Electrical

Motor Sizing Calculator

Select motor HP/kW based on torque and speed requirements.

Input Parameters

Units:

Calculation Mode

Required Torque

lb-ft

Operating Speed

RPM

Supply Voltage

Phase

Motor Efficiency

Power Factor

Results

Enter motor parameters and click Calculate

What is Motor Sizing?

Motor sizing determines the correct power rating, speed, and electrical characteristics needed to drive a mechanical load while accounting for efficiency and operating conditions.

Proper sizing ensures the motor can handle the required torque at operating speed without overheating, while not being oversized and wasting energy.

How to Use

Select calculation mode (power from torque or torque from power)
Enter torque requirement or power rating
Enter operating speed in RPM
Select supply voltage and phase
Enter expected efficiency and power factor
Click Calculate for motor requirements

FAQs

Power = Torque × Speed. In imperial units: HP = (lb-ft × RPM) / 5252. In metric: kW = (N-m × RPM) / 9549. To get more torque at the same power, reduce speed (use gearbox). Motors produce constant torque up to base speed.

Synchronous speed depends on poles and frequency: RPM = (120 × freq) / poles. At 60 Hz: 2-pole = 3600 RPM, 4-pole = 1800 RPM, 6-pole = 1200 RPM. Actual speed is slightly less (slip). More poles = more torque but lower speed.

Service factor (SF) is the overload capacity built into a motor. SF of 1.15 means the motor can continuously deliver 115% of rated power. Use this margin for occasional overloads, high ambient temperatures, or altitude derating - not as additional capacity.

Branch circuit protection (breaker/fuses): 175-250% of FLA depending on motor type. Overload protection: 115-125% of FLA (or up to SF × FLA). Conductor sizing: 125% of FLA minimum. Always verify with NEC tables and local codes.

Variable Frequency Drives are beneficial when: speed control is needed, soft starting reduces mechanical stress, energy savings on variable loads (fans, pumps), or precise speed/position control is required. They also reduce starting current and can improve power factor.

Limitations

• Current values are estimates - verify with motor nameplate
• Does not account for altitude or temperature derating
• Starting torque requirements not considered
• Actual efficiency varies with load and motor design
• Consult NEC and motor catalogs for final specification