Locked rotor current (LRC) is the maximum steady-state RMS current an AC motor draws from the power supply at rated voltage when its shaft is mechanically prevented from turning. When you energize a motor, the rotor is essentially locked at zero RPM for the first few milliseconds until it begins to spin and generate back-EMF. During this window, the motor acts like a short-circuited transformer, pulling massive current. Understanding LRC dictates how you size your branch circuit wiring, select your overcurrent protection to prevent nuisance tripping, and calculate voltage drop on long feeder runs.
What Locked Rotor Current Actually Is (And What It Isn't)
To size motor circuits correctly per NEC Article 430, you must separate the motor's operating current from its starting current. Full Load Amps (FLA) is the current the motor draws at rated horsepower and speed. LRC is the current it draws at zero speed. Because there is no back-EMF generated by a stationary rotor, the only things limiting current are the stator winding resistance and the leakage reactance. This results in an LRC that is typically 5 to 8 times higher than the FLA.
What LRC changes in a real installation is the thermal and magnetic stress on your protective devices. A standard thermal-magnetic breaker uses a bimetallic strip for long-term overloads (sized near FLA) and a solenoid for instantaneous short circuits. If you size the breaker's magnetic trip too close to the FLA, the LRC will instantly trip the breaker every time you hit the start button. Conversely, if you size it purely to survive the LRC, you might leave the motor unprotected against a mild mechanical jam.
The Math: Calculating LRC from a NEMA Nameplate
Motor manufacturers governed by the NEMA MG-1 standard provide two ways to determine LRC on the nameplate: the direct Locked Rotor Amps (LRA) value, or a NEMA Code Letter.
If the nameplate explicitly states 'LRA', use that number for your breaker sizing. If it only provides a Code Letter, you must calculate it. The Code Letter indicates the locked-rotor kilovolt-amperes per horsepower (kVA/HP).
Let's size the protection for a 15 HP, 460V, 3-phase TEFC motor.
1. The nameplate lists an FLA of 21A and a NEMA Code Letter 'H'.
2. Per NEMA MG-1, Code Letter 'H' spans 6.3 to 7.09 kVA/HP. We will use the midpoint: 6.7 kVA/HP.
3. Total locked-rotor kVA = 6.7 kVA/HP × 15 HP = 100.5 kVA.
4. Apply the 3-phase current formula: I = (kVA × 1000) / (Volts × √3)
5. LRC = 100,500 / (460 × 1.732) = 100,500 / 796.72 = 126.1 Amps.
This 126.1A LRC is exactly 6 times the 21A FLA. Your overcurrent device must tolerate 126.1A for the duration of the motor's start-up ramp (usually 1 to 3 seconds) without tripping.
Where You Meet LRC in Practice
You will encounter LRC in three specific jobsite or bench scenarios, and each requires a different technical response.
1. Sizing Motor Starters and Contactors
Contactors are rated by NEMA sizes (Size 0, 1, 2, etc.) based on continuous FLA. However, the contacts must physically withstand the mechanical and thermal forces of the LRC during closure. A NEMA Size 1 contactor is rated for 27A continuous, but its making/breaking capacity is tested against high-multiple starting currents. Never use a lighting contactor for a motor; the contacts will weld shut when they close into a locked rotor condition.
2. Voltage Drop on Long Feeder Runs
This is where LRC ruins installations. If you run a 200-foot feeder to a well pump using wire sized only for the FLA, the massive LRC will cause severe voltage drop at the motor terminals during startup. Because motor starting torque is proportional to the square of the applied voltage ($T \propto V^2$), a 20% voltage drop caused by LRC results in a 36% loss of starting torque. The motor fails to overcome the load inertia, stays locked, continues drawing LRC, and eventually burns out the windings. Always calculate voltage drop using the LRC value, not the FLA.
3. Mechanical Jams and Seized Bearings
If a conveyor belt jams or a pump impeller seizes, the motor instantly returns to drawing LRC. Your overload relay (which is sized to the FLA) must detect this and drop the contactor out within seconds. If the overload is bypassed or incorrectly dialed, the stator insulation will melt.
Decision Tree: Sizing Breakers and Starters Around LRC
Choosing the right overcurrent protection requires matching the breaker's magnetic trip curve to the motor's LRC profile. Use this decision path to select your hardware.
| Condition / Load Profile | LRC Duration | Required Protection Strategy | Concrete Hardware Pick |
|---|---|---|---|
| Low inertia load (fans, small pumps) | < 1 second | Standard Inverse-Time Thermal-Magnetic Breaker. Set to 250% of FLA max per NEC 430.52. | Eaton C320 thermal-magnetic breaker (e.g., C320F250A) |
| High inertia load (large blowers, compressors, conveyors) | 2 to 5 seconds | Motor Circuit Protector (MCP). Magnetic-only breaker paired with a separate thermal overload relay. Allows precise magnetic trip setting above LRC. | Square D PowerPact H-Frame MCP (e.g., HJL36040M) with adjustable magnetic trip dial. |
| Extreme starting duty / high code letter (Code V or W) | > 5 seconds | Reduced voltage starting (Star-Delta, Autotransformer, or VFD) to artificially lower the LRC drawn from the mains. | ABB ACS580 Variable Frequency Drive (VFD) |
Troubleshooting Nuisance Trips on Startup
Why does my breaker trip instantly when I press start, but the motor spins fine if I test it unloaded?
This is the classic signature of an LRC magnetic trip. Under load, the motor takes 2 seconds to reach full speed, drawing LRC the entire time. The breaker's magnetic solenoid sees 6x FLA and trips in milliseconds. The Fix: Switch from a standard thermal-magnetic breaker to an MCP (like the Square D H-Frame listed above) and dial the magnetic trip threshold to 12x or 15x the motor FLA, or install a soft-starter to ramp the voltage and limit the LRC.
I measured the starting current with my clamp meter and it's way higher than the nameplate LRA. Is the motor failing?
Not necessarily. Standard RMS clamp meters cannot capture the sub-cycle asymmetrical inrush peak. If you are using a high-end power quality meter (like a Fluke 435) set to capture the absolute peak transient, you will see the DC offset inrush, which can be up to 2.2 times the symmetrical LRC. This is normal physics, not a failing motor. For sizing standard breakers, always rely on the symmetrical RMS LRC value calculated from the nameplate, not the transient peak captured by an oscilloscope or power analyzer.
My motor hums, draws massive current, and trips the overload after 10 seconds. What's wrong?
You have a true locked rotor condition. The motor is energized but physically cannot turn. Disconnect power, lock out the panel, and check the mechanical load. Look for seized bearings, a jammed gearbox, or single-phasing (one blown fuse on a 3-phase supply). If single-phasing occurs, the motor will attempt to run as a single-phase motor, draw unbalanced LRC, and rapidly overheat. Verify all three phases at the contactor output with a multimeter before resetting.
Ultimately, respecting the locked rotor current value on the nameplate is what separates a reliable motor installation from one that nuisance-trips or catches fire. Calculate the exact LRC, verify your voltage drop under that specific load, and select an MCP or appropriately rated breaker that holds the line during startup while still protecting the windings when the shaft stops.
