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Wednesday, June 19, 2019
Question:
Keep up the good work. I have a new project to an old plant, 1976 construction. The power provided is 480-volt, 3 phase, open delta, B phase grounded, hopefully at the service disconnect. The NEC seems to assume that all 3 phase systems are 4 wire wye. Shouldn’t the ground be separated from the grounded phase to the transfer switch and the B phase treated as any other phase and the ground a separate wire? Should the grounded phase be connected to the line side of the motor starters and continue to the motor? Where in the code is this situation covered?
Thanks, Have a good day!!
Lowell I. (Laddie) Wolfe Jr.
A
Answer:
Hey Laddie thanks for your question and the compliment. The NEC includes rules for corner grounded systems in some places such as 250.24(C)(3). And yes, the service grounded conductor is not allowed to be re-connected to ground on the load side of the service disconnecting means as stated in 250.24(A)(5). Overload protection for motors is covered in 430.36, which includes 3 phase 3 wire systems with one wire that is grounded, and in 430.37 for other devices. In general overcurrent devices are not allowed in series with grounded conductors but they are allowed under the conditions stated in 240.22. So yes the grounded conductor in your example should be connected to the motor controller ("starter") for the motor.