System Grounding 1

[cbark]

We are installing a new 2500 kVA, 13,800-480Y/277 V transformer (solidly grounded) that will feed a new 480 V, 3000 A lineup of switchgear (3 phase, 4 wire). The transformer is located outside the building and will be cable connected to the new switchgear inside the building via underground conduit. The secondary main breaker is in the switchgear and it is about a 50’ run from the transformer secondary to the switchgear. We are going to run 8 parallel sets of 500 kcmil cables (3 phase and 1 neutral). The question I have is about where the system should be connected to ground. Should the neutral be connected to the system ground at the transformer X0 bushing or in the switchgear? Do ground/bonding conductors need to be run from the transformer to the switchgear along with the phase and neutral conductors?

 

[7anoter4]

In my opinion, it depends on ground fault protection system. Since the neutral uses as grounding wire also you have to provide a suitable protection which includes current transformers and other equipment and these will be located in the switchgear. Then the grounding point has to be in the switchgear.

 

[dpc]

If you own the transformer (not the utility) it is a separately derived systems in NEC parlance. Review the requirement in the NEC. You should run a ground wire in addition to the neutral unless the transformer belongs to the utility.

 

[EEJaime]

cbark,
We do this all the time. It is important to establish your Grounding system for the building at the building main service point. Since you will be bonding the neutral and ground busses in the main switchboard, this is where you want to originate your Grounding Electrode Conductor to your Grounding Electrode system. The service transformer can be locally grounded, and the multiple parallel sets of service entrance conductors may be run as 4-wire sets without an equipment grounding conductor. At your main, once you establish your service ground point, you will distribute from there throughout your facility.

We have even had some AHJ's state that if you establish the ground at the transformer and run equipment grounding conductors in your secondary service feeders, you are establishing two parallel ground paths, as you are running the neutrals from the secondary of the transformer, alongside the equipment grounding conductors, which are in essence connected at the same point at the transformer.

In my opinion, keep the two separate, establish your service ground point from your main switchboard, and go from there. Ths works for an incoming utility service as well as for a multi-building campus type installation.

Regards,
EEJaime

 

[cbark]

So what you are saying is that if the system ground is established at the switchgear, I don't need the bonding conductors from the transformer to the switchgear (only the phase and neutral conductors need to be installed from transformer to switchgear)? I assumed that they would still be needed per NEC 250.30(A)(2) [the supply side bonding jumpers]; is this not the case?

 

[hdp]

Look at NEC 250.142 (A) which allows the grounded conductor (neutral) to serve as the equipment grounding conductor on the supply side of the service disconnecting means.

 

[Mbrooke]

Generally speaking, if the transformer is poco owned and the service disconnect point is at the switchgear than no EGC or bond needs to be run back to the transformer.

If the transformer is customer owned and the point is service (metering and disconnect) is before the transformer practice is to treat is as separately derived source and have a neutral to ground bond at the transformer with separate neutral and ground to the gear. No N-G bond is present at the gear.