Calculator Inputs
Formula Used
The calculator supports three common engineering approaches.
How To Use This Calculator
- Enter the three phase line to line voltage.
- Enter motor power in horsepower or kilowatts.
- Add efficiency and power factor from the nameplate.
- Select multiplier, code letter, or impedance method.
- Enter starter factor for reduced voltage starting.
- Add transformer data if voltage sag is needed.
- Press the calculate button.
- Download CSV or PDF results when needed.
Example Data Table
| Motor Size | Voltage | Efficiency | Power Factor | Method | Typical Input |
|---|---|---|---|---|---|
| 10 hp | 230 V | 90% | 0.84 | Multiplier | 6.0 × FLA |
| 25 hp | 480 V | 92% | 0.86 | Code Letter | G |
| 50 hp | 480 V | 94% | 0.88 | Impedance | 0.18 Ω per phase |
| 75 hp | 600 V | 95% | 0.89 | Code Letter | H |
Understanding Locked Rotor Current
Locked rotor current is the current drawn when a three phase motor is energized while the rotor is not moving. At that moment, the motor has no counter electromotive force. The stator therefore pulls a very high inrush current. This current is often five to eight times the full load current. Large motors may create deeper voltage dips.
Why This Calculator Matters
This calculator helps estimate starting current before a motor is installed. It supports three practical methods. You can use a direct multiplier when field data is known. You can use a NEMA code letter when the motor nameplate includes one. You can also estimate current from locked rotor impedance. These options make the tool useful for design checks, troubleshooting, and starter selection.
Design Uses
Engineers use locked rotor current to size protective devices, conductors, transformers, and generators. A motor starter must handle the short starting interval without nuisance trips. A supply system must also support the voltage dip caused by inrush. The calculator includes an optional source short circuit estimate. It gives a quick view of expected voltage sag. This is helpful for pumps, compressors, fans, conveyors, and HVAC equipment.
Reading The Results
The result panel shows full load current, locked rotor current, apparent starting power, and the selected method. It also shows the current multiple. When the code letter method is used, the calculator applies the average kVA per horsepower range. The chart compares full load current with locked rotor current. This makes the severity of startup easy to see.
Good Practice
Use nameplate data whenever possible. Enter line to line voltage for three phase systems. Use realistic efficiency and power factor values. Check the result against manufacturer data before final design. Site conditions can change the actual starting current. Cable length, reduced voltage starters, variable frequency drives, and transformer impedance all affect startup behavior. Treat the calculator as a planning aid. Use certified electrical standards and local codes for final equipment selection. For critical loads, compare across normal voltage, low voltage, and emergency supply cases. Document assumptions clearly, so later maintenance teams understand each rating and safety margin during future service reviews.
FAQs
What is locked rotor current?
Locked rotor current is the current a motor draws when voltage is applied and the rotor is not yet turning. It is usually much higher than normal running current.
Why is locked rotor current important?
It affects breaker selection, starter sizing, transformer loading, generator sizing, and voltage dip. High starting current can cause nuisance trips and weak motor acceleration.
Which voltage should I enter?
Enter the three phase line to line voltage at the motor terminals. Use adjusted voltage if you are checking low voltage or high voltage conditions.
What multiplier should I use?
Many motors draw about five to eight times full load current during full voltage starting. Use manufacturer data when available for better accuracy.
What is a code letter?
A code letter represents locked rotor kVA per horsepower. It is often printed on motor nameplates and helps estimate starting current.
Can I use this for reduced voltage starters?
Yes. Enter a starter current factor below 100 percent. This estimates the reduced line current caused by star delta, soft starter, or autotransformer starting.
Does this replace manufacturer data?
No. This tool gives planning estimates. Final design should use manufacturer curves, site measurements, electrical standards, and local code requirements.
Why is voltage sag estimated?
Voltage sag shows how much supply voltage may drop during startup. Large dips can affect contactors, drives, lighting, controls, and nearby equipment.