. Replace it with starter, motor or project data when a better basis is available.

Should I enter transition current?

Enter transition current only when the project has a documented transition basis. Leave it blank when the transition value is not known.

Can this decide whether a star-delta starter suits the load?

No. Load torque, acceleration, transition timing, motor data and starter data need separate review.

Can this calculate voltage dip?

No. Use the resulting current as one input to a separate motor voltage-dip worksheet when source data is available.

What should be kept with the exported record?

Keep the DOL current source, star current ratio, transition basis, voltage and any entered review current.