grounding and bonding multiple panel boards

[ald]

we have a existing installation. from a 480 to 120/208 3 phase transformer we have 1 m.b. panelboard and one fusible disconnect, fed seperately directly from the transformer. the in xo is not bonded in the transformer but in the panelboard the neutral and ground are bonded together.also in the disconnect the neutral is bonded to the ground. is that creating parallel path for the neutral current since it is bonded in both locations

 

[RUWired]

is that creating parallel path for the neutral current since it is bonded in both locations

ald, welcome to the forum. Although both feeders are parallel to each other, they are not creating parallel paths because both feeders are radial feeds from a single source. The current will only go back to the windings indiviualy. If there was an additional bond in the transformer, then that would create a parallel path.
Rick

 

[ald]

Rick,thank you for your reply, i understand that if xo was also bonded in the xfmer that would create a parallel path. but if you draw out my situation on paper you see that there is a physical connection between the panelboard and disconnect grounds and neutrals, that to me looks also like a parallel path for the neutral current. are you saying that the proper way to bond this is to do it in both the disonnect and panelboard? I cant find this situation addressed in any literature. thanks

 

[don_resqcapt19]

It is my opinion that in this case you are required to install the system bonding jumper at the transformer.

250.30(A)(1) System Bonding Jumper An unspliced system bonding jumper in compliance with 250.28(A) through (D) that is sized based on the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices. (2005 NEC)

 

[RUWired]

Ald, I am attaching a sketch of what you describe in your post. There is a possibility that the load of the one panel could draw from the system bonding jumper of the other disconnect. See what you think.

Rick

View attachment 2727

 

[jeremysterling]

Ald, I am attaching a sketch of what you describe in your post. There is a possibility that the load of the one panel could draw from the system bonding jumper of the other disconnect. See what you think.

The odds are very good

It is my opinion that in this case you are required to install the system bonding jumper at the transformer.

I agree. Does it also have to be tied to the building's grounding electrode system at this point?

 

[don_resqcapt19]

I agree. Does it also have to be tied to the building's grounding electrode system at this point?

Yes, see 250.30(A)(3).

(3) Grounding Electrode Conductor, Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived phase conductors and shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(7). This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.

 

[ald]

Yes the way you have drawn the circuit is the way it is! I beleive the possibilty for parallel paths exist what do you think?

 

[A/A Fuel GTX]

Yes the way you have drawn the circuit is the way it is! I beleive the possibilty for parallel paths exist what do you think?

Sure looks like the EGC would be carrying objectionable current to me. BTW...Thanks RU for drawing the diagram.

 

[RUWired]

Yes the way you have drawn the circuit is the way it is! I beleive the possibilty for parallel paths exist what do you think?

Even though the MDP and the disc will be with in the 10-25' range from the transformer (except for underslab feeds), the potential for current on the ground can be there and the difference of potential between different metal object or concete floors can exist that could lead to a shock. The best way i see it, is to follow the rule Don quoted and eliminate any potential problems.

Rick