Locked rotor amperage (LRA) is the peak current an AC motor draws the instant power is applied before the rotor begins to spin, or when the rotor is mechanically stalled. When you energize an induction motor, the stator creates a rotating magnetic field, but the stationary rotor acts essentially like a short-circuited transformer secondary. This results in a massive inrush of current—often 5 to 8 times the motor's normal running current—until the rotor catches up to the magnetic field's speed and generates back-EMF (electromotive force) to limit the current flow.
Understanding LRA is not just an academic exercise; it dictates the physical hardware you install. If you size a standard thermal-magnetic breaker based solely on a motor's running current, the breaker's instantaneous magnetic trip will see the LRA spike as a dead short and trip immediately every time the motor tries to start. Below, we break down the exact numbers, how LRA alters your circuit design, and the math required to keep your motor running and your breakers closed.
Reading the Nameplate: LRA vs. FLA vs. RLA
The most common mistake DIYers and junior technicians make is confusing LRA with FLA (Full Load Amps) or RLA (Rated Load Amps). FLA/RLA is your continuous running current under maximum rated mechanical load. LRA is your instantaneous stall or startup current. If your clamp meter reads anywhere near the LRA value while the motor is running, the rotor is locked, the start winding is engaged, and the motor will burn out in seconds if the thermal overload doesn't drop the circuit.
To ground this in real-world hardware, here is a spec-sheet table of common fractional and integral horsepower motors you will encounter in residential and light commercial HVAC, pool, and shop compressor applications.
| Motor Application | Rating (HP) | Voltage / Phase | FLA / RLA (Amps) | LRA (Amps) | LRA Multiplier |
|---|---|---|---|---|---|
| Condenser Fan Motor | 1/2 HP | 230V / 1Φ | 3.2 A | 18.5 A | ~5.8x |
| Pool Pump Motor | 1.5 HP | 230V / 1Φ | 9.8 A | 58.0 A | ~5.9x |
| HVAC Scroll Compressor | 3.0 HP | 230V / 1Φ | 17.0 A | 102.0 A | 6.0x |
| Shop Air Compressor | 5.0 HP | 230V / 3Φ | 15.2 A | 91.0 A | ~6.0x |
| Industrial Blower | 10.0 HP | 460V / 3Φ | 14.0 A | 88.0 A | ~6.3x |
Notice that the LRA multiplier hovers right around 6.0x the FLA across almost all standard NEMA Design B induction motors. While the absolute amperage drops as voltage increases (compare the 3 HP single-phase at 102A to the 10 HP three-phase at 88A), the ratio of starting inrush to running current remains remarkably consistent due to the physics of rotor impedance at 100% slip.
What Locked Rotor Amperage Changes in Your Installation
LRA forces you to change three specific components in your electrical installation compared to a standard resistive load circuit:
1. Breaker Sizing and Trip Curves
Standard branch circuit breakers protect wire from melting. But motor circuits are governed by NFPA 70 (NEC) Article 430, which separates motor overload protection (handled by the motor's internal thermal switch or external overload relay) from short-circuit protection (handled by the breaker). Because of the LRA spike, NEC 430.52 allows you to drastically oversize the breaker relative to the wire's ampacity, provided the motor's dedicated overload device protects the wire from continuous overcurrent. You must use HACR (Heating, Air Conditioning, and Refrigeration) rated breakers or time-delay fuses designed to tolerate the LRA inrush for the 200 to 500 milliseconds it takes the motor to spin up.
2. Voltage Drop and Wire Sizing
Even though wire size is technically calculated using 125% of the FLA, LRA exposes voltage drop vulnerabilities. If a 3 HP compressor draws 102A during startup through a long run of undersized wire, Ohm's law dictates a massive voltage drop. If the voltage at the contactor drops below 80% of nominal during the LRA phase, the motor loses starting torque, fails to spin, and remains in a locked rotor state until the thermal overload trips.
3. Contactor and Starter Ratings
Relays and contactors must be rated for the motor's LRA to prevent the contacts from welding shut during the inrush spike. Always verify the contactor's AC-3 utilization category rating, which specifically certifies the device can handle the high inrush of squirrel-cage motors and break the circuit while the motor is still running.
Worked Numeric Example: Sizing a Circuit for a 3 HP Compressor
Let's walk through a real-world bench and jobsite scenario. You are wiring a new 3 HP, 230V, single-phase HVAC scroll compressor. The nameplate reads:
- RLA: 17.0 A
- LRA: 102.0 A
Step 1: Wire Sizing (NEC 430.22)
Motor circuit conductors must be sized at 125% of the continuous running current (RLA/FLA).
17.0 A × 1.25 = 21.25 A.
Looking at the 75°C column of NEC Table 310.16, 12 AWG THHN is rated for 25A, which is sufficient. However, because of the severe voltage drop risk during the 102A LRA spike, best practice dictates bumping to 10 AWG THHN (rated 35A at 75°C) for runs over 30 feet to ensure the compressor gets adequate startup voltage.
Step 2: Breaker Sizing (NEC 430.52)
The breaker must hold the 102A LRA spike without nuisance tripping, while still providing short-circuit protection. For an inverse-time breaker, the code allows up to 175% of the FLA.
17.0 A × 1.75 = 29.75 A.
We round up to the next standard breaker size: 30 A.
Step 3: The LRA Exception
If the 30A breaker still trips on startup (common in high-inertia loads or cold weather where oil is thick), NEC 430.52(C)(1) Exception No. 1 allows you to increase the breaker size up to 225% of FLA.
17.0 A × 2.25 = 38.25 A.
We round up to a 40 A breaker. A standard 40A breaker has an instantaneous magnetic trip threshold of roughly 5x to 10x its rating (200A to 400A). It will easily ignore the 102A LRA spike, allowing the motor to start, while the motor's internal thermistor protects the 10 AWG wire from a sustained mechanical stall.
Where You Meet LRA in Practice (and Common Confusions)
You will rarely measure LRA directly unless you are troubleshooting a failure. Standard True-RMS clamp meters sample over a few hundred milliseconds and will completely miss the 200ms LRA startup spike, often displaying a falsely low 'average' number. To actually capture LRA on a bench or in the field, you need an inrush-capable clamp meter (like the Fluke 376 FC) that features a dedicated inrush button to trigger a high-speed sampling mode synced to the AC waveform.
Hard Start Kits and Failing Capacitors
The most common place DIYers interact with LRA concepts is when an HVAC compressor 'hard starts' (hums loudly, lights in the house dim, then the breaker trips). This happens when the run capacitor degrades, dropping the starting torque. The motor fails to overcome the mechanical load, stays at 100% slip, and remains stuck drawing LRA until the thermal overload drops the contactor. Installing a hard start kit (a start capacitor paired with a 5-2-1 potential relay) injects a massive phase-shifted current boost into the start winding for the first few milliseconds. This increases starting torque, gets the rotor spinning faster, generates back-EMF quicker, and drastically reduces the time the circuit spends in the high-LRA state.
The 'Running at LRA' Confusion
Novices often look at the nameplate, see 'LRA: 102A', and assume they need to wire the entire system to handle 102A continuously. This leads to massive overspending on 2 AWG wire and 100A disconnects. Remember: LRA is a transient state lasting fractions of a second. If your system is drawing LRA for more than 3 seconds, you do not have a wiring problem; you have a mechanical failure or a dead capacitor.
Frequently Asked Questions
Is LRA the exact same thing as 'inrush current'?
In motor terminology, yes. However, in broader electronics, 'inrush current' can also refer to the spike caused by charging empty capacitors in a power supply or the cold-filament resistance of incandescent bulbs. LRA specifically refers to the electromagnetic stall current of an AC induction motor.
Can a soft starter reduce LRA?
Yes. A solid-state soft starter uses back-to-back SCRs (silicon-controlled rectifiers) to phase-angle chop the voltage applied to the motor during the first few seconds of startup. By reducing the voltage, the LRA spike is proportionally reduced (though starting torque is also reduced by the square of the voltage). This is common in industrial 3-phase setups but rare in residential 1-phase HVAC due to cost and complexity.
Why is my LRA slightly lower than the nameplate states?
Nameplate LRA is calculated at nominal voltage. If your utility is delivering low voltage (e.g., 210V instead of 230V), the actual measured LRA will be proportionally lower. However, this is dangerous: lower voltage means lower starting torque, which increases the time the motor spends in the locked rotor state, generating excess heat in the windings.
