Question 1

What is inrush current?

Accepted Answer

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.

Question 2

Why is inrush current a problem?

Accepted Answer

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.

Question 3

What is locked rotor current (LRC)?

Accepted Answer

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.

Question 4

How do I calculate inrush for a given motor?

Accepted Answer

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.

Question 5

What is a soft-starter?

Accepted Answer

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.

Question 6

What is a VFD (Variable Frequency Drive)?

Accepted Answer

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.

Question 7

How do I size a breaker for motor inrush?

Accepted Answer

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.

Question 8

How long does inrush current last?

Accepted Answer

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.

Question 9

What is the thermal effect of inrush?

Accepted Answer

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.

Question 10

How do I protect against voltage sag during motor start?

Accepted Answer

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.

Question 11

What's the difference between FLC, FLA, and LRC?

Accepted Answer

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.

Question 12

Can I start a large motor with a standard breaker?

Accepted Answer

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.

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

Motor Inrush Current Calculator

Typical NEMA Motor Inrush Multipliers

Soft-Starter vs. VFD vs. Direct Start