🔧

Motor Current Calculator

Calculate motor full load amps (FLA)

V
Result
—

Formulas

1φI = P / (V × η × PF)
3φI = P / (√3 × V × η × PF)

1 HP = 746 Watts

Typical Motor Values

Motor SizeEfficiencyPF
< 1 HP0.70-0.820.65-0.75
1-10 HP0.82-0.900.75-0.85
10-100 HP0.90-0.950.85-0.90
> 100 HP0.95-0.970.88-0.92

Related

Star-DeltakW to AmpsTransformer

Understanding Motor Full Load Current

Full Load Amps (FLA) is the current a motor draws when operating at its rated horsepower and voltage. This value is essential for sizing cables, breakers, starters, and overload protection. The motor nameplate FLA is always the most accurate reference.

Key Motor Current Values

Full Load Amps (FLA): Normal operating current at rated load. Use for wire sizing and overload protection.

Locked Rotor Amps (LRA): Starting current, typically 5-7× FLA. Use for breaker sizing and inrush calculations.

Service Factor Amps (SFA): Maximum continuous current if motor has service factor > 1.0. SFA = FLA × Service Factor.

Wire and Breaker Sizing

Per NEC requirements: Wire ampacity should be at least 125% of motor FLA. Breakers are sized at 175-250% of FLA depending on motor type. Overloads are set at 115-125% of FLA (or SFA if motor has service factor).

Frequently Asked Questions

For 3-phase motors: FLA = (HP × 746) / (√3 × V × Efficiency × PF). For single-phase: FLA = (HP × 746) / (V × Efficiency × PF). However, the motor nameplate FLA is always the most accurate source and should be used when available.

Locked Rotor Amps (LRA) is the current drawn when the motor is energized but rotor is stationary - the initial starting surge. LRA is typically 5-7 times FLA. This high inrush lasts 1-10 seconds depending on motor size and load inertia.

First convert HP to watts (HP × 746), then divide by voltage, efficiency, power factor, and phase factor (√3 for 3-phase). Example: 10 HP at 480V 3-phase with 90% efficiency and 0.85 PF = (10 × 746) / (1.732 × 480 × 0.9 × 0.85) = 11.7 amps.

Per NEC 430.22, motor branch circuit conductors must have ampacity of at least 125% of motor FLA. Check NEC Table 310.16 for wire ampacity ratings. Also calculate voltage drop for long runs - keep below 3% to prevent motor overheating.

Per NEC Table 430.52, inverse time breakers for Design B motors should be 250% of FLA. Energy-efficient motors use 175%. If that size doesn't hold, you can go up to the next standard size but not exceed 400%.

Service Factor (SF) is a multiplier indicating how much a motor can be overloaded continuously. A 10 HP motor with SF 1.15 can run continuously at 11.5 HP. Overloads can be set at SF × FLA instead of just FLA when SF > 1.0.

Motors are constant power devices - they need a certain amount of power to produce rated torque. Since P = V × I, if voltage drops, current must increase to maintain power. A 10% voltage drop can increase current by 10-15%, causing overheating.

Higher efficiency motors draw less current for the same mechanical output because less energy is lost as heat. A 95% efficient motor draws about 11% less current than an 85% efficient motor of the same HP rating.