Convert LRA to amps: estimate running amps (RLA/FLA) from locked rotor amps with a typical 6:1 ratio or your motor’s own ratio, plus starting kVA.
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LRA to Amps Formula
The following formula is used to estimate a motor’s running (full-load) current from Locked Rotor Amps (LRA) using a known motor-specific ratio k (since LRA is already measured in amps).
Variables:
- A is the estimated running (full-load) current in amps (A)
- LRA is the Locked Rotor Amps (A)
- k is the multiplier (LRA ÷ running amps). Use manufacturer or measured data for the same motor, voltage and running condition; no universal ratio is assumed.
To estimate running amps from LRA, divide the Locked Rotor Amps by the multiplier k.
| Running Amps (A) | Locked-Rotor Amps (A) |
|---|---|
| 0.5 | 3 |
| 1 | 6 |
| 1.5 | 9 |
| 2 | 12 |
| 3 | 18 |
| 4 | 24 |
| 5 | 30 |
| 7.5 | 45 |
| 10 | 60 |
| 12 | 72 |
| 15 | 90 |
| 20 | 120 |
| 25 | 150 |
| 30 | 180 |
| 40 | 240 |
| 50 | 300 |
| 60 | 360 |
| 75 | 450 |
| 100 | 600 |
| 125 | 750 |
| * LRA = k × Running Amps. Table is an arithmetic illustration assuming k = 6.0, not a universal motor rating. For other multipliers, scale LRA accordingly. Units in amperes (A). | |
What is Locked Rotor Amps (LRA)?
Locked Rotor Amps (LRA) describes current with the rotor stationary at specified electrical conditions. It differs from running current and does not by itself determine conductor or protective-device sizing. Actual initial inrush and acceleration depend on the motor, supply, starter and load. Applicable codes and manufacturer instructions require separate evaluation.
How to Calculate Amps from LRA?
The following steps outline how to estimate the running current in amps from LRA.
- First, determine the Locked Rotor Amps (LRA) of the motor.
- Next, determine the multiplier k (the ratio LRA ÷ running/full-load amps). Use manufacturer or measured data when you have it; otherwise 6 is a common rule-of-thumb estimate, not a universal value.
- Use the formula from above: A ≈ LRA / k.
- Finally, calculate the estimated running current in amps (A).
- After inserting the variables and calculating the result, check your answer with the calculator above.
Example Problem :
Use the following variables as an example problem to test your knowledge.
Locked Rotor Amps (LRA) = 50 A
Multiplier (k) = 6
Estimated Running Amps: A ≈ 50 ÷ 6 ≈ 8.33 A
