What is motor FLA?
Full-load amps (FLA) — also called full-load current (FLC) — is the current a motor draws from the supply when running at its rated power, voltage, and speed. It’s the number stamped on the nameplate under “Amps” or “FLA” and it’s the reference point for sizing wire, overload relays, contactors, and VFDs.
How to use this calculator
Enter the motor’s rated power in kW or HP, pick the system type (three-phase is default because most industrial motors are), enter the voltage, and set efficiency (η) and power factor (PF). If you don’t know the motor’s exact efficiency and PF, hit the auto buttons to fill in typical values for a modern IE3-class induction motor of that size. The calculator returns FLA plus wire ampacity and VFD sizing guidance.
The formula
For a three-phase motor:
FLA = P / (√3 × V × PF × η)
For single-phase:
FLA = P / (V × PF × η)
For DC (no power factor):
FLA = P / (V × η)
where P is the motor’s rated shaft power (mechanical output, not input power) in watts, V is line-to-line voltage, PF is the power factor (cos φ), and η is the motor efficiency (0 to 1). The √3 factor appears because three-phase current is measured per phase but power is delivered across three phases.
Why the calculator might disagree with a code table
NEC Table 430.250 in the United States lists standard FLA values for common motor sizes. Those values are systematically about 10–15% higher than a pure formula calculation, because NEC values assume worst-case older-motor efficiency and power factor to give conductor and breaker sizing a safety margin. If you want to match a NEC table result for wire and breaker sizing, use the “+15% NEC design margin” option in the calculator. If you want the actual current a modern high-efficiency motor draws — which is what you need for VFD sizing and energy calculations — use the plain formula.
VFD sizing
A variable frequency drive (VFD) is normally sized at 100% to 125% of motor FLA. The lower end works for centrifugal loads (fans, pumps) where starting torque is low. The upper end applies to high-inertia loads or applications where you need reserve headroom for overload conditions. Always use the motor’s actual nameplate FLA when sizing a VFD, not the formula result — the nameplate reflects the manufacturer’s tested value for that specific motor design.
Starting current — the number that trips breakers
An induction motor started direct-on-line (DOL) draws about 6 times its FLA for the first few seconds, until it reaches operating speed. That’s the “locked rotor current” or “inrush.” This is why motor circuits use time-delay (“D” curve or motor-rated) breakers instead of standard ones — a 10 A motor may briefly draw 60 A during startup. Star-delta starters and soft starters reduce this, and a VFD reduces it further because a VFD ramps up voltage and frequency together.
Important limitations
This calculator assumes a modern induction motor at rated load. It doesn’t account for altitude derating, high-ambient-temperature derating, variable speed operation with a VFD (where the motor may draw more than nameplate FLA at low speeds due to reduced cooling), or specific motor topologies like permanent magnet synchronous motors that have different characteristics. For final wire and breaker sizing, always cross-check against the applicable code (NEC in the US, IEC 60034 / IEC 60364 in Europe) and consult the motor manufacturer’s data.