NEC 250.30(A)(1) Exception No. 2

[Codified]

2017 NEC 250.30(A)(1) Exception No. 2 includes the words "If a building or structure is supplied by a feeder from an outdoor separately derived system, a system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor". What would be a good example where no "parallel path for the grounded conductor" is established by having "a system bonding jumper at both the source and the first disconnecting means"?

 

[electrofelon]

2017 NEC 250.30(A)(1) Exception No. 2 includes the words "If a building or structure is supplied by a feeder from an outdoor separately derived system, a system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor". What would be a good example where no "parallel path for the grounded conductor" is established by having "a system bonding jumper at both the source and the first disconnecting means"?

Its essentially the same scenario of a feeder to a detached structure where we used to be able to bond the EGC to the grounded conductor at the structure. If you had a parallel path in either case, such as a metal water pipe, metal conduit, communication line, etc, than neutral current will also take that unintended path which is bad.

 

[Codified]

Yes - but I'm asking for an example of when one could legitimately use the stated exception.

 

[david luchini]

Yes - but I'm asking for an example of when one could legitimately use the stated exception.

What comes to my mind is a campus type medium voltage loop. A padmount transformer is supplied on the primary from the MV loop, the secondary is a(n outdoor) separately derived system which supplies a building.

 

[electrofelon]

Yes - but I'm asking for an example of when one could legitimately use the stated exception.

I would say one could use the exception most of the time. How often is there a metallic path between an outdoor transformer and a building? Of course you would have to have a customer owned outside transformer and likely a MV primary, so probably a situation like David mentioned is where you would see it.

 

[Codified]

How often is there a metallic path between an outdoor transformer and a building?

Let's say (1) if a transformer's 480Y/277V secondary conductors to the building are enclosed in metallic raceways; and/or (2) if the grounding electrode system is bonded to the X-O of this transformer's secondary as well as the ground bar at the building's service panel.

Wouldn't either of these situations prevent the use of NEC 250.30(A)(1) Exception No. 2?

 

[electrofelon]

Let's say (1) if a transformer's 480Y/277V secondary conductors to the building are enclosed in metallic raceways; and/or (2) if the grounding electrode system is bonded to the X-O of this transformer's secondary as well as the ground bar at the building's service panel.

Wouldn't either of these situations prevent the use of NEC 250.30(A)(1) Exception No. 2?

1. Maybe. Typically the conduits just stub into a vault under the transformer and do not connect to any metal enclosure, thus I see no path. Now if they are bonded with jumpers to the transformer then yes of course that would be a deal-breaker. Also, in general I don't know about you but I hardly ever see rigid conduit in this manner, usually PVC

2. That would certainly be a deal-breaker, but seems like an unlikely scenario, but yes possible.

 

[jim dungar]

The potential to use this exists at most of the guard shack/gate houses I have seen in the past decade, 480V underground direct buried feeder to a small dry type transformer which is then goes to an underground non-feed to the building which contains a main breaker panel.

 

[LarryFine]

Yes - but I'm asking for an example of when one could legitimately use the stated exception.

When you drive rods at the generator and the building, each connected to the neutral, but only earth between them.

Earth alone is not considered to be an interconnection, and thus is not a parallel pathway for neutral current.