Locked rotor amps (LRA) is the maximum current an AC motor draws the instant power is applied while the rotor is held completely stationary at zero RPM. In a real circuit, this massive, brief current surge dictates the instantaneous magnetic trip setting of your circuit breaker and the withstand rating of your motor contactor, ensuring the system survives the 1-to-3-second startup phase without nuisance tripping.
The Physics of LRA and Common Nameplate Confusion
When an AC motor first energizes, the stator windings act essentially like a short-circuited transformer secondary. Because the rotor is not yet spinning, it generates zero back-electromotive force (back-EMF). Think of back-EMF like a speed-dependent toll booth on a highway; at zero RPM, the toll is zero, so the full line voltage pushes maximum current through the low-resistance copper windings. As the rotor accelerates, back-EMF builds and chokes off the current until it settles at its normal running level.
- Full Load Amps (FLA): The continuous current the motor draws at its rated horsepower and voltage. This is your baseline for wire sizing.
- Running Load Amps (RLA): A mathematical derivation used specifically in HVAC compressors to determine maximum continuous load for breaker sizing. It is not a physical test limit like LRA.
- Inrush Current: Often used interchangeably with LRA by hobbyists, but technically 'inrush' applies to the magnetic core saturation of transformers, whereas LRA is specific to the mechanical stall of a motor.
According to the NEMA MG 1 standard for motors and generators, LRA is typically 5 to 7 times higher than the motor's FLA. If you size your breaker's instantaneous trip below the LRA, the breaker will violently snap open every time the motor tries to start.
Worked Numeric Example: Sizing a 5 HP Compressor Circuit
Let's look at a real-world scenario to see how LRA changes the installation. You are wiring a 5 HP, 230V single-phase air compressor. You check the nameplate and note the following FLA: 28A and LRA: 165A.
Step 1: Wire Sizing (Based on FLA)
NEC Article 430 requires motor branch circuit conductors to be sized at 125% of the FLA.
Calculation: 28A × 1.25 = 35A.
Pick: 10 AWG THHN copper wire (rated 35A in the 75°C termination column). LRA does not dictate wire size because the surge is too brief to heat the copper to dangerous levels.
Step 2: Breaker Sizing (Where LRA Matters)
If you used a standard 35A breaker, the 165A LRA would trip it instantly. NEC 430.52 allows inverse-time thermal-magnetic breakers for motor circuits to be sized up to 250% of the FLA to accommodate the LRA startup surge.
Calculation: 28A × 2.50 = 70A maximum.
Pick: A 60A inverse-time breaker.
Where You Meet LRA in Practice
You will directly interact with locked rotor amps in three specific jobsite or bench scenarios:
- Hard-Start Kit Installation: When an aging HVAC compressor struggles to start, it stays at LRA for too long, tripping the thermal overload. Adding a hard-start capacitor doesn't lower the peak LRA; it shifts the phase angle to increase starting torque, getting the rotor off zero RPM faster so the current drops to FLA before the thermal protector trips.
- Voltage Drop on Long Feeder Runs: If you run a 100-foot feeder to a well pump, voltage drops under load. Since motor current is inversely proportional to voltage during startup, a 10% voltage sag at the motor terminals can cause the LRA to persist longer and run hotter, often requiring you to bump the wire size up one AWG gauge to maintain the voltage needed to overcome the locked rotor state.
- Contactor Selection: Motor contactors are rated by utilization categories. An AC-3 rated contactor (used for starting squirrel-cage motors) must be physically robust enough to make and break the circuit while the motor is drawing LRA, preventing the internal contacts from welding shut.
Decision Tree: Selecting Breakers and Contactors Using LRA
Use this decision matrix to select the correct overcurrent protection and switching gear based on your motor's nameplate FLA and LRA. This framework assumes standard 230V single-phase or 460V three-phase industrial motors.
| Motor Nameplate FLA | Expected LRA Range (Approx 6x) | Max NEC Breaker Size (250% FLA) | Recommended Breaker Pick | AC-3 Contactor Pick (Part Number) |
|---|---|---|---|---|
| 10A - 15A | 60A - 90A | 37.5A | 35A Inverse-Time | Schneider TeSys LC1D18 |
| 16A - 22A | 96A - 132A | 55A | 50A Inverse-Time | Schneider TeSys LC1D25 |
| 23A - 28A | 138A - 168A | 70A | 60A Inverse-Time | Schneider TeSys LC1D32 |
| 29A - 40A | 174A - 240A | 100A | 90A Inverse-Time | Schneider TeSys LC1D40 |
Note: Always verify the exact LRA on your specific motor nameplate. High-efficiency (IE3/IE4) motors often have higher LRA multipliers than older legacy designs, sometimes reaching 8x to 10x FLA. If your LRA exceeds the magnetic trip threshold of the recommended breaker, you must switch to a Motor Circuit Protector (MCP) with adjustable magnetic trips.
FAQ: Common Locked Rotor Questions
What happens if a motor is mechanically jammed and stays at LRA?
If the rotor cannot turn, the motor will continuously draw LRA. The copper windings will rapidly overheat. Relying on the branch circuit breaker to save the motor is a mistake; the breaker is sized to ignore short LRA surges. The motor's internal thermal overload or an external motor protection relay will trip in 10 to 30 seconds to prevent a winding fire.
Can I measure LRA with my standard digital multimeter?
No. Standard DMMs sample too slowly to capture the 1-to-3-second LRA transient accurately. You need a power quality analyzer or an oscilloscope with a current clamp to capture the peak startup waveform. For standard troubleshooting, a True-RMS clamp meter with an 'inrush' button (like the Fluke 376 FC) is required to capture the startup surge.
Does a soft starter reduce Locked Rotor Amps?
Yes. Unlike a hard-start kit, a solid-state soft starter uses SCRs (silicon-controlled rectifiers) to chop the voltage waveform during the first few seconds of startup. By reducing the voltage applied to the stator, the LRA is proportionally reduced (often down to 2x or 3x FLA), at the cost of reduced starting torque.
When designing or troubleshooting any motor circuit, always default to sizing your conductors for 125% of the FLA, but size your breaker's magnetic trip and contactor AC-3 rating to comfortably swallow the LRA. If your motor nameplate is faded and LRA is illegible, the default engineering assumption is to multiply the FLA by 6.5 to estimate the LRA for breaker magnetic trip verification, ensuring your installation starts reliably without nuisance interruptions.
