System bonding jumper at xo or first point disconnect?

[Jpflex]

TL- DR - It is a shame that it doesn't get the same treatment as sections like solar.

I understand where you are coming from. Art. 450 is in "Equipment for General Use" which does not normally include more intricate installation details and remains vague so that it does not infringe on many earlier sections. But I think it does more of a disservice to the installer, designer and code enforcer.

The equipment for general use section remains indifferent to how that equipment is treated in earlier sections of the code. Where "Special Equipment" is very thorough on how those devices modify earlier sections. As an example, the exception for 250.97 as it applies to solar is not in Art.250 but rather in art. 690. That is because it applies to solar. Transformers being a "Equipment for General Use", means it does not get the same treatment.

I think this leads to more misunderstandings of the code and the way it applies to it. If instead of having the exceptions for outdoor transformers in the separately derived section of Art. 250.30, along with other specific transformer related codes (Art. 240.21(C), it would make understanding the codes easier. When the code is easier to understand, it is easier for designers, installers, and code enforcers. That ultimately makes the installation safer.

I am relatively new to this forum, but have noticed that transformers typically create confusion and disagreement even among the more seasoned professionals. The confusion is not to be solely blamed on the way the code is written, obviously, but it could be easier to interpret.

P.S. I am using NEC 2020.

Well said

 

[infinity]

If a system bonding jumper is not installed at the transformer and therefore the xo is not bonded to the outdoor transformer, then the system bonding jumper would be installed at the first point of disconnect only and this is where the fault to the case would be cleared as I’m sure we both know.

How would that clear a fault? The neutral is used for fault clearing at the transformer and at the service disconnect. There is no EGC between the transformer and the service disconnect so there would be no connection at the transformer if the neutral were unbonded.

 

[Jpflex]

How would that clear a fault? The neutral is used for fault clearing at the transformer and at the service disconnect. There is no EGC between the transformer and the service disconnect so there would be no connection at the transformer if the neutral were unbonded.

The connection for ground fault clearing would be a circuit loop from the shorted ungrounded conductor to the disconnect metal cabinet to the system bonding jumper (located only at the first point of disconnect) and back to the source secondary windings via the neutral conductor. There is no EGC but there may be a supply side bonding jumper which serves a similar purpose to the EGC

 

[tom baker]

For a small five kVA 4 80 x 1 20/240 transformer that had wire leads I would always do the system bonding jumper in the first disconnecting means. And the easy way to remember where the GEC connection goes is the GEC connection follows the SBJ.

 

[don_resqcapt19]

If a system bonding jumper is not installed at the transformer and therefore the xo is not bonded to the outdoor transformer, then the system bonding jumper would be installed at the first point of disconnect only and this is where the fault to the case would be cleared as I’m sure we both know.

However, where this system bonding jumper is required to be located is the question. I have a bonding and grounding book which specifically states that the system bonding jumper MUST be located within and remain within the out door transformer. This is what NEC implies but won’t state word for word

What about the following from 250.30(A)(1)?

... If the source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection in compliance with 250.30(C).

250.30(C) requires the grounding electrode connection at the outside transformer and 250.30(A)(1) requires a system bonding jumper at that location.
Then the exception to 250.30(1) permits a second system bonding jumper at the building disconnect.
If you choose not to install the second system bonding jumper a supply side bonding jumper from the transformer to the equipment grounding bar at the building disconnect will be required to provide a fault clearing path. If the second system bonding jumper is installed at the building disconnect, the grounded conductor will be the fault clearing path just like it is for a service supplied building.