Motors & Loads calculator
Motor Starting Current Calculator
Use this motor starting current calculator to compare inrush current from the starting method, full-load current, locked-rotor multiple, reduced-voltage tap, soft-starter or VFD current limit, and optional source-stiffness data. The page keeps DOL, star-delta, soft-starter, VFD, and autotransformer assumptions separate so the result comes from the entered motor and starter data rather than a universal inrush answer.
Updated July 10, 2026
Enter FLA, locked-rotor multiple, starting method, current limit or tap value, and source data to compare DOL, star-delta, soft-starter, VFD, and autotransformer inrush.
DOL: Istart ≈ FLA × LRA multiple | Star-delta line current ≈ DOL ÷ 3
Enter motor FLA, voltage, motor type, and starting method below to compare DOL, star-delta, soft starter, VFD, and autotransformer starts
Calculator Inputs
Calculation Results
Enter values above to see calculation results
Field kit
Tools for starting checks
Use the estimate to plan an inrush measurement or motor-control review, then confirm tool ratings before live work.
Compare inrush metersLook for inrush captureOpens in a new tabCompare clamp metersFor running current checksOpens in a new tabFind overload relaysMatch the motor nameplateOpens in a new tab
As an Amazon Associate I earn from qualifying purchases.
Calculation history
Example Calculations
More examples. Open to review 2 additional calculation examples.
How to Use
How to use the motor starting current calculator
- Select the starting method you want to review: DOL, star-delta, soft starter, VFD, or autotransformer.
- Select the motor type. This page is limited to induction-motor starts, with a typical squirrel-cage option and a wound-rotor option.
- Enter the motor rating, voltage, and full-load current.
- If you know the actual locked-rotor current multiple, enter it. Otherwise the calculator uses a typical induction-motor default.
- For soft starters, VFDs, and autotransformers, enter the current limit or tap value used for the screening result.
- If available, enter the fault level at the motor bus to compare the starting current against source stiffness.
Method reference after using the calculator
Use the calculator above for the actual motor FLA, voltage, locked-rotor multiple, starting method, current limit, tap value, and source data. The reference below explains how each method treats those inputs after you have a calculator output.
| Method | Modeled Basis | How to read it |
|---|---|---|
| Direct online | Istart ≈ FLA × locked-rotor multiple | Use the full line current drawn by a DOL start. |
| Star-delta | Line current ≈ one-third of DOL | Good for a quick current screen when the load can accelerate with reduced torque. |
| Soft starter | Line current follows the entered current limit | Use the programmed current limit instead of assuming a fixed universal multiplier. |
| VFD | Line current follows the entered drive limit | Use the actual drive overload rating or current limit whenever it is known. |
| Autotransformer | Line current ≈ tap² × DOL | Useful for screening source current at common 65%, 80%, or 90% taps. |
What this calculator does well
- It gives a clear DOL inrush current estimate from FLA and locked-rotor current multiple.
- It compares reduced-voltage starting methods without pretending that every motor uses the same current limit.
- It converts the selected starting method into a line-current multiple and starting kVA that are easy to compare against the source.
What this calculator does not model
- Motor torque-speed curves, acceleration time, or thermal limits of the driven load.
- Drive harmonics, soft-starter ramp shape, or detailed protection coordination.
- Utility flicker limits, project-specific voltage-dip studies, or manufacturer commissioning settings.
Preset examples
Use the presets above to compare direct-on-line, star-delta, soft-starter, and autotransformer cases. Enter the actual FLA, locked-rotor multiple, tap or current limit, and source data before carrying the calculator output to charts or downstream motor tools.
After the starting-current result
Use the Motor Starting Current Chart after the result to document FLA, locked-rotor multiple, starting method, line starting current, starting kVA, source-stiffness notes, and the manufacturer or utility review that remains open.
Keep the current basis separate before moving downstream: use the HP to Amps Chart when the input is horsepower, voltage, efficiency, and power factor; use the Motor Full-Load Amps Chart when the next step is keeping NEC table FLC, nameplate FLA, and formula current sources separate; and use the Motor Power HP/kW Chart when the open question is power-unit conversion rather than starting duty.
After the current basis is documented, use the Motor Current Calculator for formula running-current checks, the Full Load Current Calculator for NEC table FLC references, the Motor Starter Calculator for starter and overload screening, and the Motor Branch Protection Calculator when the next question is short-circuit and ground-fault protection.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
How do you calculate motor starting current?
Enter motor full-load current and locked-rotor multiple; the calculator multiplies those inputs for the DOL screen, then applies the selected starting-method reduction, current limit, or tap value where applicable.
What is a typical locked-rotor current multiple for an induction motor?
Many squirrel-cage induction motors started direct online land in the general range of about 5x to 7x full-load current, while wound-rotor machines often start lower. The safest approach is still to replace the default with actual nameplate, code-letter, or manufacturer data whenever it is available.
How does star-delta starting reduce current?
For a quick screening comparison, star-delta line current is commonly treated as about one-third of the DOL line current. That lower current comes with reduced starting torque, so the method is best applied only where the driven load can accelerate successfully under that reduced-torque condition.
Why compare starting current against available fault level or source stiffness?
Because the source matters. A motor starting from a stiff bus sees less voltage dip risk than the same motor on a weak source. Comparing available fault current against starting current is a simple way to see whether the source is likely to tolerate the start comfortably.
Can I rely only on the soft-starter or VFD default current limit?
No. Defaults are useful for early screening, but actual devices use programmed current limits, overload ratings, acceleration times, and application-specific settings. Replace the default with the real drive or starter data before final design decisions are made.
What should I document after a motor starting-current screen?
Document the FLA basis, locked-rotor multiple, starting method, line starting current, starting kVA, available source information, and whether manufacturer or utility review is still required. The starting-current chart keeps that handoff attached to the calculator result.
Related Calculators
Motor Current Calculator
Estimate formula running current from HP or kW, voltage, efficiency, and power factor, then compare against NEC table FLC when needed.
Full Load Current Calculator
Owns NEC Table 430.248/430.250 FLC lookup by HP, voltage, and phase, including 10 HP and 200 HP at 460V field checks.
Motor Starter Calculator
Screen NEMA starter size, controller current class, overload-relay basis, and basic control-package notes.
Circuit Breaker Sizing Calculator
NEC-oriented standard breaker review-point screen with simplified conductor protection checks.
Motor Cable Size Calculator
Screen NEC table motor conductor size with ampacity derating, termination limits, and branch-circuit voltage drop.
Electrical Load Calculator
Estimate demand load and service ampacity using NEC-style demand and diversity factors.