What is locked-rotor current?

Locked-rotor current, or LRA, is the high inrush current a motor draws at the instant of starting, before the rotor turns. It is typically five to eight times the running full-load current and determines voltage dip, generator sizing, and the instantaneous trip setting of protective devices.

How does the NEMA code letter work?

Each motor is stamped with a code letter from A to V that NEC Table 430.7(B) maps to a band of locked-rotor kVA per horsepower. A code G motor, for example, draws 5.6 to 6.29 kVA per HP at start, which the tool multiplies by horsepower to get starting kVA.

Why does three-phase divide by the square root of three?

For a three-phase motor, total power equals the square root of three times line voltage times line current. Solving for current gives amps equals kVA times 1000 divided by 1.732 times voltage, which is why the three-phase result is lower than the single-phase result at the same kVA.

Should I use the midpoint or the high end of the band?

The tool reports the band midpoint as the estimate and shows the full minimum-to-maximum range. For conservative generator or protective-device sizing, use the high end of the range, and always confirm against the actual nameplate LRA when it is printed.

What do I use this number for?

Locked-rotor current sizes standby generators, sets the instantaneous magnetic trip of motor breakers, checks voltage-dip on starting, and helps select soft starters and VFD bypass contactors so the equipment can handle the inrush without nuisance tripping.

Motor Starting (LRA) Current Calculator

When an induction motor starts, it briefly draws a large inrush, the locked-rotor current, that sizes generators, protective devices, and starting equipment. This calculator uses the NEMA code letter from NEC Table 430.7(B) together with the motor horsepower and voltage to estimate the starting kVA and locked-rotor amperes for single-phase and three-phase motors.

How it works

The code letter gives a band of locked-rotor kVA per horsepower; the tool uses the band midpoint and converts to amperes:

starting kVA   = HP × kVA-per-HP (midpoint of the code-letter band)
LRA single     = starting kVA × 1000 / V
LRA three-phase = starting kVA × 1000 / (√3 × V)

Because each code letter is a range rather than a single value, the result panel also shows the locked-rotor current at the minimum and maximum of the band so you can size conservatively.

Example and tips

A 10 HP, 460 V three-phase motor with code letter G uses a midpoint of about (5.6 + 6.29) / 2 = 5.945 kVA per HP, giving 10 × 5.945 ≈ 59.5 kVA of starting power. Dividing by √3 × 460 gives roughly 75 A of locked-rotor current. Tips: prefer the actual nameplate LRA when it is printed, since the code-letter method is an estimate; use the high end of the band for generator and breaker sizing; and remember that soft starters and VFDs dramatically reduce this inrush, which is often the reason they are specified in the first place.