Motor Full-Load Current Calculator

Find the full-load current of a three-phase motor from the NEC table, or estimate line current from output power, efficiency and power factor.

NEC table lookup (three-phase induction motors)

Estimate current from power

Check the nameplate. The NEC table gives standard values used for sizing conductors and short-circuit and ground-fault protection. Real motors often draw less. Overload protection is set from the motor nameplate current, not from the table. Table values are entered from NEC Table 430.250; verify them against the edition your jurisdiction uses.

How it works

The NEC table lists a typical full-load current for each horsepower and voltage, so designers get the same answer for any motor of that size. The 125% figure is the minimum conductor ampacity for a single continuous-duty motor under NEC 430.22. The power estimate works back from output power: the motor draws more input power than it delivers because of losses (efficiency), and the current rises when power factor is low.

Three-phase: I = P × 1000 / (√3 × V × efficiency × PF)
Single-phase: I = P × 1000 / (V × efficiency × PF)
P in kW, efficiency and PF as decimals

Worked example

A 10 hp, 460 V three-phase motor has a table full-load current of 14 A. The minimum conductor ampacity is 14 × 1.25 = 17.5 A.

A 7.5 kW motor at 460 V, 90% efficiency and 0.85 power factor draws 7500 / 0.90 / (1.732 × 460 × 0.85) = 12.3 A.

Common questions

Why is the table current higher than my motor's nameplate?

The table is conservative on purpose. Modern high-efficiency motors usually draw less current than the table value.

Is this valid for a VFD-fed motor?

The table still applies for sizing conductors, but drive input current and the motor's nameplate are what matter for the drive and its protection. Follow the drive manufacturer's guidance.

Calculations are provided for convenience and education. Verify results against the applicable code edition and manufacturer data before relying on them.