Motor Inrush Current Calculator
Calculate motor starting (inrush) current, breaker sizing, and soft-starter requirements for AC induction motors. Design robust motor control and protection systems.
Typical NEMA Motor Inrush Multipliers
| Motor Type | LRC (× FLC) | Typical FLC (0.75 HP) |
|---|---|---|
| Standard induction (Design B) | 6–7 | 1 A → 6.5 A inrush |
| High-slip (Design D) | 8–10 | 0.8 A → 7.5 A inrush |
| Synchronous | 3–4 | 0.5 A → 2 A inrush |
Soft-Starter vs. VFD vs. Direct Start
Direct (DOL): Inrush = 6–10× FLC. Cheapest but harsh on motors, power lines. Best for small motors, robust power systems.
Soft-Starter: Inrush = 2–4× FLC. Moderate cost, reduces mechanical stress. No speed control. Good for large motors in weak grids.
VFD: Inrush = 1.5–2× FLC. Highest cost, smooth ramp, speed control, energy savings. Best for variable-load applications.
Frequently Asked Questions
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Inrush current is the very high current that flows when a motor starts from rest. Because rotor resistance is near zero at startup, inrush is typically 6–10 times the motor's rated full-load current (FLC). This spike lasts 100–300 ms until the rotor reaches speed.
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High inrush causes voltage sag on the power line, which can trip breakers, dim lights, or harm other equipment. It also creates mechanical shock stress on the motor shaft and mechanical coupling. Inrush must be limited with starters (soft-starters, VFDs) or the power supply must be robust.
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LRC is the steady-state current that flows if the rotor is held stationary (locked). It approximates the peak inrush current. LRC is typically 5–8 × FLC for standard induction motors and is found in motor datasheets or NEMA tables.
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Inrush ≈ LRC = (HP × 746 W/HP) ÷ (√3 × V × PF × eff). For a 3-phase 5 HP 460V motor with PF 0.85 and efficiency 0.88, LRC ≈ (5×746) ÷ (√3 × 460 × 0.85 × 0.88) ≈ 8.4 × FLC. Typical NEMA tables show LRC directly.
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A soft-starter ramps up voltage to the motor over 5–30 seconds, gradually increasing current instead of a hard switch. This reduces inrush to ~2–4 × FLC and mechanical shock. Soft-starters are more economical than VFDs but offer no speed control.
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A VFD converts AC to DC, then back to variable-frequency AC. This allows motor speed control and smooth ramps. VFDs limit inrush to ~1.5–2 × FLC because acceleration is gradual. More expensive than soft-starters but offer precise control.
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Use 125% of motor FLC for standard breakers per NEC Article 430. For example, a 10 A motor requires 10 × 1.25 = 12.5 A breaker (next standard size: 15 A). Use a motor-rated breaker (Class D, adjustable trip) that tolerates inrush without nuisance tripping.
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Typical inrush duration is 100–300 ms for direct-on-line (DOL) starts. With soft-starters, inrush is spread over 5–30 seconds, reducing peak magnitude. Duration depends on motor inertia, load inertia, and starter ramp time.
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Although short-lived, inrush creates significant heat in the motor windings due to I²R losses. Even one cold-start can degrade insulation slightly. Frequent starts (>10/hour) or extreme climates require derating or thermal overload protection.
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Use a soft-starter or VFD to reduce inrush, upgrade the utility power supply (larger transformer, higher capacity), or use a local power source with lower impedance. Capacitor banks may also help stabilize voltage.
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FLC = Full Load Current (running current at rated load). FLA = Full Load Amps (synonym for FLC, used in nameplate ratings). LRC = Locked Rotor Current (inrush when rotor is stationary). LRC is typically 5–8× FLC.
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Standard breakers may trip on inrush. Use a motor-rated breaker (higher inrush tolerance) or a soft-starter/VFD to reduce inrush. Always verify breaker trip curves and motor inrush specs before connecting.