Enter the Locked Rotor Amps (LRA) and the multiplier (k = LRA ÷ running (full-load) amps) into the calculator to estimate the motor’s running current in amps. You may also enter running amps and k to estimate LRA.
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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 typical ratio k (since LRA is already measured in amps).
A \approx \frac{LRA}{k}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). For many induction motors it is often around 5–8, but it can vary widely (roughly 3–10+ depending on motor design and size).
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 assumes k = 6.0 (typical for many motors). For other multipliers, scale LRA accordingly. Units in amperes (A). | |
What is Locked Rotor Amps (LRA)?
Locked Rotor Amps (LRA) is the current drawn by a motor when the rotor is not turning. It occurs at the moment power is applied to the motor and is significantly higher than the running or full-load amps. LRA is an important specification for starting electric motors and is used to size circuit breakers and fuses to handle the initial inrush current safely.
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). If you do not know it, a common rule-of-thumb is around 6 for many induction motors (but it varies by motor).
- 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
