Calculator Inputs
Enter nameplate data, supply information, starter reduction, and feeder details.
Formula Used
Full Load Current
For three phase motors:
FLA = Output watts / (√3 × V × Efficiency × Power factor)
For single phase motors:
FLA = Output watts / (V × Efficiency × Power factor)
Locked Rotor Current by Code Letter
LRC = kVA per hp × hp × 1000 / (phase factor × voltage)
The phase factor is √3 for three phase and 1 for single phase.
Locked Rotor Current by Multiplier
LRC = Full load current × selected multiplier
Adjusted Starting Current
Starting current = selected LRC × starter current factor / 100
Voltage Dip Estimate
Source dip % = Starting current / source rated current × source impedance %
Feeder drop = current × conductor resistance × feeder length factor
How to Use This Calculator
- Enter the motor rating from the nameplate.
- Select horsepower or kilowatts as the input unit.
- Enter voltage, phase, efficiency, and power factor.
- Select a code letter, or enter a custom kVA per horsepower value.
- Choose the code method or multiplier method.
- Add starter reduction if a soft starter, drive, or star delta starter is used.
- Enter source and feeder details for voltage dip estimation.
- Press the calculate button and review the result section above the form.
- Use CSV or PDF export to save the result.
Example Data Table
| Motor Size | Voltage | Phase | Code Letter | Basis | Estimated LRC |
|---|---|---|---|---|---|
| 10 hp | 230 V | Three phase | F | Upper | About 140 A |
| 25 hp | 460 V | Three phase | G | Upper | About 198 A |
| 50 hp | 480 V | Three phase | H | Upper | About 426 A |
| 7.5 kW | 400 V | Three phase | E | Middle | About 69 A |
Locked Rotor Current Guide
Understanding Locked Rotor Current
Locked rotor current is the current drawn by a motor when the rotor is not turning. It appears at the first instant of starting. The motor has no back electromotive force yet. Current can rise many times above full load current. This short surge affects cables, breakers, fuses, contactors, generators, and transformers. It also affects voltage stability. A small motor may start easily. A larger motor can disturb a weak electrical system.
Why the Estimate Matters
A correct estimate helps prevent nuisance trips. It also protects equipment from heating. Motors on weak supplies can pull voltage down. That dip may dim lights or stop controls. Pumps, compressors, fans, crushers, and conveyors need extra care. Their starting torque and load inertia can stretch acceleration time. Longer starts create more thermal stress. This is why current and time should be reviewed together.
Choosing the Right Inputs
The code letter method uses locked rotor kilovolt amperes per horsepower. It is useful when the nameplate shows a motor code letter. Use the upper value for conservative sizing. The multiplier method is helpful when expected start ratio is known. Across the line starts often use five to eight times running current. Soft starters and drives can reduce line current. Enter realistic voltage, efficiency, and power factor values.
Reading the Results
First compare full load current with estimated starting current. Then review starting kVA. This value shows demand placed on the supply. Next check voltage dip. A high dip can reduce starting torque. It can also disturb nearby loads. Feeder drop depends on conductor resistance, route length, and parallel sets. Shorter and larger conductors reduce drop.
Practical Planning Tips
Use this calculator early during design. Check the breaker, overload range, starter size, and feeder. Compare the estimate with manufacturer data when it is available. Real motors vary by design, voltage, temperature, and driven load. Critical sites need a proper study. Generators may need oversizing or staged starting. Transformers need short time capacity. Record every assumption. Good notes make future troubleshooting much easier.
FAQs
What is locked rotor current?
Locked rotor current is the current a motor draws when power is applied and the rotor is still stopped. It is usually much higher than running current.
Is locked rotor current the same as starting current?
They are closely related. Locked rotor current is the theoretical stalled current. Actual starting current may be lower when reduced voltage starters or drives are used.
What does the code letter mean?
The code letter represents a range of locked rotor kVA per horsepower. It helps estimate starting current from the motor nameplate.
Which code value should I use?
Use the upper range for conservative planning. Use the middle range for general estimates. Use custom data when the manufacturer provides exact locked rotor kVA.
Why is voltage dip important?
Voltage dip reduces available starting torque. It can also affect lighting, controls, electronics, and other motors connected to the same supply.
Can a soft starter reduce locked rotor current?
A soft starter reduces line starting current by lowering applied voltage. The entered starter factor models this reduction in a simple planning format.
Does this calculator replace manufacturer data?
No. It provides an engineering estimate. Use manufacturer data, local rules, and professional review for final protection and feeder decisions.
Why include start duration?
Start duration helps estimate heating stress. A high current for a long time can damage motors, starters, and conductors.