Formula Used
Three phase full load current:
FLA = Watts ÷ (√3 × Voltage × Efficiency × Power Factor)
Single phase full load current:
FLA = Watts ÷ (Voltage × Efficiency × Power Factor)
Manual inrush method:
Locked Rotor Current = FLA × Inrush Multiplier
Locked rotor code method:
Locked Rotor Current = Code kVA per hp × hp × 1000 ÷ (Phase Factor × Voltage)
Line inrush current:
Line Inrush = Locked Rotor Current × Starter Factor
Starting kVA:
Starting kVA = Phase Factor × Voltage × Line Inrush ÷ 1000
Estimated voltage drop:
Voltage Drop Percent = Line Inrush × Source Impedance ÷ Voltage × 100
How To Use This Calculator
Enter motor power from the nameplate or datasheet.
Select horsepower or kilowatt as the power unit.
Choose single phase or three phase supply.
Enter rated voltage, efficiency, and power factor.
Choose a locked rotor code when it is known.
Use a manual multiplier when the code is unknown.
Select the starting method used at the installation.
Enter impedance, breaker setting, and start data if available.
Press calculate. The result appears above the form.
Use CSV or PDF buttons to save the result.
Example Data Table
| Motor Size |
Voltage |
Phase |
Starter |
Multiplier |
Estimated Use |
| 10 hp |
230 V |
Three phase |
Direct online |
6.0 |
Small pump planning |
| 25 hp |
460 V |
Three phase |
Soft starter |
6.5 |
Compressor review |
| 75 hp |
480 V |
Three phase |
Autotransformer |
7.0 |
Generator sizing check |
| 5 hp |
240 V |
Single phase |
Direct online |
5.5 |
Workshop motor estimate |
Motor Inrush Current Guide
Why starting current matters
Motor inrush current is the short surge drawn when a motor starts. The rotor is still stationary. Back electromotive force is low. Current can rise many times above normal running current. This surge may last only cycles or seconds. Still, it affects cables, breakers, contactors, fuses, generators, and transformers. A good estimate helps avoid nuisance trips. It also helps protect equipment during repeated starts.
What the calculator estimates
This calculator first estimates full load current from motor power, voltage, phase, efficiency, and power factor. It then applies either a selected locked rotor code value or a manual multiplier. Starter type is included because starting method changes the line current. Direct online starting usually gives the highest current. Star delta, autotransformer, soft starter, and drive starting can reduce it.
Inputs that improve accuracy
Use nameplate data whenever possible. Enter rated voltage, motor output, efficiency, and power factor from the motor plate or datasheet. Choose the phase connection used at site. For three phase motors, voltage means line to line voltage. For single phase motors, voltage means supply voltage. If a code letter is unknown, use the multiplier method. Many motors start around five to eight times full load current.
Reading the results
The result shows estimated running current, inrush current, locked rotor current, starting kVA, and apparent starting power. It also shows an approximate voltage dip when source impedance is entered. This value is only a planning estimate. Real dips depend on transformer size, feeder length, cable impedance, utility strength, and motor design.
Practical use
Compare the inrush value with breaker instantaneous settings and fuse curves. Check contactor making capacity. Review generator starting capability. Long feeder runs may need larger conductors. Weak supplies may need reduced voltage starting. Soft starters and drives can reduce stress, but settings matter. Always confirm final protection with local electrical codes, manufacturer curves, and qualified engineering review. The calculator supports early design checks and field planning.
Limitations
Use measured data after installation when available. Clamp meters with inrush capture can verify the estimate. Trend several starts, because temperature and load affect results. Document assumptions beside each export, so maintenance teams understand every number and later can compare changes.
FAQs
What is motor inrush current?
Motor inrush current is the starting surge drawn when the rotor is not yet moving. It is often several times higher than full load current.
Why is inrush current high?
At startup, the motor has little back electromotive force. The winding impedance limits current, but the current remains high until speed builds.
What multiplier should I use?
Many induction motors use five to eight times full load current. Use the nameplate code letter or manufacturer data when available.
Does a soft starter reduce inrush current?
Yes. A soft starter reduces applied voltage during starting. Actual current reduction depends on settings, motor load, and acceleration time.
Is locked rotor current the same as inrush current?
Locked rotor current is a base starting value. Line inrush current may be lower when reduced voltage starters or drives are used.
Can this size a breaker exactly?
No. It supports planning only. Final breaker sizing needs code rules, device trip curves, conductor ratings, and engineering review.
Why enter source impedance?
Source impedance helps estimate voltage dip during starting. Higher impedance can cause larger dips and lower available starting torque.
Can I export the calculation?
Yes. After calculation, use the CSV or PDF button to download a simple report for records, review, or field notes.