Motor Full-Load Current Calculator

How to use this calculator

1. Choose single-phase or three-phase.
2. Enter the rated power (HP or kW).
3. Enter the voltage, efficiency (%) and power factor from the nameplate.
4. Click Calculate for the full-load current plus the 125% conductor-sizing value.

Full-load current formula

Because a motor has losses (efficiency η) and reactive draw (power factor PF), the input current at rated output is — three-phase: I = (HP × 746) / (√3 × V × η × PF); single-phase: I = (HP × 746) / (V × η × PF). For a kW rating, replace HP × 746 with kW × 1000. One horsepower equals 746 W of mechanical output and η is entered as a fraction (90% → 0.90).

Quick reference: HP to full-load amps (3Ø, 400 V)

Estimated at η 0.9 and PF 0.85 — confirm against the motor nameplate:

Why the 125% value matters

For a single continuous-duty motor, the NEC (Article 430) requires branch-circuit conductors to be rated at least 125% of the full-load current. For final NEC sizing, code generally requires the table values in 430.248/430.250 rather than a calculated figure — use this result for estimating and confirm against the table and nameplate.

Frequently asked questions

How many amps does a 10 HP three-phase motor draw?

At 400 V with efficiency 0.9 and PF 0.85, about 14 A. Always confirm against the motor's nameplate full-load current.

Why size conductors at 125%?

The NEC requires branch-circuit conductors and overload protection for a continuous-duty motor to be rated at least 125% of the full-load current.

Should I use the calculated value or the NEC table?

Use this calculation for estimating. For code compliance, the NEC generally requires the table values in 430.248 (single-phase) or 430.250 (three-phase).