What is Inrush Current and How Does it Affect Motor Sizing?

Sometimes called input surge current or switch-on surge, inrush current refers to the maximum, instantaneous input current drawn by an electrical device when it's first turned on. Things like incandescent light bulbs have high inrush current when first turned on and so do AC motors. In fact, they draw several times their normal full-load current when first energized. 

The heavier the duty of a motor, the more inrush current we can expect-and plan for with respect to using appropriate fuses and circuit breakers. As skilled electricians, knowing how to size motors and their circuits properly can protect equipment and personnel from damage and injury.  Blown fuses are a typical sign of breakers that are too small from improperly sized motors during start-up. 

How is an electric motor sized?  

Most electric motors will have code letter designations found on their nameplates that classify motors by the ratio of locked rotor KVA per horsepower. Electricians can refer to the National Electric Code (NEC) to find the appropriate KVA/HP range based on the code letter.

"By properly interpreting the motor nameplate information, we can use the NEC code book to size it correctly, the first time for the specific application without any problems when inrush current happens," said Mark Milne, San Diego Apprenticeship Instructor.

"Fourth year WECA Commercial Apprentice courses provide apprentices with hands-on lab instruction on how inrush current works, reading motor name and specification plates, and why it matters. This knowledge will help apprentice graduates make an install go smoothly and save time and money on the job."