Locked Rotor Amperes Explained: Sizing Breakers and Motor Contactors

Locked rotor amperes (LRA) is the maximum instantaneous current an AC motor draws the moment voltage is applied before the rotor begins to spin. When you throw the switch on a table saw, an HVAC compressor, or a workshop dust collector, the motor does not instantly pull its running current. Instead, it pulls a massive, brief spike of current to overcome the physical inertia of the stationary rotor and the connected mechanical load. Understanding this spike is the difference between a reliable installation and a system that nuisance-trips every time it starts—or worse, melts your switching components.

The Physics of the Spike: Why LRA is So High

To understand LRA, you have to look at back-electromotive force (back-EMF). When an AC motor is running at full speed, the spinning rotor generates a voltage that opposes the incoming supply voltage. This back-EMF acts as a natural current limiter, keeping the running draw relatively low.

However, at the exact millisecond you apply power, the rotor is stationary. Speed is zero, which means back-EMF is zero. The only things limiting the current flow are the DC resistance and the leakage reactance of the stator windings—both of which are intentionally kept very low to make the motor efficient. The result is a massive inrush of current.

LRA vs. FLA vs. RLA: Clearing Up the Confusion

What people commonly confuse LRA with are the running current metrics printed right next to it on the motor nameplate. Sizing a continuous-duty breaker or wire based on LRA will result in massively oversized components that fail to protect the motor from burning up during a sustained mechanical overload. Conversely, sizing a breaker based only on running amps will cause it to trip instantly on startup.

Where You Meet This in Practice

Here is exactly what LRA changes in a real circuit or installation: it forces you to upsize the breaker's magnetic trip threshold, dictates the physical contact mass inside your contactor, and determines whether you can use Direct-On-Line (DOL) starting or need a soft starter.

1. Breaker Sizing (NEC 430.52): Standard thermal-magnetic breakers have an instantaneous magnetic trip designed to catch short circuits. If your motor's LRA exceeds this magnetic threshold, the breaker will trip before the motor reaches full speed. The NEC allows you to size inverse-time breakers up to 250% of the motor's FLA specifically to accommodate the LRA startup spike without nuisance tripping.
2. Contactor Utilization Categories: You cannot use a standard lighting or resistive contactor for a motor. Under IEC standards, you must select an AC-3 rated contactor, which is specifically engineered with heavier contacts and arc chutes to safely make and break the high inductive currents of an LRA event.
3. Starting Methods: If the LRA is so high that it causes severe voltage drop on the feeder (dimming lights, tripping sensitive VFDs on the same bus), you must implement a reduced-voltage starter, a star-delta configuration, or a Variable Frequency Drive (VFD) to ramp the voltage and limit the inrush.

Worked Numeric Example: Sizing the Protection

Let's size the protection for a standard workshop machine. We are wiring a 7.5 HP, 230V, 3-phase dust collector motor. The nameplate provides the following data:

• Full Load Amps (FLA): 22A
• Locked Rotor Amperes (LRA): 132A (NEMA Code Letter J, approx. 6x FLA)
• Service Factor: 1.15

Step 1: Wire Sizing (Based on FLA)
NEC 430.22 requires conductors to be sized at 125% of the FLA.
22A × 1.25 = 27.5A.
Using the 75°C column of NEC Table 310.16, we select 10 AWG THHN copper (rated for 35A).

Step 2: Overload Relay Sizing (Based on FLA)
The thermal overload protects the motor from sustained mechanical overloading. Per NEC 430.32, for a 1.15 SF motor, we trip at 125% of FLA.
22A × 1.25 = 27.5A trip setting.

Step 3: Breaker Sizing (Accommodating LRA)
If we used a standard 30A breaker, its magnetic instantaneous trip (typically 10x rating, or 300A) might hold, but the thermal element could nuisance trip on a hard start. NEC 430.52 allows an inverse-time breaker up to 250% of FLA.
22A × 2.50 = 55A.
We select the next standard size down to ensure protection: a 50A 3-pole inverse-time breaker. This breaker's thermal curve will protect the 10 AWG wire during a sustained stall, while its magnetic trip (set around 500A) will ignore the 132A LRA spike during the 2-second startup phase.

Real-World Scenario: The Welded Contactor Failure

Abstract numbers on a nameplate translate directly to physical failures on the bench. Here is a walkthrough of a common, destructive mistake in small cabinet shops.

The Numbers: The motor's FLA is 15A, but its LRA is 95A. The pressure switch is set to cut out at 150 PSI.

The Outcome: The system runs fine for three weeks. Then, the compressor happens to hit the 150 PSI cut-out pressure at the exact moment the motor is struggling through a high-inertia restart cycle (still pulling near LRA). The pressure switch opens, signaling the relay to drop out. A massive arc forms across the relay contacts. The contacts weld shut. The compressor runs continuously until the mechanical safety valve pops, or the motor thermal overload eventually trips.

What Went Wrong: The builder looked at the 32A continuous resistive rating (an IEC AC-1 category) and ignored the motor starting category. Breaking 95A of highly inductive LRA current requires an AC-3 rated motor contactor with specialized arc-extinguishing chambers. The surplus relay was physically incapable of snapping the magnetic field, resulting in a sustained plasma arc that melted the silver-alloy contacts into a single solid mass. Always match the contactor's AC-3 horsepower rating to the motor, never its generic amperage rating.

Frequently Asked Questions