Understanding Lock Rotor Amps: Sizing Breakers and Wire for Motor Startup

What Lock Rotor Amps Actually Means (And Why It Matters)

Lock rotor amps (LRA) is the absolute maximum current an AC motor draws the instant power is applied while the shaft is physically prevented from turning. When you first energize an induction motor, the rotor is stationary, meaning it generates zero back-electromotive force (back-EMF). Without back-EMF to oppose the supply voltage, the motor windings act essentially as a dead short, limited only by the DC resistance and leakage reactance of the copper wire.

In a real circuit or installation, LRA changes everything about your upstream protection and conductor sizing. It dictates the instantaneous magnetic trip curve of your circuit breaker, forces you to calculate voltage drop at 500% to 700% of normal running current, and determines whether you need a soft starter or variable frequency drive (VFD) to prevent severe voltage sag on the local grid. If you ignore LRA and design solely for running current, your breakers will nuisance-trip on startup, or worse, your motor will stall and burn out its windings due to severe voltage drop at the terminals.

The LRA vs. FLA vs. RLA Confusion

The most common mistake DIYers and junior technicians make is confusing LRA with Full Load Amps (FLA) or Rated Load Amps (RLA). Sizing a breaker for FLA guarantees it will trip the millisecond the motor starts. Here is how these distinct nameplate values actually function in circuit design:

According to the NFPA National Electrical Code, your wire size is based on MCA/FLA, but your breaker size must accommodate the LRA without tripping, while still protecting the wire from continuous overloads. This balancing act is why motor circuits use specialized HACR (Heating, Air Conditioning, and Refrigeration) breakers or D-curve miniature circuit breakers (MCBs), which have delayed magnetic trip thresholds specifically designed to swallow the LRA spike.

Worked Numeric Example: Sizing for a 5 HP Compressor

Let's look at a real-world bench calculation for a 5 HP, 230V, single-phase air compressor motor.

Notice that the LRA (168A) is exactly 6 times the FLA (28A). This 6x multiplier is standard for NEMA Design B motors. Now, let's calculate the voltage drop if we run this compressor 100 feet from the panel using 10 AWG THHN copper wire, which is perfectly adequate for the 28A running current.

1. Find wire resistance: 10 AWG copper at 75°C has a resistance of roughly 1.24 ohms per 1,000 feet. For a 100-foot run, the total loop (hot + neutral) is 200 feet. Total resistance = 0.248 ohms.
2. Calculate running voltage drop: At 28A (FLA), V_drop = 28A × 0.248Ω = 6.9V. The motor sees 223V. Perfectly fine.
3. Calculate startup voltage drop: At 168A (LRA), V_drop = 168A × 0.248Ω = 41.6V.
4. Determine terminal voltage: 230V - 41.6V = 188.4V at the motor terminals during startup.

Motor starting torque drops with the square of the applied voltage. Dropping from 230V to 188V reduces your starting torque to roughly 66% of normal. If this compressor is starting against high head pressure (e.g., it wasn't fully pumped down), that 34% loss in torque might be exactly enough to prevent the rotor from breaking inertia. The motor stalls, continues drawing 168A, and eventually melts the winding insulation. This is why long runs to high-LRA motors often require upsizing wire to 8 AWG or 6 AWG, not for thermal ampacity, but to preserve startup voltage.

Where You Meet This in Practice

You will encounter LRA constraints in three primary areas on the jobsite or in the workshop:

• Breaker Selection and Nuisance Tripping: Standard thermal-magnetic breakers trip magnetically at 5x to 10x their rated current. A 40A standard breaker might magnetically trip at 200A. If your motor's LRA is 220A, the breaker will trip instantly on startup. You must select a breaker with a higher magnetic threshold or a specialized motor-protection curve.
• Generator Sizing: When sizing a portable generator for a jobsite, you cannot just add up the running wattages. A 5 HP motor drawing 28A running requires about 6.4 kW. But to overcome the LRA of 168A at a degraded power factor during startup, the generator's alternator must be able to supply upwards of 15 to 20 kVA momentarily without the engine bogging down and stalling. As noted by Fluke's electrical testing guides, inrush and locked rotor conditions are the primary reasons undersized generators fail to start heavy inductive loads.
• Reading NEMA Code Letters: If LRA isn't explicitly printed on the nameplate, look for a NEMA Code Letter (e.g., Code G, Code H). This letter designates the locked-rotor kVA per horsepower. For a Code H motor (6.3 to 7.09 kVA/HP), you can calculate LRA by multiplying the HP by the kVA factor, then dividing by (Voltage × √3 for 3-phase).

Real-World Scenario: The Stalled Well Pump

To see how ignoring LRA destroys equipment, let's walk through a real-world failure involving a submersible well pump.

The Setup: A homeowner replaces a 1.5 HP, 230V submersible well pump. The pump sits 250 feet down the well casing, and the trench to the wellhead is another 100 feet from the main panel. Total wire run: 350 feet. The existing underground wire is 12 AWG UF-B. The new pump nameplate reads FLA: 10A, LRA: 65A.

The Numbers: 12 AWG wire is rated for 20A, which easily covers the 10A FLA. The total circuit loop is 700 feet. At 75°C, 12 AWG resistance is about 1.98 ohms per 1,000 feet. Total loop resistance = 1.38 ohms.

The Outcome: The homeowner flips the breaker. The contactor clicks. The pump emits a low hum, the lights in the house dim severely, but the pump never spins up. After 10 seconds, the thermal overload in the control box pops. The homeowner resets it, and the exact same thing happens.

What Went Wrong: The voltage drop was calculated for the running load, not the LRA. At 65A (LRA), the voltage drop across the 1.38-ohm loop is a massive 89.7V (65A × 1.38Ω). The voltage actually reaching the pump terminals is only 140V. Because starting torque is proportional to voltage squared, the pump only produces about 37% of its required starting torque. It cannot overcome the static head pressure of the water column. The rotor remains locked, drawing 65A continuously until the thermal protector saves the motor from catching fire.

The Fix: The homeowner had to pull the pump and replace the 350-foot drop wire with 6 AWG copper, reducing the loop resistance to 0.28 ohms. At 65A, the new voltage drop was only 18.2V, delivering 211V to the motor—enough to break inertia and spin the pump up to speed.

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