Motors and the NEC

SIDEBAR 1: Starting, Running, and Locked

A motor draws significantly more current when starting than when running. It draws even more in a locked-rotor condition.

• Starting current. When voltage is first applied to the field winding of an induction motor, only the conductor resistance opposes the flow of current through the winding. Because conductor resistance is very low, the motor has a large inrush current.
• Running current. Once the rotor reaches rated speed, the starting current reduces to running current due to the counter-electromotive force (CEMF).
• Locked-rotor current (LRC). If the rotating part of the motor winding (armature) can’t rotate (for instance, due to a jam), then the winding produces no CEMF. Consequently, conductor impedance decreases until it’s effectively a short circuit. The motor operates at LRC, often six times the full-load ampere rating, depending on the motor code letter rating [430.7(B)]. The resulting overheating of the motor winding will destroy the winding if the current isn’t quickly reduced or removed.