Generator Grounding

msu-ee

Member
Location
Columbia, MO
Occupation
Electrical Engineer
I have a complex grounding/generator question and would like to bounce this off some other experts.

I have a new two story building under construction.
The owner decided, during construction, they would like to add a generator to pick up all building loads.

The building was set up with three services, 208v 3 phase, 4 wire.
There are two services feeding the upper floor and one feeding the lower floor (a bank).

The bank has a 600A CT metered service with an MDP located just inside the building.

The upper floor has two tenants and thus two services.
One service is 200A and one is 400A, both fed from a meter center on the outside wall.

I would like to split this building up and supply it with two generators, for cost reasons.
One for the upper level and one for the lower level.

Each generator would have a breaker at the generator.
So here is the question.

Because the neutral and ground are bonded together at the meter center, I was looking as doing a separately derived generator for the upper level.
This would feed through a 250A disconnect on the lower level, splitting it into two feeds, run up to two 4 pole ATS on the upper level.
I would be grounding the generator at it’s pad and provide a grounding electrode conductor from the generator to the ATS’s.

For the lower service, the generator would be a non-separately derived with a service rated 3 pole ATS.
I would also have a grounding electrode conductor from the generator to the ATS.
This ATS would be grounded to the water line, building steel, and ground rod.

Do you see any issues with this?
Any suggestions?

thanks,
Jim
 

Peter Furrow

Member
Location
Cape canaveral Fl
Occupation
Electrical contractor
Let me get this straight you have two generators and the neutral is bonded to generator one and two? I think you would have to isolate these neutrals for generator one and generator two?
Please correct me if I’m wrong.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The neutral/ground bonding should either be:

A: At each source, switched by the T/S, and not at all after the T/S

B: Lifted in each genny, not switched by the T/S, and made in the service.

To keep the service bond in place, B is the better method, so lifted in both gennys.


Response to Peter:

As long as only one conductor from each source is bonded, the two sources' neutrals can be effectively connected together with no issue. Picture two metallic flashlights touching; what would be the issue?

However, with two EGCs and two neutrals in use and connected together at the load ends, both genny bonds should be lifted to prevent neutral current on their EGCs., so you/re correct.
 
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