Locked rotor amps (LRA) is the maximum current an electric motor draws when power is applied but the rotor is physically prevented from turning or is at zero RPM during startup. When you flip the switch on a heavy compressor or well pump, the motor momentarily acts like a near-dead short circuit, pulling a massive surge of electricity to break the rotor's inertia and establish the stator's magnetic field. Think of pushing a heavy car: the initial, back-straining shove required to get it moving from a dead stop takes significantly more force than keeping it rolling at 10 mph. In electrical terms, that initial shove is the LRA, and the rolling cruise is the Full Load Amps (FLA).
The Physics: Why the Current Spikes at Zero RPM
To understand LRA, you have to understand Back-Electromotive Force (Back-EMF). When an AC induction motor spins, its rotor cuts through the stator's magnetic field, generating a voltage that opposes the incoming line voltage. This Back-EMF is what naturally limits the current draw to a manageable level while the motor is running.
However, at the exact millisecond you energize the circuit, the rotor is sitting at 0 RPM. Because it isn't moving, Back-EMF is exactly 0V. The only things limiting the current flow are the DC resistance and the leakage reactance of the stator windings—which are intentionally kept extremely low to make the motor efficient. With almost zero opposition to the incoming 230V or 460V line, Ohm's law dictates that the current will spike dramatically. As the rotor accelerates, Back-EMF builds, opposing the line voltage, and the current rapidly tapers off to the motor's normal running amperage.
LRA vs. FLA vs. RLA: Clearing Up the Confusion
What people most commonly confuse with LRA are the running current metrics found on a motor's nameplate. Sizing a breaker based on the wrong acronym is the number one cause of nuisance tripping on the jobsite. Here is how they break down:
| Metric | Full Name | Definition | Used For Sizing... |
|---|---|---|---|
| LRA | Locked Rotor Amps | Current drawn at 0 RPM (startup or stalled). | Breaker magnetic trip thresholds, voltage drop calculations. |
| FLA | Full Load Amps | Current drawn at rated horsepower and voltage. | Wire ampacity, thermal overload heaters. |
| RLA | Rated Load Amps | Maximum continuous current under specific HVAC conditions. | HVAC compressor wire and breaker sizing (per UL 1995). |
| MCA | Minimum Circuit Ampacity | FLA plus 25% of the largest motor in a multi-motor system. | Minimum wire gauge for HVAC equipment. |
The Math: A Worked Numeric Example
Motor manufacturers don't always stamp the exact LRA on the nameplate. Instead, they often use a NEMA Code Letter (defined in the NEMA MG 1 standard) that indicates the locked-rotor kVA per horsepower. Let's calculate the LRA for a 5 HP, 230V, single-phase motor with a NEMA Code Letter 'G'.
- Identify the kVA/HP range: NEMA Code G corresponds to 5.6 to 6.29 kVA per HP. We will use the midpoint: 5.95 kVA/HP.
- Calculate Total Locked Rotor kVA: 5 HP × 5.95 kVA/HP = 29.75 kVA (or 29,750 VA).
- Calculate LRA: Divide the total VA by the line voltage. 29,750 VA ÷ 230V = 129.3 Amps.
Even though this motor only draws about 28A while running (FLA), it demands nearly 130A the moment the contactor closes. According to data from the Engineering Toolbox, LRA is typically 5 to 7 times the FLA for standard induction motors, which aligns perfectly with our 129.3A calculation (roughly 4.6x to 5x, factoring in single-phase efficiency losses).
Where You Meet This in Practice
What does LRA actually change in a real circuit or installation? It dictates three critical hardware choices:
1. Breaker Sizing and Magnetic Trips
Standard thermal-magnetic breakers have two tripping mechanisms. The thermal bimetallic strip handles long-term overloads (based on FLA). The magnetic solenoid handles instant short-circuits. If you size a breaker too close to the FLA, the LRA inrush will trigger the magnetic solenoid, and the breaker will trip with a loud 'clack' before the motor even reaches 50% speed. The NFPA National Electrical Code (NEC Article 430.52) allows you to size an inverse-time breaker up to 250% of the motor's FLA specifically to let the LRA pass without tripping the magnetic element.
2. Voltage Drop and Stalling
Pushing 130A through a long, undersized wire run causes severe voltage drop. If the voltage at the motor terminals drops too low during startup, the motor's starting torque (which drops with the square of the voltage) may fall below the load's breakaway torque. The motor stalls, stays at 0 RPM, and remains in a continuous locked rotor state until the thermal overload melts.
3. Contactor Utilization Categories
When selecting a contactor or relay, you must look at the IEC utilization category. An AC-1 rated contactor is for resistive loads. An AC-3 rated contactor is specifically designed to handle the high LRA making-current and the FLA breaking-current of squirrel-cage motors.
Real-World Scenario Walkthrough: The Compressor Nuisance Trip
Let's look at a classic jobsite failure to see how ignoring LRA ruins an installation.
- The Setup: A DIYer installs a 3 HP (230V) air compressor in a detached garage. The motor nameplate shows an FLA of 17A and an LRA of 105A. The run from the main panel is 80 feet. The DIYer uses 12 AWG copper wire and installs a 20A double-pole breaker, reasoning that 17A FLA fits safely under 20A.
- The Numbers: 12 AWG wire has a resistance of roughly 1.98 ohms per 1,000 feet. The round-trip distance is 160 feet (0.16 kft), yielding a circuit resistance of about 0.31 ohms.
- The Outcome: Every time the pressure switch closes, the breaker trips instantly with a sharp snap. The compressor never starts.
- What Went Wrong: The DIYer ignored the LRA. When the motor starts, it pulls 105A. Pushing 105A through 0.31 ohms of wire causes a 32.5V drop. The voltage at the motor terminals plummets to 197V. At this lower voltage, the LRA actually drops slightly, but the starting torque drops drastically (by roughly 26%). The compressor head is under load and fails to break away. The motor stays locked, continuing to pull high current. Furthermore, a standard 20A breaker's magnetic trip threshold is typically set at 5x to 10x its rating (100A to 200A). The 105A inrush hit the lower bound of the magnetic trip curve, snapping the breaker open instantly.
- The Fix: Upsize the breaker to 40A (per NEC 430.52 allowances for motor starting) and upsize the wire to 8 AWG to mitigate the voltage drop, ensuring the motor gets enough terminal voltage to generate the torque needed to escape the locked rotor state.
Frequently Asked Questions
Can I measure LRA with my standard digital multimeter?
No. A standard DMM samples too slowly to catch the transient inrush, which peaks in the first half-cycle (8.3 milliseconds on a 60Hz system) and decays rapidly. You need a clamp meter with a dedicated 'Inrush' or 'Peak Hold' button (like the Fluke 376 FC) that triggers on the current threshold and captures the maximum value over the first 100 milliseconds.
Is Locked Rotor Amps the same as Short Circuit Current?
No. Short circuit current (SCC) is the fault current that flows when a hot wire physically touches a ground or neutral, limited only by the utility transformer's impedance and the wire's resistance. SCC can be thousands of amps. LRA is the normal, expected current drawn by the motor's intact windings when the rotor is stationary. LRA is usually 5 to 7 times FLA; SCC can be 50 to 100 times FLA.
Will a soft starter reduce the LRA?
Yes. A soft starter works by temporarily reducing the voltage applied to the stator during the first few seconds of startup using phase-angle control (thyristors). Because LRA is directly proportional to the applied voltage, dropping the voltage to 70% during startup reduces the LRA draw to roughly 70% of its normal locked-rotor value. However, keep in mind that starting torque also drops significantly, so soft starters are best for fans and pumps, not high-breakaway-torque loads like conveyors or loaded compressors.
