Mike Dolt Enterprises
Mike Dolt Enterprises
User: eric9822, Location: Camarillo, CA
Asked on: Tuesday, April 5, 2011 at 11:25 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:50 am EDT
Question: I have an existing installation of a 1200 HP 2300V motor. A project manager that likes to think way outside of the box would like to install a backup for this motor but avoid the expense of adding another motor starter. His proposal is to add a "switch" at the location of the motors that would allow the existing starter to feed either motor one at at a time. Section 430.87 specifies that each motor shall have its own controller with a few exceptions, none of which says you have a transfer switch downstream of the controller, I would like to kill this idea quickly. My gut instinct says that by the time I spec out an adequate transfer switch and add the proper interlocking to prevent switching while the motor is running and prevent running both motors from running I will exceed the cost of a starter. The whole thing sounds ridiculous anyway. Is 430.87 enough to kill the idea?
Yes, Section 430.87 of the National Electrical Code (NEC) can be used to effectively kill the idea of installing a transfer switch to share a single motor starter between two 1200 HP 2300V motors.
NEC Section 430.87"Each motor shall have an individual motor controller. Exception: A single controller may serve a group of motors under any of the following conditions: (1) Motors of a machine are functionally interdependent for a specific purpose. (2) Motors operate only from a common source of supply and starting of one motor does not adversely affect the operation of the other motors in the group. (3) Motors are each rated 600 volts, nominal, or less, are grouped together and are located in a single room or enclosure, and the aggregate horsepower of all motors in the group does not exceed 7-1/2 horsepower. (4) Motors are each rated not more than 100 horsepower, are grouped together in a single room or enclosure, and the aggregate horsepower of all motors in the group does not exceed 5 horsepower."Application to Your Scenario
Your scenario does not meet any of the exceptions listed in Section 430.87. The two 1200 HP motors are not functionally interdependent, do not operate from a common source of supply, and exceed the aggregate horsepower limits specified in exceptions 3 and 4.
Conclusion
Based on NEC Section 430.87, it is not permissible to install a transfer switch to share a single motor starter between the two 1200 HP 2300V motors. The proposal of the project manager is therefore not in compliance with the NEC and should be rejected. Flag for review