Motor Inrush & LRA Calculator
Enter horsepower, voltage, code letter and starting method to get the inrush current and breaker coordination guidance.
Step-by-step
- Enter valid values to begin.
FLA comes from NEC Table 430.250 for 200/208/230/460/575 V and matching table horsepower; other voltages scale the 460 V value, and untabulated horsepower falls back to a formula estimate. The breaker figures are starting-current guidance for coordination, not a replacement for the NEC 430.52 maximums, the manufacturer’s trip curves or a coordination study.
Motor Inrush and LRA: What the Breaker Has to Ride Through
Core Engineering Principles
At the instant a motor is energised the rotor is standing still, so the stator sees a short-circuited transformer. Current jumps to five to eight times full-load amps and stays there until the rotor accelerates. That is locked-rotor current, LRA. NEMA tells you how big it will be through the code letter stamped on the nameplate, which gives starting kVA per horsepower. The letter A is under 3.15 kVA/HP and V is over 22.4. Most design B industrial motors sit between F and H.
The starting method changes what the supply actually sees. Direct-on-line gives you the full LRA. Star-delta starts with the windings in star, which cuts line current to one third, though also cuts starting torque to a third. A soft starter limits current electronically, commonly to 300–400% of FLA, at the price of less torque and a longer ramp. And on the first half-cycle the current is asymmetrical, so the peak can be roughly 1.7 times the steady locked-rotor value. A magnetic trip set on the LRA will nuisance-trip.
Instantaneous pickup ≈ 1.5 × (Istart / FLA) × FLA
NEC & Standard References
NEC Table 430.7(B) lists the code letters and kVA/HP. NEC 430.52 and Table 430.52 set the maximum short-circuit device: 250% of FLC for an inverse-time breaker, 800% for an instantaneous-trip breaker, and 1100% for design B energy-efficient motors, and an exception to 430.52(C)(3) allowing a higher setting where the standard one still trips on starting current. An instantaneous-trip breaker is only permitted as part of a listed combination motor controller. FLC comes from Table 430.250, not the nameplate. Verify the percentages against your adopted edition.1. Starting kVA = 6.29 × 50 = 314.5 kVA.
2. LRA = 314,500 / (1.732 × 460) = 395 A, a ratio of 6.1 × FLA.
3. Star-delta: 395 / 3 = 132 A. Soft starter at 3.5 × FLA: 228 A.
4. Magnetic pickup on direct-on-line ≈ 1.5 × 6.1 = 9.1 × FLA, which is above the 800% base value, so check the combination controller and the motor design.
- Don’t set the trip at LRA. Asymmetrical first-cycle current is higher, and the breaker will open every start.
- Voltage dip is the real constraint on weak supplies. A 400 A inrush on a small generator or long feeder can drop the bus enough to drop contactors out.
- Star-delta transitions can spike. An open transition that switches too early can produce a current surge near the original LRA.
- Frequent starts cook windings. The inrush heats the rotor as much as many seconds of running; check starts per hour.