Motor Full-Load Amps Calculator

NEC 430.6(A)(1) mandates table values because they represent worst-case, thermally-equivalent currents under standard test conditions. Nameplate FLA reflects the specific motor's design tolerances. Table values give conservative sizing that ensures conductors and protection survive real-world heating over the motor's service life. The NEC deliberately uses the higher of nameplate or table FLA for branch-circuit conductors.

Full-load amperage (FLA) is the steady-state current at rated load. Running current (actual running amps) is measured under the specific load conditions present — often 60–90% of FLA. A motor running at 60% load draws approximately 60% of FLA. Nameplate FLA equals FLA by NEMA definition.

Per NEC 430.32, motor overload protection must not exceed 125% of FLA for motors with a marked service factor (SF ≥ 1.15), or 115% for all other motors. Set the overload relay at no more than these percentages. For example, a 10hp, 480V motor with FLA = 14A and SF = 1.15: maximum overload setting = 14 × 1.25 = 17.5A.

Three-phase apparent power is S = √3 × V × I. Real power is P = √3 × V × I × PF. Rearranging for current: I = P / (√3 × V × PF). Without PF, the formula calculates apparent current, not real current. Standard motor FLA tables already embed PF and efficiency; when using the formula with explicit PF, ensure efficiency is also included or use a combined PF × η value.

Common three-phase motor voltages in North America: 208V (VFD only), 230V (older standard), 460V (most common industrial), 480V (standard modern). IEC voltages: 220V/380V, 230V/400V, 240V/415V. Always use the motor's nameplate voltage. VFD-controlled motors should use the motor's rated voltage on the nameplate, not the supply voltage.

Motors lose approximately 1% of capacity per 330m (1,000ft) above 1,000m due to reduced cooling air density. A motor rated for 14A at sea level draws slightly more current at 2,000m altitude for the same mechanical load. For conductor and protection sizing at altitude, apply the altitude correction factor from NEC Table 310.15(B)(3)(a) — typically 0.96 at 3,000m — to derate ampacities.