System bonding jumper at xo or first point disconnect?

[Jpflex]

This concept is still not clear and needs to be simplified by NEC. In Mike Holts video, someone asked him if the grounding electrode conductor can be connected directly to the X0 terminal of a separately derived system (transformer secondary). Mike Holt explains that you must either connect the grounding electrode conductor to the XO lug or connect the grounding electrode connector to the XO lug with a system bonding jumper (wire jumper).

If a system bonding jumper is to be installed at the transformer XO lug to bond the transformer secondary windings, the transformer cabinet, grounding electrode conductor and finally the supply side bonding jumpers / EGC, then this becomes the point of the SYSTEM BONDING JUMPER. Therefore, with this condition, and even with an indoor transformer, in order to avoid objectionable current and adhere to NEC code compliance, another systems bonding jumper CANNOT be installed at the first (building or structure) point of disconnect as stated in 250.30 A1.

Excluding the exemption where a grounded neutral is used to serve both the function as a grounded conductor and the supply side bonding jumper, a system bonding jumper can only be installed at one location.

In other words if it is required that the system bonding jumper be located at the separately derived transformer XO terminal, then it wouldn’t make sense for NEC mentioning system bonding jumpers to be allowed at either the source or first point of disconnect.

If a grounding electrode conductor is not required to be connected to a separately derived system’s XO lug, then do we just bond the grounding electrode conductor only to the transformer case and EGC and not the XO lug?

For an outdoor transformer, 250.30 A 1 located outside its says a SYSTEM BONDING JUMPER SHALL BE INSTALLED AT THE GROUNDING ELECTRODE CONNECTION according to 250.30 (C) Before checking 250.30 (C) it would appear that this system bonding connection can be at ANY grounding electrode connection.



However, in 250.30 (C) it says: for an outdoor transformer, the grounding electrode connection must be made at the source but the NEC does not specify whether they are referring to the transformer windings or case.

This seems to imply that the system bonding jumpers for outdoor transformers cannot be made at a remote first point of disconnect to a building or structure if the transformer is located outside???

So then my question for most practical situations is whether this is to mean that the system bonding jumper can only be installed at the first point of disconnect, when the separately derived transformer is located indoors and the transformer’s XO lug does NOT have to be connected to the grounding electrode conductor and grounding system as explained by Mike holt.

See video below

 

[Jpflex]

 

[infinity]

Almost 40 years in and I've never connected the SBJ anywhere but within the transformer. Run the GEC there too and you're done.

 

[Jpflex]

Almost 40 years in and I've never connected the SBJ anywhere but within the transformer. Run the GEC there too and you're done.

But if you didn’t go this way on a former project and instead bonded the system bonding jumper at the first breaker panel, then it would have been ok to not have not bonded the transformer XO lug to the grounding electrode conductor.

If this were ok would bonding a grounding electrode conductor to the transformer cabinet with bonded EGC suffice?

 

[augie47]

Per Code 250.30(A)(5): "Grounding Electrode conductor..........This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected. "

Picture is worth 1,000 words.......

 

[Jpflex]

Per Code 250.30(A)(5): "Grounding Electrode conductor..........This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected. "

Picture is worth 1,000 words.......
View attachment 2571000

Ok but since the system bonding jumper can be located at multiple locations, the grounding electrode also can be at different locations.

It is apparent that the system bonding conductor locations determines the grounding electrode conductor location. However, it also appears that with the transformer location being inside or out doors might determine the system bonding jumper location. This is why it appears that the system bonding jumper cannot just be randomly installed at the first point of disconnect or transformer source.

This criteria loop is not written well by NEC

 

[Jpflex]

With 250.30 A 1 and its reference to 250.32 (C) for outdoor separately derived transformers I’m pretty confident now that a system bonding jumper is required to be within the enclosure of the outside transformer rather than the first point of disconnect.

I just wish NEC would blatantly state this and stop making ambiguous code

 

[don_resqcapt19]

The key is Exception #2 to 250.30(A)(1). This rule, like the rule in 240.21(C)(4) were added to place the customer owned transformers on a campus type distribution system on the same footing as an utility transformer supplying that building.

In other words if there are no metallic paths between the transformer and the building disconnect, other than the circuit conductors, you are permitted to install a system bonding jumper at both locations. This makes the installation almost identical to a service.

If there are metallic paths, such as a metallic raceway, there will only be one system bonding jumper and that will be at the outside transformer. A supply side bonding jumper will be required between the transformer and the building disconnect, however the non-flexible metallic raceway is permitted to serve as the required supply side bonding jumper.

 

[Elect117]

I agree that the rules around transformers are too ambiguous and sporadically mentioned. Between 240.21(C), 450, and 250.30, it is one of the only utilizations equipment's to not have most, if not all, of it's requirements in the section. There is no reason why the transformer section couldn't elaborate on the grounding and bonding requirements in 450 to expand/modify section 250 rather than have all these exceptions. Not to mention, the tap requirement could just exist in 450 where the tap pertains solely to transformers.

 

[Jpflex]

In other words if there are no metallic paths between the transformer and the building disconnect, other than the circuit conductors, you are permitted to install a system bonding jumper at both locations. This makes the installation almost identical to a service.

you mean using the exemption to use the grounded conductor also as a supply side bonding jumper, where this allows two system bonding jumpers at two locations transformer and first point of disconnect only because a parallel path for ground fault current is not made, excluding ground electrodes from transformer and first point of disconnect through earth ground?

 

[Jpflex]

So once again if code says that if a separately derived transformer source is located outside then a grounding electrode connection must be made here but NEC doesn’t say that this grounding electrode connection MUST be connected to the system windings XO terminal.

Then NEC says if the transformer is located outside the system bonding jumper must be connected to the grounding electrode conductor.

HOWEVER, THERE ARE TWO GROUNDING ELECTRODE CONDUCTORS AND THEREFORE LOCATIONS - one at the source outside and one at the first point of disconnect so why doesn’t NEC plainly say which grounding electrode conductor they are referring to so we can solve where the F they want the system bonding jumper?

 

[augie47]

It does...
If an outside transformer with no metallic path to the disconnecting means a SBJ may be installed at both the transformer and the disconnect per 250.30(A)(1) Exception 2. If you do so you do not need a SSBJ between the two per 250.30(A)(2) Exception.
Grounding electrodes would be installed per 250.30(C) and ((A)(4)

It's there but, as with many Articles, it could be made clearer. If you have better wording submit a PI and perhaps they will adopt it,

 

[don_resqcapt19]

you mean using the exemption to use the grounded conductor also as a supply side bonding jumper, where this allows two system bonding jumpers at two locations transformer and first point of disconnect only because a parallel path for ground fault current is not made, excluding ground electrodes from transformer and first point of disconnect through earth ground?

That is what the exception says you can do. However I see no reason to call the grounded conductor a supply side bonding jumper. We don't do that for services and this exception is intended to make the campus distribution transformer installation be almost identical to the installation if that same transformer was owned by the utility.

 

[Jpflex]

It does...
If an outside transformer with no metallic path to the disconnecting means a SBJ may be installed at both the transformer and the disconnect per 250.30(A)(1) Exception 2. If you do so you do not need a SSBJ between the two per 250.30(A)(2) Exception.
Grounding electrodes would be installed per 250.30(C) and ((A)(4)

It's there but, as with many Articles, it could be made clearer. If you have better wording submit a PI and perhaps they will adopt it,

Yes and like I was saying, the part that throws me off is where the code mentions outdoor transformers. It says a grounding electrode conductor must be made here. Then the code says the system bonding jumper MUST be installed at the grounding electrode conductor location WHICH WAS DESCRIBED TO BE MANDATORY AT THE OUTDOOR TRANSFORMER.

clearly this is the issue and has double meanings but I'm not sure if its too late to submit a change request for wording for the next NEC cycle?

When you said a system bonding jumper may be installed at both locations where a transformer has no bonding path to a disconnect, then wouldn’t this force you to use the grounded electrode conductor as a supply side bonding jumper?

The reason I ask is because installing both a supply side bonding jumper and a grounded conductor with system bonding jumpers at both the transformer and first point of disconnect creates a code violation due to establishing a paralkel current path for objectionable current?

Also if you do not install a system bonding jumper in an outdoor transformer, how do you treat the grounding electrode conductor? Do you only install it to the transformer case? Is it ok to not connect it to XO terminal because once you connect the XO and grounding electrode conductor while the grounding electrode conductor is bonded to the transformer case you create a system bonding jumper ( and possible parallel path for objectionable current if this set up is not correct)?

 

[infinity]

Also if you do not install a system bonding jumper in an outdoor transformer, how do you treat the grounding electrode conductor? Do you only install it to the transformer case? Is it ok to not connect it to XO terminal because once you connect the XO and grounding electrode conductor while the grounding electrode conductor is bonded to the transformer case you create a system bonding jumper ( and possible parallel path for objectionable current if this set up is not correct)?

If the X0 is not bonded at the outdoor transformer how will a fault to the case clear?

 

[augie47]

when it comes to outside transformers, don's post #13 hits the nail on the head.
If it's a POCO transformer they bond XO and install a ground rod at the transformer and you have a MBJ and ground rod at the building.
We do that every day without questioning it.

The exceptions in 250.30 basically allows you to treat your SDS just like it was a POCO installation.

 

[electrofelon]

I agree that the rules around transformers are too ambiguous and sporadically mentioned. Between 240.21(C), 450, and 250.30, it is one of the only utilizations equipment's to not have most, if not all, of it's requirements in the section. There is no reason why the transformer section couldn't elaborate on the grounding and bonding requirements in 450 to expand/modify section 250 rather than have all these exceptions. Not to mention, the tap requirement could just exist in 450 where the tap pertains solely to transformers.

I don't agree. It would be confusing to have a bunch of non transformer requirements thrown into article 450. 450 is about transformers. Yes a transformer installation will have conductor over current protection requirements (240) and panelboard protection requirements (408). You need to also go to the other applicable sections.

 

[Elect117]

I don't agree. It would be confusing to have a bunch of non transformer requirements thrown into article 450. 450 is about transformers. Yes a transformer installation will have conductor over current protection requirements (240) and panelboard protection requirements (408). You need to also go to the other applicable sections.

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.

 

[don_resqcapt19]

Yes and like I was saying, the part that throws me off is where the code mentions outdoor transformers. It says a grounding electrode conductor must be made here. Then the code says the system bonding jumper MUST be installed at the grounding electrode conductor location WHICH WAS DESCRIBED TO BE MANDATORY AT THE OUTDOOR TRANSFORMER.

clearly this is the issue and has double meanings but I'm not sure if its too late to submit a change request for wording for the next NEC cycle?

When you said a system bonding jumper may be installed at both locations where a transformer has no bonding path to a disconnect, then wouldn’t this force you to use the grounded electrode conductor as a supply side bonding jumper?

The reason I ask is because installing both a supply side bonding jumper and a grounded conductor with system bonding jumpers at both the transformer and first point of disconnect creates a code violation due to establishing a paralkel current path for objectionable current?

Also if you do not install a system bonding jumper in an outdoor transformer, how do you treat the grounding electrode conductor? Do you only install it to the transformer case? Is it ok to not connect it to XO terminal because once you connect the XO and grounding electrode conductor while the grounding electrode conductor is bonded to the transformer case you create a system bonding jumper ( and possible parallel path for objectionable current if this set up is not correct)?

The next opportunity to submit Public Inputs to change the NEC will start in the around August or September of 2025 and end in September of 2026.

The code does not see current paths through the earth as objectionable current path. The very reason the exception #2 to 250.30(A)(1) says:

...For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

 

[Jpflex]

If the X0 is not bonded at the outdoor transformer how will a fault to the case clear?

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