Calculate Locked Rotor Amps
Formula Used
Code letter method:
LRA = HP × kVA per HP × 1000 ÷ (Voltage × phase factor)
Multiplier method:
FLA = HP × 746 ÷ (Voltage × phase factor × efficiency × power factor)
LRA = FLA × starting multiplier
Known kVA method:
LRA = locked rotor kVA × 1000 ÷ (Voltage × phase factor)
For single phase, the phase factor is 1. For three phase, the phase factor is √3.
How to Use This Calculator
- Select the best method from the dropdown.
- Enter motor horsepower, voltage, and phase.
- Add efficiency and power factor for the running current estimate.
- Choose the motor code letter when available.
- Use the multiplier method when the code letter is unknown.
- Enter known locked rotor kVA when you already have it.
- Press calculate to view results above the form.
- Download CSV or PDF for records.
Example Data Table
| Motor HP | Voltage | Phase | Code | kVA/HP Used | Estimated LRA |
|---|---|---|---|---|---|
| 5 HP | 230 V | 1 | G | 5.945 | 129.24 A |
| 10 HP | 460 V | 3 | G | 5.945 | 74.63 A |
| 25 HP | 480 V | 3 | H | 6.695 | 201.34 A |
| 50 HP | 575 V | 3 | J | 7.545 | 378.80 A |
Understanding Locked Rotor Amps
Locked rotor amps, or LRA, shows the current drawn when a motor starts and the rotor has not begun turning. This moment is short. It can still stress conductors, starters, contactors, breakers, generators, and transformers. A running motor may use modest current. A starting motor can pull several times more current.
Why It Matters
LRA helps you compare motor starting demand with available electrical capacity. It is useful for pumps, compressors, fans, lifts, shop tools, and HVAC equipment. A high value can cause voltage sag. Lights may dim. Controls may reset. Protective devices may trip before the motor reaches speed. Knowing this value supports better planning.
Choosing an Input Method
This calculator supports three practical methods. The code letter method uses kVA per horsepower. It is useful when the nameplate includes a motor code letter. The multiplier method starts with estimated full load amps and applies a starting factor. It works when you know a typical inrush multiplier. The total kVA method is useful when design documents already list locked rotor kVA.
Reading the Result
The main result is estimated locked rotor amps. The page also shows estimated full load amps, starting kVA, LRA to FLA ratio, and optional reduced-current starting. These values help with early checks. They do not replace manufacturer tables, local rules, or field measurements. Treat them as planning values.
Practical Use
Use nameplate voltage and phase. Use realistic efficiency and power factor. Select the code letter printed on the motor. When the code letter is unknown, compare the multiplier method with a conservative value. For critical work, confirm results with the motor supplier, drive vendor, or a qualified electrician. Starting behavior can change with load torque, wiring length, utility stiffness, starter type, and ambient conditions. Record your assumptions with every result.
FAQs
What are locked rotor amps?
Locked rotor amps are the current a motor may draw at startup before the rotor turns. It is usually much higher than running current and lasts for a short time.
Is LRA the same as FLA?
No. FLA is full load running current. LRA is starting current with the rotor stopped. LRA is normally several times higher than FLA.
Which method should I use?
Use the code letter method when the motor nameplate shows a code letter. Use the multiplier method for quick estimates. Use known kVA when it is supplied.
Why does phase affect the result?
Three phase power uses a √3 factor in current calculations. Single phase does not. That changes the current for the same voltage and power level.
Can this size a breaker?
This calculator helps with starting-current estimates. Breaker and conductor sizing should follow local electrical rules, equipment listings, and professional design review.
What multiplier is common?
Many motors use a starting multiplier near five to eight times FLA. Actual values depend on motor design, load, voltage, and starting equipment.
Does a soft starter reduce LRA?
A soft starter can reduce starting current. The exact reduction depends on setup, motor load, ramp time, and torque needs during acceleration.
Are the results exact?
No. Results are estimates for planning. Use the motor nameplate, manufacturer data, and measured values for final electrical decisions.