Motor Locked-Rotor (Inrush) Amps Calculator
Calculate starting (inrush) current of an AC motor using its NEMA code letter (kVA/HP). Essential for sizing breakers, soft starters, VFDs, and generators that must handle motor starting loads without nuisance tripping.
How to Use
- Enter the motor's rated horsepower.
- Enter line-to-line voltage at the motor terminals.
- Select phase: single-phase for fractional-HP and small commercial; three-phase for most industrial motors.
- Pick the NEMA code letter from the nameplate (A = lowest kVA/HP, V = highest). Common commercial motors are F–H.
- Result shows locked-rotor amps (LRA), the starting kVA/HP code value, starting kVA, and typical LRA/FLA ratio.
Show Work
Formulas
History of Motor Inrush and Starting
The huge starting current of AC induction motors was recognized the moment they went into practical service in the 1890s. A motor at standstill presents essentially zero back-EMF — the rotor is locked electromagnetically equivalent to a short-circuited secondary of a transformer — so line voltage drives a current limited only by winding impedance. The NEMA MG-1 code letter system was developed in the 1930s to help electricians size starting circuits without measuring individual motors.
Across-the-line (DOL, direct-on-line) starting was the original method: close a contactor, dump full line voltage onto the motor, accept the inrush. It\'s still the most common starting method for motors up to about 50 HP because it\'s simplest, but supply network voltage sags from DOL starts on larger motors historically caused lights to dim and sensitive equipment to reset on distant circuits.
Star-delta (wye-delta) starting emerged in the 1920s as the first inrush-reduction technique: run the motor in wye connection for the first few seconds (giving 1/3 the voltage and 1/3 the current), then switch to delta once the rotor is near synchronous speed. Autotransformer starters (reduced-voltage starts) followed, then solid-state soft starters in the 1980s using SCR phase control, and finally full-featured variable-frequency drives (VFDs) in the 1990s/2000s that eliminate the inrush problem entirely by ramping frequency from zero.
About This Calculator
Enter rated HP, line voltage, phase count, and the nameplate NEMA code letter. The tool returns locked-rotor amps, starting kVA, starting kVA per HP (mid-range of the code letter), and the typical LRA/FLA ratio for that size and code class.
Apply the result to size breakers (use D-curve thermal-magnetic or time-delay fuse), select soft starters (look for continuous rating ≥ 50% of LRA), size generators (starting kVA × 2 = gen rating), and plan weak-line installations where supply voltage drops during starting. All math runs client-side.
Frequently Asked Questions
What is the NEMA code letter?
NEMA MG-1 and NEC 430.7(B) specify a letter code (A through V, excluding I, O, Q) that represents locked-rotor kVA per HP. Letter A is 0–3.15 kVA/HP (lowest starting current); letter V is 22.4+ kVA/HP (highest). Most general-purpose motors fall in F (5.0–5.6) through H (6.3–7.1). The nameplate includes this letter for electricians to size starting circuits.
How long does inrush last?
Typical induction motors draw 6–10× FLA for 1–3 seconds while the rotor accelerates from zero to running speed. High-inertia loads (large fans, flywheels, conveyor drums) can take 10–30 seconds; motor overload protection uses thermal time constants to distinguish normal starts from actual overloads.
How do I size a breaker for a motor?
NEC 430.52 allows breaker ratings up to 250% of motor FLA for instantaneous-trip breakers, or 175–400% for time-delay fuses, specifically to avoid nuisance trips during starting. "D-curve" thermal-magnetic breakers (10–20× rating for 100ms) are designed specifically for motor applications.
Can I reduce inrush?
Yes — a soft starter reduces starting voltage (and therefore current) to 50–70% of nominal during startup, roughly halving the inrush but extending startup time. A VFD provides the cleanest soft-start (ramped frequency, constant V/f ratio) with inrush limited to 100–150% of FLA — no more surge than normal running current.
How does this interact with generator sizing?
A generator must handle motor starting kVA, not just running. A 20 HP motor with code letter G (5.6–6.3 kVA/HP) needs ~120 kVA during start — far exceeding its ~18 kW running load. Undersized generators sag badly and can fail to start large motors. Rule of thumb: gen kVA ≥ 2× largest motor\'s starting kVA.
Common Use Cases
Breaker Sizing
A 10 HP 3-phase motor at 480 V, code letter G (6.0 kVA/HP) draws ~72 A inrush — vs. ~14 A FLA. A 50 A thermal-magnetic breaker trips on start; a 30 A D-curve breaker allows 6-sec inrush and runs fine.
Soft-Starter Selection
A 50 HP pump starting across-the-line spikes at 8× FLA = ~500 A briefly. A soft starter ramped at 70% voltage cuts peak to ~350 A — critical for weak supply lines.
Generator Backup
A 25 kW genset cannot start a 15 HP motor (needs ~90 kVA inrush). A 45 kVA generator is required, even though the running load is only 15 HP / ~11 kW.
VFD Selection
Replacing across-the-line start with a VFD drops inrush from 8× FLA to 1.5× FLA. Wire and breaker sizing can follow FLA × 125% instead of starting-current sizing — substantial savings on large installs.
Troubleshooting Starting Problems
Motor struggles to start? Measure voltage at motor terminals during inrush. If supply sags more than 10%, the line or transformer is undersized — explains why bigger motors "trip breakers."
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