Three-Phase Motor Full-Load Current

A motor's rating is what comes out of the shaft; the line must supply that plus the losses, and it must supply it through a phase angle. So the input volt-amperes are P_out/(η · PF), and dividing by √3 V gives the line current. A 10 hp, 460 V motor at 89% efficiency and 0.85 PF draws 7457/(1.732 × 460 × 0.85 × 0.89) ≈ 12.4 A — reassuringly close to the 14 A that NEC Table 430.250 lists for sizing.

Use the code table, not this calculation, for conductor and overload sizing in North America: the NEC deliberately tabulates conservative currents and requires them for branch-circuit design, reserving nameplate amps for overload protection. Use this formula instead when you want to know what a specific machine really draws, to sanity-check a clamp-meter reading, or to see how an unloaded motor's collapsing power factor pushes current up out of proportion to the work being done.