How do I select the right NEMA motor size?

Calculate the required shaft power (brake horsepower) of the driven equipment, apply a service factor (typically 1.15 for normal service), then select the next standard NEMA motor size that meets or exceeds the required power. Standard NEMA sizes are 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 350, 400, 450, and 500 HP.

What is the difference between TEFC and ODP motor enclosures?

TEFC (Totally Enclosed Fan Cooled) motors are sealed against dust and moisture, cooled by an external fan. ODP (Open Drip Proof) motors allow air circulation through the windings but prevent liquid drops from entering at angles up to 15 degrees. TEFC is preferred for outdoor, dusty, or wet environments. ODP is suitable for clean indoor locations and typically costs less.

How do I calculate Full Load Amps (FLA) for a three-phase motor?

For three-phase motors, FLA = (HP x 746) / (sqrt(3) x Voltage x Efficiency x Power Factor). For example, a 50 HP motor at 460V with 93% efficiency and 0.85 power factor: FLA = (50 x 746) / (1.732 x 460 x 0.93 x 0.85) = 59.6 amps.

When should I derate a motor for altitude or temperature?

Motors should be derated when operating above 3,300 feet (1,000 meters) elevation or above 104 degrees F (40 degrees C) ambient temperature per NEMA MG-1. Above 3,300 ft, derate approximately 1% per 330 ft of additional elevation. For high ambient temperatures, NEMA provides derating factors that reduce the allowable continuous load.

Electric Motor Sizing Calculator | NEMA MG-1, NEC 430, IEEE 841

Driven Equipment

Equipment Type

Required Shaft Power

Brake horsepower of driven equipment

Service Factor

1.0 = unity, 1.15 = NEMA standard, 1.25 = heavy duty

Motor Specifications

Motor Speed

Voltage

Frequency

Enclosure Type

Efficiency & Power

Efficiency Class

Power Factor

Operating Environment

Ambient Temperature

°F

NEMA standard: 104°F (40°C). Derate above this.

Site Altitude

NEMA standard: 3,300 ft (1,000 m). Derate above this.

Energy Cost Analysis

Annual Operating Hours

hrs/yr

Average Load Factor

Fraction of rated load (0.1-1.0)

Electricity Rate

$/kWh

Understanding Electric Motor Sizing

NEMA Standard Sizes
Motors are manufactured in standard HP ratings: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 350, 400, 450, 500 HP. Always select the next size above the calculated requirement.

Key Electrical Parameters:
FLA: Full Load Amps at rated HP
LRA: Locked Rotor Amps (6-8x FLA)
Service Factor: Continuous overload capacity
Efficiency: Shaft output / electrical input

Derating Considerations:
Above 3,300 ft elevation or 104°F ambient temperature, motor output must be derated per NEMA MG-1. Wire sizing follows NEC Article 430 based on FLA with 125% continuous duty factor.

📚 Learn the Theory

Understand NEMA motor standards, efficiency classes, derating factors, and motor selection best practices

Read Engineering Guide →

Results

Selected Motor

HP (NEMA Standard)

NEMA Frame Size -

Full Load Amps (FLA) -

Locked Rotor Amps (LRA) -

Motor Efficiency -

Input Power -

Starting Torque -

Altitude Derating -

Temperature Derating -

Recommended Wire Size -

Recommended Breaker -

Estimated Weight -

Annual Energy Use -

Annual Energy Cost -

Formula

FLA = (HP × 746) / (√3 × V × η × PF)

FLA = Full Load Amps (A)

HP = Motor horsepower

V = Line voltage (V)

η = Motor efficiency (decimal)

PF = Power factor (decimal)

746 = W per HP conversion

Standards & References

NEMA MG-1
Motors and Generators - Standard frame sizes, efficiency tables, derating
NEC Article 430
Motors, Motor Circuits, Controllers - Wire and breaker sizing
IEEE 841
Severe Duty Motors for Petroleum and Chemical Industry
NEMA Premium
Energy efficiency program for motors 1-500 HP
DOE 10 CFR 431
Federal motor efficiency standards

Engineering Notes

Service Factor: NEMA standard 1.15 allows 15% continuous overload
Altitude: Above 3,300 ft, derate 1% per 330 ft additional elevation
Temperature: Above 104°F (40°C) ambient, apply NEMA derating
Wire Sizing: NEC 430.22 requires 125% of FLA for continuous duty
Breaker: NEC 430.52 allows up to 250% FLA for inverse-time breakers
Explosion Proof: Class I motors per NEC 500/505 for hazardous areas

Quick Reference — Typical FLA (460V)

10 HP, 460V → FLA ≈ 14 A
25 HP, 460V → FLA ≈ 34 A
50 HP, 460V → FLA ≈ 65 A
100 HP, 460V → FLA ≈ 124 A
200 HP, 460V → FLA ≈ 240 A

Electric Motor Sizing Calculator