Motor Starting Current Calculator
Calculate the full-load running current of a three-phase AC motor from its horsepower, voltage, and efficiency ratings. Useful for selecting circuit breakers, wire gauge, and motor starters.
About this calculator
The full-load current (FLA) of a three-phase motor is derived from its mechanical power output converted to electrical input power, then divided by the three-phase power relationship. The formula used is: I = round((HP × 746) / (√3 × V × (η / 100) × 0.85) × 10) / 10, where HP × 746 converts horsepower to watts, √3 ≈ 1.732 accounts for the three-phase power factor geometry, V is line-to-line voltage, η is motor efficiency as a decimal, and 0.85 is a typical power factor for induction motors. Inrush (starting) current is typically 6–8× the FLA for across-the-line starts; soft starters and VFDs can reduce this to 1.5–3×. The calculated FLA is the baseline for selecting a breaker rated at 250% FLA (NEC 430.52) and conductors rated at 125% FLA (NEC 430.22).
How to use
Consider a 10 HP motor running at 460 V with 92% efficiency. Step 1 — convert HP to watts: 10 × 746 = 7,460 W. Step 2 — calculate three-phase denominator: √3 × 460 × (92/100) × 0.85 = 1.732 × 460 × 0.92 × 0.85 ≈ 624.7. Step 3 — divide: 7,460 / 624.7 ≈ 11.94 A FLA. The calculator rounds this to 11.9 A. For breaker sizing: 11.9 × 2.5 ≈ 29.8 A → select a 30 A breaker. Conductor sizing: 11.9 × 1.25 ≈ 14.9 A → use #14 AWG minimum (verify with NEC tables).
Frequently asked questions
How do I calculate starting inrush current for an electric motor?
Inrush current is the large surge of current drawn when a motor first starts, before the rotor reaches operating speed. For a direct across-the-line start, inrush is typically 6–8 times the full-load amperage (FLA). If your motor has an FLA of 12 A, expect 72–96 A at startup. This is why motor branch-circuit breakers are sized at 250% of FLA — they must tolerate the inrush without tripping. Soft starters and variable frequency drives (VFDs) limit inrush to 150–300% of FLA by ramping up voltage gradually.
What size circuit breaker do I need for a three-phase motor?
NEC Table 430.52 specifies that inverse-time circuit breakers for three-phase induction motors must be rated at no more than 250% of the motor's full-load amperage. Calculate your FLA using the motor's nameplate data or this calculator, then multiply by 2.5 and round up to the next standard breaker size. For example, a 12 A FLA motor needs a breaker no larger than 30 A (12 × 2.5 = 30). Always verify against the motor nameplate and your local electrical code, as some motor types and starting methods allow higher multipliers.
Why does power factor affect motor current calculations?
Power factor (PF) represents how efficiently a motor converts apparent power (kVA) into real working power (kW). An induction motor with a PF of 0.85 draws 1/0.85 ≈ 17.6% more current from the supply than it would if PF were 1.0. This extra current does no useful mechanical work but still heats conductors and loads the utility transformer. That is why PF appears in the denominator of the FLA formula — lower PF means higher current for the same horsepower output. Power factor correction capacitors can be added at the motor terminals to reduce this penalty and lower energy costs.