Q. A new Design E motor has been chosen to replace an existing 480V, 50-hp, 3–phase AC motor. The existing motor was fed from an existing size 3 motor starter (controller) located in an existing motor control center. When is it permissible to use the existing size 3 starter (controller), which is in very good condition, for this new Design E motor, which is also 50-hp, 480V, 3-phase?
A) when the controller is rated as Type 2.
B) only when the motor control center is 10 years or older
C) any time
D) in general, it isn't permitted at any time to use the same size starter (controller) for a Design E motor.
Explanation: This was a 1996 NEC change to Section 430-83(a)(Ex)(1). A controller for a Design E motor rated more than 2 hp shall comply with a. or b. below:
a. It shall be marked as rated for use with a Design E motor. (I am not aware of any manufacturers who manufactured Design E controllers).
b. It shall have a horsepower rating not less than 1.4 times the rating of a motor rated 3 through 100 hp, or not less than 1.3 times the rating of a motor rated over 100 hp. For this application, a standard controller may be used if it is sized as such: 1.4 X 50 hp = 70 hp rating. A 70-hp rating is consistent with a size 4 controller (480VAC) for this application, which would be the minimum size required for this application.
The same rule still applies in the 2002 NEC in Section 430.83(A)(1).
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states.