Locked rotor amperage (LRA) is the maximum electrical current an AC motor draws the instant power is applied while the rotor is physically prevented from turning. In a real circuit, this massive current spike dictates the magnetic trip threshold of your circuit breaker and the instantaneous let-through current your contactors and wire terminations must survive without melting. Most DIYers and junior techs confuse LRA with Full Load Amps (FLA) or Running Load Amps (RLA), leading to undersized breakers that nuisance-trip every time the motor starts or, worse, undersized thermal overloads that fail to protect the windings during a mechanical jam.
The Physics of a Stalled Rotor and NEMA Code Letters
When an AC motor runs at its rated speed, the spinning rotor generates a reverse voltage called back-EMF (Electromotive Force). This back-EMF opposes the incoming line voltage, effectively choking off the current draw to a manageable level (the FLA). However, at the exact millisecond you energize a motor, the rotor speed is zero. Because there is no movement, there is zero back-EMF. The only things limiting the current flow are the bare DC resistance and leakage reactance of the stator windings, which are intentionally kept extremely low to maximize efficiency.
Think of it like a manual transmission car stopped at a red light: if you floor the accelerator while the clutch is fully engaged and the brakes are locked, the engine dumps maximum torque and fuel into a stalled system until something breaks or the clutch burns out. In a motor, that "fuel dump" is the locked rotor current.
A Worked Numeric Example: Calculating LRA from the Nameplate
Motor manufacturers don't always print the exact LRA on the nameplate. Instead, they use NEMA Standard MG 1 Code Letters to indicate the locked-rotor kVA per horsepower. Let's calculate the exact LRA for a common workshop compressor motor.
- Motor Specs: 3 HP, 230V, Single-Phase, NEMA Code Letter 'J'
- Code Letter 'J' Range: 7.1 to 7.99 kVA per HP
To find the worst-case LRA, we use the top of the 'J' range (7.99 kVA/HP):
Calculated LRA = 23,970 VA ÷ 230V = 104.2 Amps
Even though this motor only draws about 17 Amps while running (FLA), the circuit must be prepared to handle a 104.2A surge the moment the contactor closes. If your breaker's magnetic trip is set too low, it will interpret this 104A surge as a dead short and trip instantly.
Where You Meet Locked Rotor Amperage in Practice
You will encounter LRA constraints in three primary real-world scenarios:
1. HVAC Compressors and HACR Breakers
Air conditioning compressors are notorious for high LRA. If you look at the condenser nameplate, you will see a specification for "Max Fuse Size" or "HACR Type" breaker. HACR (Heating, Air Conditioning, and Refrigeration) breakers are specifically designed with a delayed magnetic trip curve. They allow the massive LRA to flow for the 1 to 3 seconds it takes the compressor to overcome internal pressure and start spinning, without tripping the instantaneous short-circuit mechanism.
2. Hard-Start Kits and Potential Relays
When an older compressor struggles to start (drawing LRA for too long and tripping the breaker on thermal overload), techs install a hard-start kit. A hard-start kit (a start capacitor paired with a potential relay) does not actually lower the absolute peak LRA. Instead, it shifts the phase angle of the start winding to produce massive starting torque. This gets the rotor off zero RPM faster, drastically reducing the duration of the LRA spike so the breaker doesn't overheat.
3. Variable Frequency Drives (VFDs)
In industrial settings, VFDs eliminate LRA entirely. By starting the motor at 2 Hz and slowly ramping up the frequency and voltage, the VFD ensures the rotor is always spinning just slightly slower than the magnetic field. The slip never reaches 100%, back-EMF is generated immediately, and the starting current is typically limited to 110% to 150% of FLA.
LRA vs. FLA vs. RLA: The Motor Current Matrix
Confusing these three acronyms is the fastest way to fail an electrical inspection or burn out a compressor. Here is how they differ and where each value is used in your installation.
| Metric | Definition | Typical Value (Relative) | Used For Sizing / Setting |
|---|---|---|---|
| LRA (Locked Rotor Amps) | Current drawn at 0 RPM with full voltage applied. | 500% to 800% of FLA | Breaker magnetic trip curves, contactor let-through ratings, VFD fault limits. |
| FLA (Full Load Amps) | Current drawn at rated HP, voltage, and frequency under maximum continuous load. | 100% (Baseline) | Wire ampacity (sized at 125% FLA), thermal overload heater selection. |
| RLA (Running Load Amps) | Maximum current a hermetic compressor will draw under normal operating conditions (specific to HVAC). | Usually 60% to 80% of FLA | HVAC nameplate calculations, sizing contactors and relays in refrigeration circuits. |
Frequently Asked Questions About Locked Rotor Amperage
How do I calculate locked rotor amperage from the motor nameplate code letter?
Locate the NEMA Code Letter (A through V) on the motor nameplate. Refer to NEMA MG 1 Table 10-7 to find the kVA/HP range for that letter. Multiply the top end of that kVA range by the motor's horsepower to get total locked-rotor kVA. Finally, divide that kVA value (multiplied by 1000 to get VA) by the motor's rated voltage. For three-phase motors, divide by the voltage multiplied by 1.732 (the square root of 3).
What is the difference between locked rotor amperage and starting current?
While often used interchangeably on the jobsite, they are technically different. LRA is the absolute theoretical maximum current drawn at exactly zero RPM with full line voltage applied. "Starting current" (or inrush current) is the actual current profile measured during a real-world start. Because voltage drop across the supply wires occurs the moment the motor energizes, the actual starting current is usually 10% to 20% lower than the calculated nameplate LRA. Furthermore, as the motor accelerates, the starting current drops rapidly, whereas LRA assumes the rotor never moves.
Why does my breaker trip instantly when my AC compressor kicks on?
If a breaker trips instantly (with a loud snap, within milliseconds) rather than tripping after a few seconds or minutes, the magnetic short-circuit mechanism is activating. This happens because the compressor's LRA is exceeding the breaker's instantaneous magnetic trip threshold. This is usually caused by using a standard thermal-magnetic breaker instead of an HACR-rated breaker, a weak grid causing severe voltage sag (which forces the motor to draw even more current to produce starting torque), or a mechanically seized compressor that is genuinely stuck at zero RPM, holding the circuit in a continuous LRA state.
Does locked rotor amperage change as a motor gets older or gets hotter?
Yes, but only slightly. The primary limiting factor of LRA is the DC resistance of the copper stator windings. As copper heats up, its resistance increases (copper has a positive temperature coefficient). A motor that has been running hot and is then immediately restarted will have slightly higher winding resistance, which marginally reduces the peak LRA compared to a cold start. However, this change is negligible for breaker sizing. What does change drastically with age is the time it takes to get off LRA; worn bearings, degraded lubrication, and pitted contactors increase mechanical drag and voltage drop, forcing the motor to dwell in the high-current starting zone longer, which trips thermal overloads.
