Ground and Neutral - 3 Phase 480V Service

[steve.lewis@willbros.com]

The mid-point of a 480V wye connected utility transformer is solidly grounded to grounding electrode at the transformer, and the first point of disconnect is in an MCC (also grounded to grounding electrode system - the metal building housing the MCC, eufer, etc.). In this case, there is no neutral bus in the MCC - only phase bus and ground bus. There are no 277 loads supplied from the MCC. The utility transformer mid-point is grounded to provide voltage to ground stability. In this case, is it required to run a neutral conductor from the utility transformer to the MCC and where should it land since there is no neutral bus.

[raider1]

You must run a grounded conductor from the utility transformer to the service disconnecting means. (See 250.24(C))

The grounded conductor is required to provide the ground fault current path back to the utility transformer.

You would need to land the grounded conductor on the "ground bus".

Chris

[steve.lewis@willbros.com]

There would be a green "grounding" conductor ran with the un-grounded service conductors that would function as the return path to the transformer (connecting mid-point on the utility transformer to the MCC ground bus - MCC houses service disconnect). The insulation would be green rather than white. Is is still required to also (in addition to the grounding conductor) run a grounded conductor - even though there is no current expectation on the grounded conductor?

[raider1]

There would be a green "grounding" conductor ran with the un-grounded service conductors that would function as the return path to the transformer (connecting mid-point on the utility transformer to the MCC ground bus - MCC houses service disconnect). The insulation would be green rather than white. Is is still required to also (in addition to the grounding conductor) run a grounded conductor - even though there is no current expectation on the grounded conductor?

You do not run a equipment grounding conductor (green) with the ungrounded service conductors.

What you are required to run is a grounded (white) conductor with the ungrounded service entrance conductors.

Again read section 250.24(C) this section will give you the requirements for installing and sizing the grounded conductor.

On the line side of the service disconnecting means the grounded conductor is used for bonding purposes.

Chris

[steve.lewis@willbros.com]

Chris:

Thanks for the input. I am aware of 250.24 (C). I think my question was generated from not making the coorelation that the grounded conductor also serves as bonding between utility transformer and service disconnect. Thanks again.

Steve

[raider1]

You are very welcome.

By the way welcome to the forum.

Chris