The calculation, without hidden assumptions
Estimate the RMS line current a motor start can place on the supply. Start from known full-load current and a motor-specific locked-rotor multiple, then compare DOL with common reduced-current starting methods. The calculator keeps assumed multiplier estimates separate from manufacturer/nameplate LRA data.
How to use this calculator
Enter the motor full-load current from the nameplate or a verified design source.
Enter the locked-rotor/DOL current multiple from manufacturer data when available; do not treat the default as a universal motor value.
Choose the starting method and its applicable tap or current limit.
Read selected-method current, DOL reference current, current-to-FLA ratio and starting kVA.
Use the result for preliminary supply-impact comparison, then verify protection, voltage dip and starter suitability with project-specific data.
Where people use it
- •Comparing DOL and star-delta line starting current.
- •Estimating transient kVA seen by a transformer or generator before a detailed study.
- •Checking an autotransformer tap against a DOL baseline.
- •Translating a soft-starter current-limit setting into amperes.
Example: 40 A motor, 6× DOL
DOL current is 240 A. Star-delta line starting current is 80 A. At 415 V three-phase, that star-delta start is about 57.5 kVA. The 6× multiplier is an entered assumption, not a universal motor characteristic.
What the result does not assume
- •Prefer manufacturer/nameplate LRA, Istart/In or code-letter data over a generic multiplier. Motor design changes the real starting current.
- •Star-delta ÷3 applies to the line current when a six-lead motor is intended to run delta at the supply voltage; it is not valid for every motor connection.
- •Autotransformer line-side current is approximated with tap ratio squared. Starter design and transition behavior can change real measurements.
- •A soft starter is represented by its entered current limit; acceleration time and available starting torque require load and motor curves.
- •VFD starting is deliberately not reduced to a universal multiplier here because drive control, current limit and load torque determine the actual current.
- •This calculator does not size breakers, overloads, conductors or generators and does not claim a universal acceptable voltage dip.
Frequently asked questions
What is the formula for motor starting current?+
For an estimate from full-load current, DOL starting current = FLA × the motor locked-rotor current multiple. Use nameplate or manufacturer data for that multiple where possible.
Why is star-delta current one-third of DOL?+
For a motor designed to run delta at the supply voltage, star starting reduces winding voltage by √3 and the line/winding current relationship changes too, giving about one-third of DOL line current.
How does an autotransformer starter change line current?+
For an ideal reduced-voltage autotransformer start, supply line current is approximately the DOL current multiplied by the tap ratio squared.
Is starting current the same as locked-rotor current?+
DOL initial RMS starting current is commonly approximated by locked-rotor current. During acceleration current changes, and reduced-voltage or electronic starters alter the line current.
Can I use 6× FLA for every motor?+
No. It is only an estimate. Prefer motor-specific LRA, locked-rotor ratio, code-letter or manufacturer data.
Does this calculate starting voltage drop?+
No. Voltage dip needs actual source, transformer, generator and cable impedance data; a current-only shortcut would create false precision.
Semantic next steps
Continue the calculation
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