No grounding electrode available

[bsmel603]

I have a floating dock that is attached to a fiberglass bulkhead via a hinged gangway. The dock and gangway is made of aluminum and decked with trex (a plastic composite decking). I am feeding a 200KVA 480:120/240V 1-PH substation with a parallel 3/0 feed (upsized to 4/0 for voltage drop. Does 250.102C "equipment bonding jumper on the supply side of the service", provide for adequate grounding requirements for the substation feed?

 

[Volta]

Bsmel, welcome to the forum!

I think that the answer is yes and no.

Is this a service? My guess is no, and that the paralleled 4/0 are feeders, and if so, the supply side of service would not apply.

If this is a feeder, the Equipment Grounding Conductor (upsized per 250.122(B)) would 'provide adequate grounding requirements for the substation feed', and that would serve the primary side, but the newly derived system on the secondary could not rely on it. The low voltage system would need an electrode system.

 

[hardworkingstiff]

The low voltage system would need an electrode system.

No argument on the requirement, but what good does it really do? Going to take a GEC back to shore?

 

[bsmel603]

This is not a service. If I run a ground back to the shore (apx 50lf) & bond the gangway and set a ground rod or a plate electrode, would this be a path that would improve on the equipment ground that goes back to the shorepower disconnect (apx 100lf from gangway) & is already bonded at the substation?

 

[bsmel603]

Since the primary & secondary of the substation are bonded at the transformer, how would an electrode system provide a better path than the equipment ground path with the primary feeds?

 

[Volta]

No argument on the requirement, but what good does it really do? Going to take a GEC back to shore?

I think so, though I'd like to see the system bonded to the water too, to reduce any stray voltages as much as possible. I guess it is time for me to finally browse Article 555 a little.

. . .

Hmmm, seems that the transformer needs to be located at least 12" above the hinged dock, 30" above the water, and specifically approved for the location, 555.5.

Can't be a service on the floating dock, 555.7.

But not much guidance for derived systems, so back to 250.30, 250.32, and the like.

 

[shepelec]

For the sake of 50 feet, I'd put the transformer on shore and keep the install simple.

 

[Volta]

Since the primary & secondary of the substation are bonded at the transformer, how would an electrode system provide a better path than the equipment ground path with the primary feeds?

Not so much that it is a better path, but that they are serving different purposes.

The feeder's EGC is to help operate the OCPD at the source of the 480 volt circuit. The local electrode system should reduce voltages from the neutral to the ground (or water).