Selection of short-circuit protection and control for Design E motors

The 1996 National Electrical Code (Article 430-7 (9)) introduced the Design E motor to the electrical industry. A new motor designation was necessary because the Design E motor has higher efficiency, higher locked rotor current and reduced torque requirements compared to existing NEMA motor designs. Design E motors provide increased energy savings that are important to many users, but the possible additional costs of protective and control equipment, as well as more horsepower, must be part of a cost savings calculation. The purpose of this paper is to give users of Design E motors electrical application information concerning the selection of the appropriate motor disconnecting means, motor branch-circuit short-circuit protection, motor circuit conductor, motor controller and motor overload protection, and guidance concerning torque characteristics. This paper describes how motor efficiency is increased and how the design changes related to increased efficiency affect the mechanical and electrical characteristics of motors. This paper reviews the requirements of Article 430 of the 1996 National Electrical Code and provides generic application tables that compare the protection and control requirements of Design E motors with those of Designs B, C, and D.