Motor Starting Current Calculator
Calculate inrush current and locked rotor amps (LRA) for motor starting.
NEMA Code Letters (NEC Table 430.7(B))
Code letters indicate locked rotor kVA per horsepower. Higher letters = higher starting current.
| Code | kVA/HP Range | Typical Application |
|---|---|---|
| A-B | 0-3.55 | High efficiency motors |
| C-E | 3.55-5.0 | Premium efficiency |
| F-G | 5.0-6.3 | Standard motors (most common) |
| H-K | 6.3-9.0 | High torque motors |
| L-P | 9.0-14.0 | Very high torque |
| R-V | 14.0-22.4+ | Special applications |
Starting Current Formulas
Locked Rotor Amps (3-Phase):
LRA = (HP x kVA/HP x 1000) / (V x 1.732)
Locked Rotor Amps (1-Phase):
LRA = (HP x kVA/HP x 1000) / V
Starting Method Comparison
| Starting Method | Current % | Torque % | Best For |
|---|---|---|---|
| Direct On Line (DOL) | 100% | 100% | Small motors, high starting torque needed |
| Star-Delta | 33% | 33% | Low torque loads, fans, pumps |
| Autotransformer 80% | 64% | 64% | Medium reduction needed |
| Autotransformer 65% | 42% | 42% | Higher reduction needed |
| Autotransformer 50% | 25% | 25% | Maximum reduction (low torque loads) |
| Soft Starter | Variable | Variable | Controlled acceleration, pumps |
| VFD | 100-150% FLA | 150%+ | Variable speed, energy savings |