Locked rotor amps (LRA) is the absolute maximum surge of current an AC motor draws the instant power is applied while the shaft is physically prevented from turning or hasn't yet overcome inertia.
When you wire a motor circuit, you cannot simply size your breakers and wires for the motor's running current. The startup phase introduces a massive, temporary electrical spike that dictates your choice of overcurrent protection, contactor ratings, and sometimes even voltage drop calculations. Understanding LRA is the difference between a reliable installation and a breaker that nuisance-trips every time you flip the switch.
The Physics of Locked Rotor Current
To understand LRA, you have to look at how an AC induction motor generates torque. When power is first applied, the stator creates a rotating magnetic field. However, the rotor is stationary (0 RPM). In a running motor, the spinning rotor generates a counter-voltage called back-EMF (electromotive force), which naturally opposes the supply voltage and limits the current draw to a manageable level.
At the exact moment of startup, back-EMF is zero. The only things limiting the current flow are the stator winding's DC resistance and its leakage reactance—both of which are extremely low. Because impedance is minimal, the motor essentially acts like a dead short circuit for a fraction of a second.
As the rotor begins to spin, back-EMF builds up rapidly, and the current drops down the "starting curve" until it settles at the Full Load Amps (FLA) rating once the motor reaches synchronous speed and the mechanical load is applied.
Worked Numeric Example: Sizing a Breaker for a 5 HP Compressor
Let us look at a real-world scenario to see how LRA changes our component selection. You are wiring a 5 HP, 230V, single-phase air compressor in your workshop.
- Nameplate FLA (Full Load Amps): 28A
- Nameplate LRA (Locked Rotor Amps): 168A (exactly 6x the FLA)
If you sized your breaker for the LRA, you would need a 175A breaker, which would leave the 28A running motor completely unprotected against moderate overloads. If you sized it strictly for the 28A FLA using a standard fast-acting breaker, the 168A startup spike would instantly trip the magnetic mechanism.
The Solution (NEC Article 430): The National Electrical Code (NEC) solves this by separating motor overload protection from branch-circuit short-circuit protection. According to NEC 430.52, the maximum rating for an inverse-time breaker protecting a single-phase motor is 250% of the FLA.
The Math: 28A × 2.50 = 70A.
You install a 70A inverse-time breaker. But wait—won't the 168A LRA spike trip a 70A breaker? No. Inverse-time breakers have a thermal-magnetic trip curve. The magnetic (instantaneous) trip threshold on a standard 70A breaker is typically set at 5x to 10x the frame rating (350A to 700A). The 168A LRA spike falls safely below the magnetic trip threshold and into the thermal delay zone, allowing the breaker to tolerate the surge for the 1 to 3 seconds it takes the motor to spin up and generate back-EMF.
Where You Meet LRA in Practice (and What It Changes)
You will encounter LRA explicitly printed on the data plates of hermetic and semi-hermetic compressors (HVAC systems, refrigeration units, and well pumps). On these nameplates, you will see both RLA (Rated Load Amps) and LRA.
Here is exactly what LRA changes in your installation workflow:
- Breaker Trip Curves: You must select breakers with the correct magnetic trip settings. In IEC-standard regions, this means choosing a D-curve breaker (magnetic trip at 10x-20x In) rather than a standard C-curve breaker, which would trip instantly on motor startup.
- Contactor Utilization Categories: When selecting a contactor to switch the motor, you must look at the AC-3 rating (squirrel-cage motors, starting and switching off during running), not just the AC-1 (resistive) rating. The contactor's internal contacts must withstand the physical and thermal stress of making a circuit under LRA conditions without welding shut.
- Voltage Drop Calculations: Because the starting current is 6x higher than running current, the voltage drop across long wire runs during startup is also multiplied by six. If your wire is sized only for running voltage drop (e.g., 3%), your startup voltage drop might hit 18%, causing the motor to stall, overheat, and fail to generate back-EMF.
Common Confusions: LRA vs. FLA vs. Transformer Inrush
One of the most common mistakes on the jobsite is confusing LRA with other high-current phenomena. Sizing components for the wrong metric leads to either nuisance tripping or catastrophic fire hazards.
| Metric | Definition | Duration | What It Dictates |
|---|---|---|---|
| LRA (Locked Rotor Amps) | Current drawn by a motor at 0 RPM. | 0.5 to 5 seconds (until rotor spins). | Breaker magnetic trip settings, contactor AC-3 ratings, starting voltage drop. |
| FLA / RLA (Full Load / Rated Load Amps) | Current drawn by a motor running at full mechanical capacity. | Continuous (hours to years). | Wire ampacity sizing, thermal overload relay settings, continuous voltage drop. |
| Transformer Inrush | Magnetizing current surge when a transformer core is first energized. | 1 to 10 cycles (milliseconds). | Primary fuse sizing (requires time-delay fuses). Can be 10x to 40x nominal current. |
For a deeper dive into the mathematical derivation of motor starting currents and NEMA code letters, refer to the comprehensive guides on Electrical Engineering Portal and Fluke's motor testing resources.
Decision Tree: Your Motor Trips the Breaker on Startup
If you have a motor that immediately trips the breaker the moment you hit the start switch, do not just swap in a larger breaker. Follow this diagnostic decision path to find the root cause and the correct fix.
| Symptom / Measurement | Diagnosis | Required Action |
|---|---|---|
| Breaker trips instantly (loud snap), LRA is within normal range (5x-8x FLA). | Wrong breaker curve. You are using a standard/fast-acting breaker instead of an inverse-time or D-curve breaker. | Fix: Replace with a HACR-rated inverse-time breaker (NEC) or D-curve MCB (IEC). |
| Breaker trips after 2-4 seconds of humming, motor shaft is physically stiff or seized. | True locked rotor condition. Mechanical failure, bad start capacitor, or broken centrifugal switch. | Fix: Disconnect power, test start/run capacitors with a multimeter, and spin the shaft by hand to check for bearing seizure. |
| Breaker trips instantly, measured startup current is >10x FLA. | Internal stator short circuit or severe phase-to-phase fault. | Fix: Megger test the motor windings. If insulation resistance is <1 Megohm, the motor is dead and must be rewound or replaced. |
| Wiring and breaker are perfectly sized per NEC 430, but long wire runs cause voltage to dip below 85% nominal during startup, stalling the motor. | Excessive voltage drop during the LRA phase prevents the motor from developing enough torque to overcome inertia. | Fix: See default recommendation below. |
FAQ: Locked Rotor Troubleshooting
Can I use LRA to size my motor wires?
No. Wire sizing is based on the motor's Full Load Amps (FLA) multiplied by 125% (per NEC 430.22) to account for continuous thermal heating. Sizing wires to handle LRA continuously would result in massively oversized, incredibly expensive copper that serves no practical purpose, as the LRA spike only lasts for a few seconds.
Why does my HVAC nameplate say RLA instead of FLA?
RLA (Rated Load Amps) is a specific term used by the HVAC and refrigeration industry. It represents the maximum current the compressor should draw under any normal operating condition. It is mathematically derived from the motor's FLA but is often slightly lower due to specific ARI (Air-Conditioning and Refrigeration Institute) testing standards. For breaker and wire sizing, treat RLA exactly as you would FLA.
What happens if a motor actually stays in a "locked rotor" state?
If the mechanical load jams and the rotor cannot turn, the motor will continue to draw LRA continuously. Without back-EMF to limit the current, the stator windings will overheat rapidly, melting the insulation enamel and causing a dead short. This is why motor circuits must include dedicated thermal overload relays (OLRs) set to the FLA, which will physically cut the control circuit if the high current persists beyond the startup window.
