Outdoor Transformer - 250.24(A)(2) and 250.30(C)

[Flapjack]

I want to get thoughts on where to connect the GEC if both the source (outdoor transformer) and first disconnecting means/service are located within a common ground grid.

For separately derived systems with an outdoor transformer, the SBJ and GEC would be connected only at the source and you would be compliant with 250.30(C).
But for a service supplied system, how can you be code compliant to have your MBJ and GEC in your service entrance and an additional GEC connection at the outdoor transformer? Wouldn't objectionable current be present because of the common ground grid?

 

[david]

The earth itself is a common grounding grid.

As long as there is a good neutral(grounded conductor) path between the service equipment and utility transformer we really do not worry over currents that may pass through the earth back to the service transformer.

exterior transformers that have the neutral bonded at the transformer should not have additional metallic paths between the exterior transformer and the service equipment you would hope to see PVC conduit if conduit is used

 

[ActionDave]

Your right Mybeard. The unavoidable objectional. It's the same thing as all the water pipes in the neighbourhood bonded.

 

[Flapjack]

The earth itself is a common grounding grid.

The code states that connection through the earth is not considered a parallel path. But if you have a ground grid (bare copper conductor, grounding rods, building steel, etc) as your grounding electrode with a GEC to both the source and service entrance, then you have a loop with the GSC, the GECs, and the grounding electrode. So what is supposed to be done in this situation?

 

[david]

The code states that connection through the earth is not considered a parallel path. But if you have a ground grid (bare copper conductor, grounding rods, building steel, etc) as your grounding electrode with a GEC to both the source and service entrance, then you have a loop with the GSC, the GECs, and the grounding electrode. So what is supposed to be done in this situation?

Who is mandating this design, are there options?

You could earth at the transformer the un-bonded neutral direct to rods

run a bonding conductor sized 250.66 from the transformer enclosure and bond to the buildings grounded conductor that bonds to the buildings grounding electrode system/ grid

If the transformer enclosure is required to be bonded directly to the grounding grid, tie a bonding jumper from the grounding grid to the transformer enclosure.

 

[Flapjack]

I believe what you are proposing is the scenario I am questioning. A GEC at the transformer without a main bonding jumper, and a GEC in the service entrance with a main bonding jumper. The grounding electrode in the ground is common for both GECs because it is a grid (very common in industrial facilities), not just one rod by a panel and a rod by a transformer separated by the earth as the code references.

 

[Flapjack]

Or are you saying that connecting a GEC in the transformer is fine as long as you don't have a main bonding jumper to the enclosure?

Smart $, are you out there?

 

[david]

Or are you saying that connecting a GEC in the transformer is fine as long as you don't have a main bonding jumper to the enclosure?

Smart $, are you out there?

I am under the 2008 NEC looking at 250.30 exception #2 for a point of reference. When the earth is the only secondary means for neutral current between the bonded neutral at the exterior transformer and the bonded neutral at the buildings service equipment it has never been consider objectionable to bond the neutral at both locations.

In your case you have a grounding grid (wire Conductor) common to both locations.

I would say it is not considered objectionable to earth an unbounded neutral at the transformer and provide a 250.66 bonding jumper from the bonded neutral at the building to the unbounded transformer enclosure to provide a fault clearing path.

In addition to the 250.66 bonding conductor you seem to have specifications that the exterior transformers enclosure must be directly bonded to the grid.

A do not see a conflict in bonding the transformer enclosure to the grid, but I would look at the bonding jumper as the primary fault clearing path for the transformer enclosure.

 

[david]

I really do not see much difference in the grounding grid being tied to a bonded earthed neutral at the transformer than if rigid metal conduit had been the wiring method between the transformer and the building. they both would be metallic paths in direct continuous contact with the earth.