Diesel Genset 3 wire vs 4 wire 1

[CuriousElectron]

Hi Friends,

I have a site that is in process of getting upgraded with addition of 300kW diesel generator. The MCC is a 480V, with no single phase loads, although the utility feed is a 4 wire with neutral wire landing on neutral bar inside the MCC. Would it be a bad idea to bring 3 phase wires and ground from genset to MCC? I'm thinking that if in the future the owner adds a 277V single phase load to the system, the standby power source would not be able to accommodate it.

Thoughts?

EE

 

[waross]

Connect the neutral.
If the gen-set is close to the panel, do not connect the neutral to the ground at the gen-set.
There shall be ONE connection between the ground and the system neutral.
If the set is far from the panel, you may have to ground the neutral at the gen-set.
When you ground the neutral at the gen-set you must switch the neutral with a late break, early make contact on a four pole transfer switch.
Switching the neutral with a late break, early make contact on a four pole transfer switch lets you ground at the gen-set and still adhere to the There shall be ONE connection between the ground and the system neutral rule.
How far is far from the panel?
It depends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CuriousElectron]

I went back and re-assessed the plant single line. The utility feed actually comes in as 3 load wires and ground. Please see attached showing a high level representation of the system. With this being the case, I'll still have 277V available for any future single phase loads.

The question now is, should I add a ground rod next to genset. The genset is about 30 ft from MCC. It seems redundant to have two separte ground rods potentially creating potential differences in the system during ground faults. I'm already running a ground wire from genset to MCC to provide low impedence ground path in case of ground fault. Will have to refer to NEC on this.

Regards,
EE

 

[waross]

With this being the case, I'll still have 277V available for any future single phase loads.

NOT and comply with the NEC.
The ground conductor must not be used as a neutral conductor.


Bill
--------------------
"Why not the best?"
Jimmy Carter