Locked Rotor Amps (LRA) Calculator

These rows are Table 430-7(b), "Locked Rotor Indicating Code Letters", as MSHA's reprint of the 1968 National Electrical Code (NFPA 70-1968) prints them; the letters I, O and Q are not used. For a three-phase motor the calculator works LRA = kVA/hp × hp × 1,000 ÷ (1.732 × V), with 1.732 as Land and Electric prints the square root of 3; for a single-phase motor it divides by V alone. ABB, a motor maker, and Rockwell Automation, a motor-control maker, print the same code letters, A to V, with bands that differ from this table's only in how an upper edge is written, and we have not opened NFPA's own text of Table 430.7(B), so confirm the letter's band against the NEC edition your jurisdiction has adopted or your authority having jurisdiction. Compressors and high-torque applications often land in the H-K range. Check the motor nameplate for the NEMA code letter.

NEMA MG 1 prints these maximum locked-rotor currents for Design B, C and D motors at 230 volts; each sits inside the band of its own code letter, and the maximum at another voltage is "inversely proportional to the voltages." ABB's Motor Protection Fundamentals webinar puts the starting current as "typically 6 times the Full Load Current"; this calculator returns the code letter band instead. Baldor's data sheet for its EM3581T motor (1 HP, code N) prints "Starting Current 14.4 A" at 460 V ("Typical performance; not guaranteed values."); the code N band for that motor at 460 V is 14.1 to 15.7 A.

When you apply power to a stopped motor, the rotor is not turning. The motor's windings see full voltage across zero back-EMF (the voltage a spinning rotor generates to oppose the supply). With no back-EMF, the windings behave almost like a short circuit, and current surges to several times the running amps. Our explainer on why motors spike at startup walks through the physics and the code-letter method in depth.

This inrush lasts 0.5-3 seconds for most motors. As the rotor accelerates, back-EMF builds and current drops toward normal FLA. A Baldor EM3581T (1 HP, code N, 3.26 A full load at 230 V three-phase) draws 28.1 to 31.4 A by its code letter band as it starts. That band is what your breaker, wiring, and power source must survive without tripping or sagging.

The locked rotor condition is the worst case: the rotor is physically prevented from turning (locked), so inrush current never subsides. A motor that jams (seized bearing, stuck compressor, frozen pump) draws locked rotor amps continuously until the overload relay opens or the breaker trips. This is why motor circuits use dual protection: a breaker sized for starting inrush (250-300% of FLA) and an overload relay sized for running current (115-125% of FLA).

Temperature affects starting current too. A cold motor has lower winding resistance and draws slightly more inrush than a warm one. If you are sizing protection for a motor in an unheated shop that starts from cold every morning, use the top of the code letter's band.

A 5HP three-phase air compressor at 230V has a nameplate FLA of 15.2A and a NEMA code letter of H. Code H prints 6.3 to 7.09 kVA per horsepower, so the LRA is 79.1 to 89 A.

To handle this startup surge, you need:

• Breaker: 250% of 15.2A = 38A. Next standard size is 40A inverse-time breaker. NEC Article 430 allows up to 250% for inverse-time breakers because the brief 79.1 to 89 A inrush does not trip a thermal breaker fast enough to cause a nuisance trip.
• Contactor: Rated for at least the FLA (15.2A) with an inrush rating matching the LRA. A NEMA Size 1 starter is rated 27A continuous with a published closing-inrush rating of 288A peak. That covers this motor with room to spare.
• Overload relay: Set to 115% of FLA = 17.5A. This trips on sustained overcurrent but rides through the brief starting surge.
• Wiring: Must carry 125% of FLA = 19A minimum. 12 AWG copper handles that, but check voltage drop if the run exceeds 50 feet.

If you do not know your motor's FLA, our motor FLA calculator reads the printed table's current by horsepower, voltage and phase, and its page gives the nameplate formula, from horsepower, voltage, efficiency and power factor, as a second figure to check against the nameplate.

Running this compressor off a battery-powered inverter? The inverter must handle the top of the band, 89 A. An inverter whose surge rating is 2x continuous surges to about 26A at 3,000W and 230V. You would need a much larger inverter or a soft starter to reduce the inrush. Our battery size for inverter calculator can help you figure out how much battery capacity the system needs.