How do you calculate motor horsepower?

HP = Torque (lb-ft) × RPM ÷ 5,252. Or: HP = kW × 1.341. Or from electrical: HP = (V × I × Efficiency × PF) ÷ 746 (single-phase) or HP = (√3 × V × I × Eff × PF) ÷ 746 (3-phase). Example: 10 lb-ft at 1,750 RPM = 10 × 1750 ÷ 5252 = 3.33 HP.

What is motor starting current?

Starting (inrush) current is 6-8× full-load amps for standard induction motors (DOL start). A 10HP motor drawing 14A running needs 84-112A to start. This lasts 2-10 seconds. Solutions: soft starters reduce inrush to 2-4×, VFDs to 1-1.5×. Wire and breaker must handle starting current without tripping.

What is motor service factor?

Service factor (SF) is a multiplier above nameplate HP that the motor can handle continuously. SF = 1.15 (most common) means a 10HP motor can run at 11.5HP continuously. SF = 1.0 means no overload capacity. Higher SF provides margin for voltage fluctuation, high altitude, and occasional overloads. Always size for nameplate HP — use SF as safety margin, not as design capacity.

How to calculate motor torque?

Torque (lb-ft) = HP × 5,252 ÷ RPM. Torque (Nm) = kW × 9,549 ÷ RPM. Example: 5HP at 1,750 RPM = 5 × 5252 ÷ 1750 = 15.0 lb-ft = 20.3 Nm. Starting torque for standard motors is 150-300% of full-load torque. Higher starting torque needed for conveyors, crushers, and loaded starts.

How do I size wire for a motor?

Size wire for full-load amps from NEC Table 310.16 (same as circuit breaker sizing). Motor branch circuit breaker: 250% of FLA for standard breakers, 175% for inverse-time. Overload protection: 115-125% of FLA. Example: 10HP 3-phase 480V (FLA 14A): wire = 14 AWG min, breaker = 35A (250%), overloads = 16.1A (115%).

Motor Sizing Calculator — Free HP, Torque & Current Estimator | AllInOneTools

⚡ Electrical & Energy

Motor Sizing Calculator

Calculate motor HP, kW, torque, current draw, starting current, and wire size. Supports single-phase and three-phase AC motors with full NEC-compliant sizing.

Motor Parameters

Horsepower

RPM

Voltage

Efficiency

Power Factor

Service Factor

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Motor Specifications

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Electrical & Protection

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Frequently Asked Questions

How to calculate motor HP?▼

HP = Torque(lb-ft) × RPM ÷ 5,252. Or HP = kW × 1.341. From electrical: HP = (√3×V×I×Eff×PF) ÷ 746 for 3-phase.

How many amps does a motor draw?▼

3-phase: I = (HP×746) ÷ (√3×V×Eff×PF). Example: 5HP at 480V = 5.9A. Starting current is 6-8× running. Use soft starters or VFDs to reduce inrush.

How to convert HP to kW?▼

kW = HP × 0.7457. Examples: 1HP=0.746kW, 5HP=3.73kW, 10HP=7.46kW, 50HP=37.3kW, 100HP=74.6kW.

What is starting current?▼

6-8× full-load amps for DOL start (2-10 seconds). Soft starter: 2-4×. VFD: 1-1.5×. Wire and breaker must handle it. Motor branch breaker = 250% of FLA.

What is service factor?▼

SF = multiplier above nameplate HP for continuous operation. SF 1.15 = 10HP motor can run at 11.5HP. Use as safety margin, not design capacity.

How to calculate torque?▼

Torque(lb-ft) = HP × 5,252 ÷ RPM. Torque(Nm) = kW × 9,549 ÷ RPM. Example: 5HP at 1,750RPM = 15.0 lb-ft = 20.3 Nm.

How to size wire for a motor?▼

Wire for FLA per NEC 310.16. Breaker: 250% of FLA. Overload: 115-125% FLA. Example: 14A FLA → 14 AWG min wire, 35A breaker, 16.1A overloads.