When an AC induction motor starts, it briefly draws a large inrush current — the locked-rotor amperes (LRA) — that sizes standby generators, protective devices, and soft-starter or VFD equipment. This calculator uses the NEMA code letter from NEC Table 430.7(B) together with motor horsepower and voltage to estimate the starting kVA and locked-rotor current for both single-phase and three-phase motors.
What happens electrically at motor start
At the instant of starting, the rotor is stationary. From the stator’s perspective, this looks like a transformer with a shorted secondary — impedance is very low and current is very high. A typical AC induction motor draws 5–8× its full-load current at start. As the rotor accelerates and slip decreases, current drops rapidly. The entire starting transient typically lasts 1–10 seconds depending on load inertia. During that window, the supply voltage dips, which is why large motors sometimes cause lights to flicker when they start.
NEMA code letters: how the system works
Each motor is stamped with a code letter (A through V) that identifies the range of its locked-rotor kVA per horsepower according to NEC Table 430.7(B). Common code letters and their typical ranges:
| Code letter | kVA/HP range | Typical application |
|---|---|---|
| B | 3.15–3.54 | Energy-efficient designs |
| D | 4.0–4.49 | Standard NEMA design D |
| F | 5.0–5.59 | General purpose |
| G | 5.6–6.29 | General purpose (common) |
| H | 6.3–7.09 | Higher inrush |
| J | 7.1–7.99 | Compressors, hard-start loads |
Higher letters (further in the alphabet) mean higher inrush. If the nameplate LRA is printed directly, use that value — it is more accurate than the code-letter estimate.
How it works
The code letter gives a band; the tool uses the midpoint to estimate and shows the full range:
starting kVA = HP × kVA-per-HP (midpoint of code-letter band)
LRA (1-phase) = starting kVA × 1000 / V
LRA (3-phase) = starting kVA × 1000 / (√3 × V)The √3 factor (1.732) for three-phase reflects that three-phase power is √3 × V_line × I_line, so solving for current divides by both √3 and V.
Worked example: 10 HP, 460 V, 3-phase, code G
- Code G midpoint:
(5.6 + 6.29) / 2 = 5.945kVA/HP - Starting kVA:
10 × 5.945 = 59.45 kVA - LRA:
59,450 / (1.732 × 460) = 74.5 A - At the high end of code G:
10 × 6.29 / (1.732 × 460) ≈ 78.8 A
For generator sizing, use the high-end estimate to avoid undersizing. For breaker instantaneous trip setting, use the nameplate LRA if available; otherwise use the high end of the code-letter band with additional margin.
How starting equipment reduces this inrush
Across-the-line starting applies full voltage from the start, drawing full LRA. It is simple and fast but stresses the supply and connected equipment.
Soft starters ramp the applied voltage up gradually over a programmable time period (typically 3–10 seconds), reducing peak inrush to roughly 3–4× FLC instead of 6–8×. They also reduce mechanical stress on couplings and driven loads.
Variable frequency drives (VFDs) control frequency and voltage simultaneously, allowing the motor to start at essentially rated current. They provide the smoothest start and highest energy efficiency but are the most expensive option and add complexity. VFDs are standard on large pumps, fans, and compressors where energy savings during partial-load running justify the cost.
Use this calculator to understand the starting-current challenge; the severity often determines which starting method is appropriate.