Motor Inrush Current Calculator

Estimate starting (inrush) current using nameplate data, supply voltage, and start method multipliers. Includes Show Work and share links.

How to Use

  1. Select phase (single-phase or three-phase).
  2. Enter supply voltage and either motor power (HP/kW) or full-load amps (FLA).
  3. If using power input, enter efficiency and power factor (or keep defaults).
  4. Choose a start method (or enter a custom inrush multiplier).
  5. Review inrush amps, estimated kVA, and quick sizing notes. Open “Show Work” for the steps.
Start Surge View
Visual feedback: surge intensity scales with inrush multiplier and current.
FLA
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MULT
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INRUSH
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kVA
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Start Severity: —
Motor Start surge + thermal stress Inputs drive estimates (FLA → Inrush = FLA × Multiplier) Visual pulse speed scales with multiplier; heat overlay scales with estimated inrush.
Inputs & Settings
Enter supply + motor data. Use start method or custom multiplier. Results update instantly.
Three-phase uses √3 × V × I for kVA and power calculations.
Common: 120V, 240V, 208V, 480V, 600V
If you already have nameplate FLA, choose FLA mode.
Used to estimate FLA if you don’t know it.
Typical small motors: 70–90% (leave blank to use a safe default).
0.65–0.90 is common. Used only when estimating FLA from power.
Start method selects a typical multiplier range. You can override with custom.
Examples: DOL 5–8×, star-delta ~2–3×, soft starter 1.5–5×, VFD 1–2×.
Quick Adjust
6×
Show Work (step-by-step)
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Work is shown using base units. Estimates depend on motor type, load, and start method.

Reference & Formulas

Core relationship: Inrush ≈ FLA × Multiplier

  • Inrush current: I_start = I_FLA × M
  • Apparent power (1φ): kVA ≈ (V × I) / 1000
  • Apparent power (3φ): kVA ≈ (√3 × V × I) / 1000
  • Estimate FLA from power (1φ): I ≈ P_out / (V × η × PF)
  • Estimate FLA from power (3φ): I ≈ P_out / (√3 × V × η × PF)
Where η = efficiency (0–1), PF = power factor (0–1), and P_out is motor output power (W).

FAQ

Why is motor starting current so high?

At startup the rotor isn’t moving yet, so the motor behaves closer to a low impedance load. As speed rises, back-EMF increases and current drops toward normal running current.

Is inrush always 6×?

No. It varies by motor design (NEMA design, rotor type), load, and the start method. DOL is often 5–8×, while soft starters and VFDs can reduce it significantly.

Can I use this for breaker/relay sizing?

This gives a fast estimate for planning and comparisons. Final sizing should follow your applicable electrical code, motor protection tables, and equipment specifications.

Tool Info

Last updated:

Updates may include UI improvements, more motor/start presets, and edge-case handling.