What is ‘locked rotor current’?
A motor at rest acts as if its rotor is locked. The locked rotor current and the starting current of a motor are one and the same. It is the starting current drawn by a motor when full nominal voltage is applied to its terminals. Here is a simple locked rotor current calculator. Enter the motor-rated power, nominal voltage, and NEMA code, and click on calculate button.
Locked rotor current calculator
| Nominal voltage | V | |
| Motor rated power | ||
| Design Code | ||
Calculated Values | ||
| Locked rotor current | A | |
How to use the calculator:
The above tool requires the following data for calculation:
- Motor Type – Single-phase or three-phase. (Mandatory)
- Motor-rated power in HP or kW. (Mandatory)
- Input supply voltage. (Mandatory)
- NEMA code. (Mandatory)
After entering the required data, click on the “Calculate” button to find out the locked rotor current of the motor.
The appropriate selection of fuses, circuit breakers, overload relays, cables, and other switchgear is necessary to protect the motor from damage. The motor full load current forms the basis for the selection of all this equipment.
Motor FLC calculation
For single-phase AC motors
For single-phase motors, when kW is known:
![\[ LRC =\frac{P_k_W \times 1341 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-c35058807279476dcf043809e70fd092_l3.svg "Rendered by QuickLaTeX.com")
For single-phase motors, when HP is known:
![\[ LRC =\frac{P_h_p \times 1000 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-71b9617698c8d397f4dc5b9fa7084e7e_l3.svg "Rendered by QuickLaTeX.com")
For Three-phase AC motors
For three-phase motors, when kW is known:
![\[ LRC =\frac{P_k_W \times 774 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-0ba912672aa74b75ac7ba05080a91004_l3.svg "Rendered by QuickLaTeX.com")
For Three-phase motors, when HP is known:
![\[ LRC =\frac{P_h_p \times 577 \times (KVA/HP)}{Voltage} \]](https://www.electricalclassroom.com/wp-content/ql-cache/quicklatex.com-330e48add9aa1c04fcbedd37ef3173d3_l3.svg "Rendered by QuickLaTeX.com")
Where,
- Voltage: The phase-to-phase voltage for a 3-phase supply.
- Rating: The power rating of the motor in kW.
- KVA/HP: Maximum value corresponding to the NEMA code.
Locked rotor KVA per Horsepower
The National Electrical Manufacturer’s Association (NEMA) has designated code letters to classify motors by the rotor kVA per horsepower. Here is the tabulation of those code letters and kVA per horsepower values.
| Letter designation | (KVA/HP) Minimum | (KVA/HP) Maximum |
|---|---|---|
| A | 1 | 3.14 |
| B | 3.15 | 3.54 |
| C | 3.55 | 3.99 |
| D | 4 | 4.49 |
| E | 4.5 | 4.99 |
| F | 5 | 5.59 |
| G | 5.6 | 6.29 |
| H | 6.3 | 7.09 |
| I | 7.1 | 7.99 |
| K | 8 | 8.99 |
| L | 9 | 9.99 |
| M | 10 | 11.19 |
| N | 11.2 | 12.49 |
| P | 12.5 | 13.99 |
| R | 14 | 15.99 |
| S | 16 | 17.99 |
| T | 18 | 19.99 |
| U | 20 | 22.39 |
| V | 22.4 | – |
Other calculators:
- Synchronous speed calculator
- Torque-power calculator
- Power-torque calculator
- Torque-speed calculator
- Speed-torque calculator
- Induction motor slip
- Motor torque calculator
- Motor FLC calculator
- Motor Locked rotor current calculator
- Number of poles calculator
- DOL Starter Design tool
- Star-Delta Starter design tool
- HP to kW Converter
- kW to Amps Converter
- Amps to kW converter
What is the equation for three phase motors when HP is known? Your page only lists for single phase motors twice. If I assume the last equation is for three phase motors, I don’t get the same value as your calculator.
Thank you for notifying us. The equation for three-phase motors, when HP can be found above.
Why do we not use the 3 phase(ex 208 x 1.73=360v) voltage config when we divide by voltage?
Thank you,
Never mind. I see you reduced the HP multiplier from 1000 for single phase to 577 three phase.
I’m guessing there is a typo on the table at Code 7 Max Value.
I’m guessing there is a typo on the table at Code F Max Value.
When using your “Locked Rotor Current” calculator for a 240v, 1HP, single phase motor with NEMA Code A, it states that Locked Rotor Current is 7.55A.
However, when using your “Motor FLC calculation” for single phase ac motors when HP is known I calculate LRC=1x1000x3.14/240=13.08A.
Is the equation within the text trying to determine Full Load Current or Locked Rotor Current? as Locked Rotor Current should be higher than Full Load Current. Also, the text mentions efficiency, but it does not appear in the equation. Does Power Factor come into the picture, or does the NEMA Code letter incorporate the effect of both efficiency and power factor?
You are referring to a wrong formula for Motor FLC calculation. FLC when the motor power is rated in HP, FLC = 1 x 746/(240xpfxefficiency) = 3.7. Kindly refer to https://www.electricalclassroom.com/motor-fla-motor-flc-calculator/ for FLC calculation. The power factor is not taken into account during the LRC calculation.
Efficiency is not taken into consideration during the LRC calculation. That was a mention for standard notations used in the calculation.
In your Locked Rotor Current Calculator, have the tabs for Single phase and 3 phase become swapped or interposed?
I agree. It looks like they are swapped. All the answers are incorrect.
Thank you for notifying us. We have fixed the calculator.