Motor Full-Load & Starting Current Calculator

Motor Current

Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.

Methodology

Full-load current is calculated from rated power, voltage, power factor and efficiency. Starting current is estimated by applying a typical multiplier to FLC based on the starting method — these multipliers are approximate industry-typical values, not manufacturer-specific locked-rotor data. Star-delta starting reduces line current to approximately one-third of direct-on-line inrush because the winding voltage is reduced to $1/\sqrt{3}$ of line voltage during the star connection.

Worked Example

A 15kW, 415V three-phase motor with PF 0.85 and 92% efficiency: $I_{FLC} = \frac{15 \times 1000}{\sqrt{3}\times415\times0.85\times0.92} = 26.69\ \mathrm{A}$. On DOL starting at 6× FLC, starting current ≈ 160.1A. On star-delta starting at 2× FLC, starting current ≈ 54A.

Assumptions & Limitations

• Starting multipliers are typical approximations — actual locked-rotor current varies by motor design class (NEMA/IEC design letter)
• Always verify against motor nameplate locked-rotor current (LRC) or manufacturer datasheet where available
• Does not model starting torque or acceleration time
• Protection sizing shown is indicative only — a full coordination study is required for final design