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.
Last 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.
Formula
Three-phase: I = P × 746 / (√3 × V × PF × η) Single-phase: I = P × 746 / (V × PF × η)
| Symbol | Meaning | Unit |
|---|---|---|
I | Full-load current | A |
P | Mechanical output power | hp |
η | Motor efficiency | — |
PF | Power factor at full load | — |
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.
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.