Motor Current (FLA) Calculator
Calculate full-load amperes (FLA) for three-phase and single-phase AC motors. Sizing for conductors and protective breakers.
Motor Current (FLA) Calculator
Results
I = (HP × 746) / (√3 × V × PF × η)Understanding Motor Current (FLA)
Full Load Amperes (FLA), or Full Load Current (FLC), represents the nominal current drawn by an electric motor when it operates under its rated horsepower (HP) or kW output capacity, at rated terminal voltage.
Sizing vs. Nameplate Current
When designing circuits (sizing motor feeds, conductors, and breakers), standard electrical codes (like NEC Article 430) mandate using the **average standard values** in tables rather than the motor nameplate FLC. Nameplates are used for sizing thermal overload relays, but building wiring sizing must rely on code tables to account for potential motor replacements.
Formulas
To solve for motor current, we convert mechanical output power to electrical input power using the motor efficiency ($\eta$).
Single-Phase Motors:
Three-Phase Motors:
Step-by-Step Worked Example
Problem: Calculate the full load current of a 15 HP, 460V 3-phase induction motor with a power factor of 0.85 and an efficiency of 89%.
- Convert HP to Output Watts:
Output Watts = 15 HP × 746 = 11,190 W. - Calculate Electrical Input Watts (divide by Efficiency):
Input Watts = 11,190 W / 0.89 ≈ 12,573 Watts(or 12.57 kW). - Apply 3-Phase current formula:
I = 12,573 W / (1.732 * 460V * 0.85)I = 12,573 / 677.21 = 18.57 Amperes. - Conductor sizing load:
NEC requires sizing conductor at 125% FLC:18.57A × 1.25 = 23.21 A.
Frequently Asked Questions (FAQs)
Why does motor current spike during startup?
When a motor is turned on, the rotor is at a standstill. Without rotor motion, there is no back-electromotive force (Back-EMF) to oppose line voltage. This results in "Locked Rotor Amps" (LRA) or inrush current, which can be 5 to 8 times the normal operating FLA for a few seconds.
What is motor power factor and why does it change?
Power factor represents how much current is used to create electromagnetic fields (reactive power) versus rotating shaft work (active power). An induction motor has a very low power factor when unloaded, but improves to 0.80 - 0.88 when fully loaded.
How is efficiency determined?
Motor efficiency is the ratio of mechanical output energy to electrical input energy. Premium efficiency motors (NEMA Premium / IE3) waste less energy as heat, resulting in lower operating current and smaller electrical utility bills.