Why is line current based on tap ratio squared?

The worksheet applies the standard autotransformer relationship where supply line current follows the square of the tap ratio, while motor terminal current follows the tap ratio.

Should the tap percent come from the starter?

Yes. Use starter, motor or project data where available. The default is only an editable worksheet value.

Can this decide which tap should be used?

No. Tap choice depends on load torque, acceleration, voltage dip, motor data, starter data and project criteria.

Can this calculate generator suitability?

No. It reports current and kVA values that may support a later generator conversation.

What should be kept with the exported record?

Keep the full-load current source, DOL multiplier basis, tap percent, voltage and any entered review current.

Autotransformer starting current calculator

126.75A

Within the entered review current. Keep the tap and DOL multiplier basis visible with the record.

Motor reference: PMP-AT-1
Line voltage: 400V
Full-load current: 50A
DOL multiplier: 6x
Estimated DOL current: 300A
Autotransformer tap: 65%
Tap ratio: 0.65
Estimated supply starting current: 126.75A
Motor terminal current: 195A
Motor terminal voltage: 260V
Torque percent of DOL: 42.25%
Starting apparent power: 87.81kVA
Entered review current: 140A
Current margin: +13.25A
Review utilisation: 90.5%

Assumptions

DOL current is estimated from full-load current and the entered DOL multiplier.
Supply starting current uses DOL current multiplied by tap ratio squared.
Motor terminal current uses DOL current multiplied by tap ratio.
The worksheet does not model acceleration, transition timing, voltage dip or protective-device settings.

IDOL = IFLC x M; tap_ratio = tap_% / 100; Iline = IDOL x tap_ratio^2; Imotor = IDOL x tap_ratio; Vmotor = VLL x tap_ratio; torque_% = tap_ratio^2 x 100; Sstart_3ph = sqrt(3) x VLL x Iline / 1000

Full-load current and DOL multiplier are entered by the user.
Tap percent should match the starter data or documented project assumption.
Supply line current uses tap ratio squared while motor terminal current uses tap ratio.
Starting kVA is a worksheet relationship, not a supply or generator decision.

Formula variables
Variable	Meaning	Unit	Use
IFLC	Full-load current	A	Entered running current basis for the motor.
M	DOL multiplier	x	Entered multiplier used to estimate DOL current.
IDOL	Direct-on-line current	A	Estimated DOL current from full-load current and multiplier.
tap_%	Autotransformer tap	%	Entered starter tap percent.
tap_ratio	Tap ratio	ratio	Tap percent divided by 100.
Iline	Supply starting current	A	Estimated supply line current during autotransformer starting.
Imotor	Motor terminal current	A	Estimated motor terminal current under the entered tap.
Vmotor	Motor terminal voltage	V	Estimated terminal voltage at the entered tap.
torque_%	Torque percent of DOL	%	Tap-ratio-squared torque relationship for a first worksheet view.
Sstart_3ph	Starting apparent power	kVA	Three-phase apparent-power estimate using supply starting current.

Autotransformer starting current calculator technical guide

Estimate supply line current, motor terminal current and tap voltage from entered autotransformer starter assumptions for Australian motor-start records.

Use this page when a motor-start record needs a transparent autotransformer starter current estimate. The calculator estimates DOL current from full-load current and an entered multiplier, then applies the tap ratio to supply line current, motor terminal current and motor terminal voltage.

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, generator review or manufacturer instructions.

Field use cases

Practical autotransformer current use cases
Work setting	Real question	Useful action from this page
Motor schedule review	What supply current follows from the entered tap?	Enter FLC, DOL multiplier and tap percent, then export the tap basis.
Starter comparison	How does tap ratio change line current and motor terminal current?	Compare line current, motor terminal current and torque percent of DOL.
Tender estimate	What value should be carried before starter data is complete?	Use editable FLC, multiplier and tap fields with the source of each assumption visible.
Generator discussion	What starting kVA should be discussed with a supplier?	Record line starting current and kVA before separate generator review.
Voltage-dip preparation	Which current should feed a later dip worksheet?	Carry the supply starting current forward with the tap basis attached.

Confirm that the FLC, DOL multiplier and tap percent belong to the same motor and starting arrangement before using the record.

Tap interpretation

Tap-ratio outputs and meaning
Output	What the worksheet reports	Practical note
DOL current	FLC multiplied by the entered DOL multiplier.	This is the reference current before tap effects.
Supply starting current	DOL current multiplied by tap ratio squared.	This is the current normally carried into upstream supply discussion.
Motor terminal current	DOL current multiplied by tap ratio.	This helps explain the motor-side current basis.
Motor terminal voltage	Line voltage multiplied by tap ratio.	Lower tap voltage may affect starting torque and acceleration.
Torque percent of DOL	Tap ratio squared, expressed as a percent.	Use as a worksheet indicator only; real torque depends on motor and load data.

The tap relationship makes the supply current lower than the motor terminal current at reduced taps. That does not by itself show that the motor will accelerate the driven load.

Result interpretation

Reading the autotransformer result
Result state	What it means	Practical response
Autotransformer record	No 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 current	The estimated supply current is not above the entered comparison current.	Keep the FLC, DOL multiplier and tap source visible before downstream use.
Above entered review current	The estimated supply current is above the entered comparison current.	Recheck tap percent, DOL multiplier, FLC and review-current source.
Low tap ratio	Supply current falls, but torque percent also falls.	Check driven-load torque and acceleration outside this worksheet.
High tap ratio	More torque is available, but supply current rises.	Check voltage dip, source limits and equipment data outside this worksheet.

Formula basis

The DOL current and tap ratio are:

text IDOL = IFLC x M tap_ratio = tap_% / 100

The supply and motor terminal currents are:

text Iline = IDOL x tap_ratio^2 Imotor = IDOL x tap_ratio

The motor terminal voltage and torque relationship are:

text Vmotor = VLL x tap_ratio torque_% = tap_ratio^2 x 100

The three-phase starting apparent power uses supply line current and entered line voltage:

text Sstart_3ph = sqrt(3) x VLL x Iline / 1000

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

Worked records

Autotransformer current examples
Recorded situation	Calculation basis	Example result	Follow-up supported
Pump autotransformer record	50 A FLC, 6.0 x DOL multiplier, 65 percent tap and 400 V three phase.	Supply starting current about 126.75 A and motor terminal current about 195 A.	Carry the current and tap basis into voltage-dip or supply review.
Fan record without review current	28 A FLC, 5.5 x DOL multiplier and 50 percent tap.	Supply starting current about 38.5 A with no review current entered.	Add a real comparison current before downstream use.
High tap current review	80 A FLC, 6.0 x DOL multiplier and 80 percent tap compared with 300 A.	Supply starting current about 307.2 A, above the entered comparison current.	Recheck FLC, multiplier, tap percent and source 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 full-load current source is unknown or may not match the actual motor.
The DOL multiplier or tap percent is a placeholder but is being treated as product data.
Load torque, acceleration or transition behaviour 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.

Motor reference: PMP-AT-1
Line voltage: 400 V
Full-load current: 50 A
DOL multiplier: 6 x
Autotransformer tap: 65%
Review current: 140 A

Estimated DOL current300 A
Supply starting current126.75 A
Motor terminal current195 A
Motor terminal voltage260 V
Torque percent of DOL42.25%
Starting apparent power87.81 kVA
Current margin13.25 A

Estimated supply starting current126.75 A

Carry the current record forward with the tap and DOL multiplier basis.

The supply starting current is within the entered review current. Keep the tap and multiplier basis visible with the record.

400 V line-to-line three-phase supply context.
DOL multiplier is entered by the user.
Tap percent is entered from the starter basis.

Motor reference: FAN-AT-2
Line voltage: 400 V
Full-load current: 28 A
DOL multiplier: 5.5 x
Autotransformer tap: 50%
Review current: Not entered

Estimated DOL current154 A
Supply starting current38.5 A
Motor terminal current77 A
Motor terminal voltage200 V
Torque percent of DOL25%
Starting apparent power26.67 kVA

Estimated supply starting current38.5 A

Carry the current record forward with the tap and DOL multiplier basis.

The result is an autotransformer current record only. Add a real comparison current before downstream use.

Full-load current is entered by the user.
Tap ratio is 50 percent.
No review current was entered.

Motor reference: MTR-AT-9
Line voltage: 400 V
Full-load current: 80 A
DOL multiplier: 6 x
Autotransformer tap: 80%
Review current: 300 A

Estimated DOL current480 A
Supply starting current307.2 A
Motor terminal current384 A
Motor terminal voltage320 V
Torque percent of DOL64%
Starting apparent power212.83 kVA
Current margin-7.2 A

Estimated supply starting current307.2 A

The estimate is above the entered review current.

The estimate is above the entered review current, so the tap, multiplier and equipment basis need review before handoff.

The review current is an entered comparison value.
DOL current is estimated from FLC and the entered multiplier.
The worksheet does not model acceleration torque.