Motor Current Calculator
Enter motor hp, voltage, efficiency and power factor to estimate full-load current — with the formulas, and clear notes on when to use NEC tables instead.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Mechanical output (746 W per hp) is divided by efficiency to get electrical input power.
- Input power is then converted to current using the three-phase or single-phase formula with the entered power factor.
- The kVA drawn is reported so transformer and generator sizing can be checked.
Worked example
Interpreting the result
This is the steady full-load estimate. Real motors draw 6–8× this current during starting (locked rotor), which determines breaker type and voltage drop during start. For conductor sizing, code practice uses 125% of FLC; for overload protection, use the nameplate FLA.
Assumptions
- Nameplate hp is the mechanical output; efficiency and PF are at full load.
- Induction motor, sinusoidal supply, no VFD harmonics.
Limitations
- Not a replacement for NEC tables 430.248/430.250 — use those for code compliance.
- Starting (locked-rotor) current is not calculated; it is typically 6–8× FLC.
Nameplate FLA beats any calculation
The result here is an estimate from power, efficiency and power factor — the three nameplate values that drift the most in real motors. When a nameplate FLA is available, code practice (NEC 430.6) uses the table or nameplate value for conductor and overload sizing, not a computed one, because real motor efficiency and PF at operating load differ from the rating points. Use this calculator for planning, load schedules and generator sizing; use the nameplate for the final wire and breaker decision.
Starting current is a different problem
- A typical induction motor draws 6–8× FLA at locked rotor for the first seconds — that is what breaker instantaneaous settings and generator sizing must tolerate.
- Reduced-voltage starters, star-delta and soft starts exist to cut that inrush where the supply cannot handle it.
- Sizing a generator: check both the running kVA and the starting surge — the surge usually decides, and this calculator's companion generator-sizing tool accounts for it.
Frequently asked questions
What is FLA?
Full-load amperes: the current the motor draws at rated horsepower, voltage and frequency. It's stamped on the nameplate and used for overload protection settings.
Why do motor tables differ slightly from the calculation?
NEC tables use standardized values rounded to cover common designs, and older motors are less efficient than the 0.90–0.95 assumed here. When they differ, the table governs for code purposes.