250.32(B)(1) Exception #2

[Revous]

I have an existing building with a 208V service that I would like to backfeed from an outside transformer. To accommodate future mechanical loads we selected a Outdoor 480V service entrance rated switchboard and a 480V to 208V transformer to back-feed the existing 208V service. We are providing the system bonding jumpers per 250.30(A)(1) Exception #2 so that we can re-use the existing feeder and system bonding jumper at the existing switchboard.


In the future, we may bring in a feeder to a 480V switchboard for some mechanical equipment. Per 250.32 we need to connect the equipment grounding conductor to the grounding electrode system of a building served by an outside feeder. My understanding of 250.32 (B) (1) Exception #2 seems to be that we need to connect the EGC to the buildings GEC but by doing so we will create a parallel path in the grounding system. That means we won't be able to use 250.30(A)(1) Exception #2 and would need to run a system bonding jumper between the outside transformer and the existing 208V switchboard.

Is my understanding correct or have I missed something?

I created a diagram in the PDF attached of what I think the requirements of 250.32(B)(1) Exception #2 are just as a visual aid.

 

[david]

You currently have 208/120 3 ph.
You also need 480/277 3 ph

Your not wanting to remove the main bonding jumper and separate the grounded conductors from the equipment grounds for the existing 208/120.

Is there any reason you can't leave the 208/120 service and have the utility provide a 480/277 volt service for the 480 volt loads your wanting to add?

Also if your transformer was inside you would not have to have a system bonding jumper at the transformer location

 

[augie47]

I might also be overlooking something but it appears to me your diagram addresses the Sections you reference.

 

[david]

I might also be overlooking something but it appears to me your diagram addresses the Sections you reference.

The op mentioned the second metallic path from the transformer neutral through the 480 volt feeder to the building because of the required sbj at the outside transformer

 

[Revous]

You currently have 208/120 3 ph.
You also need 480/277 3 ph

Your not wanting to remove the main bonding jumper and separate the grounded conductors from the equipment grounds for the existing 208/120.

Is there any reason you can't leave the 208/120 service and have the utility provide a 480/277 volt service for the 480 volt loads your wanting to add?

Also if your transformer was inside you would not have to have a system bonding jumper at the transformer location

If I understand correctly, you mean asking the utility for a seperate 480V service and leave the 208V service alone. That could have been an option but the Utilities gives us a hard time about giving one building two services.

But thinking about this more, if we were to ask for a second service we would end up with the neutral and ground bonded at each service anyway and they would share the grounding electrode system. So from that point I don't see how it is much different from what we are doing in the diagram. Effectively the outside 480V switchgear is providing "services" as needed to the building.

 

[Revous]

I might also be overlooking something but it appears to me your diagram addresses the Sections you reference.

I think my biggest concern is the that the there is a parallel path to ground the way we are doing it and since the neutral and ground are bonded at the 480V and 208V side we are not complying with 250.30 (A) (1) Exception #2 about the parallel grounded paths.

But in the same vain, what is the difference between this system and providing the building with two services from the utility. They would be wired almost the same way.

 

[david]

You have given this alot of thought.

So you know with two services, you cannot avoid a main bonding jumper at both the 208/120 and the 480/277. So who maintains the transformers if they are utility services?

You also know with a feeder you only need a bond from the equipment grounds/ supply side bonding jumper/ grounding electrode system at the building.

As you say they are similar but not the same.

You also are aware from what you posted the neutral can be the only path other than earth to use the exception.

 

[david]

Have you considered a design having your transformer indoors?

 

[Revous]

You have given this alot of thought.

So you know with two services, you cannot avoid a main bonding jumper at both the 208/120 and the 480/277. So who maintains the transformers if they are utility services?

You also know with a feeder you only need a bond from the equipment grounds/ supply side bonding jumper/ grounding electrode system at the building.

As you say they are similar but not the same.

You also are aware from what you posted the neutral can be the only path other than earth to use the exception.

The utility would provide (2) transformers so I would assume the utility, but I am just using this as an example to show having the neutral and ground bonded when you provide (2) feeder from outside the building is the standard way we handle these situations. Also that the path to clear the faults is valid.

Its just the way 250.30 and 250.32 reference each other is strange. 250.32 acknowledges that fact that 250.30(A)(1) exists and basically says to bond the equipment ground from our outside feeder to the grounding electrode system of the building, creating a parallel ground path between the two system, which 250.30 says is not allowed. But maybe the devil is in the details because 250.30 says "...shall be permitted if doing so does not establish a parallel path for the GROUNDED conductor....". Technically only the grounds are in parallels. The neutrals just also happen to be connected by the system bonding jumpers.... but that still seems like the neutrals are in parallel to me.

250.32(B)(1) Exception No. 2: If system bonding jumpers are installed in accordance with 250.30(A)(1), Exception No. 2, the feeder grounded circuit conductor at the building or structure served shall be connected to the equipment grounding conductors, grounding electrode conductor, and the enclosure for the first disconnecting means.

TLDR; For a "will the system be able to clear a fault and the grounding system work as intended?". The way we are connecting these systems is no different than how (2) separate services from the utility would be connected (unless I'm missing something). From a code standpoint, it feels like 250.30 and 250.32 are in conflict with one another.

 

[Revous]

Have you considered a design having your transformer indoors?

Space is the limiting factor. Also backfeeding the existing service allows us to leave the existing service equipment and conductors as they are and just tap into them since we don't have to ad a ground.

 

[david]

I think my biggest concern is the that the there is a parallel path to ground the way we are doing it and since the neutral and ground are bonded at the 480V and 208V side we are not complying with 250.30 (A) (1) Exception #2 about the parallel grounded paths.

But in the same vain, what is the difference between this system and providing the building with two services from the utility. They would be wired almost the same way.

IMO
.remove the sbj @ the transformer.
.Earth just the transformer neutral @the transformer location
.bring a supply side bonding jumper from your 208 existing switchgear and bond the transformer enclosure/ eguipment ground terminal @ the transformer location

 

[Revous]

IMO
.remove the sbj @ the transformer.
.Earth just the transformer neutral @the transformer location
.bring a supply side bonding jumper from your 208 existing switchgear and bond the transformer enclosure/ eguipment ground terminal @ the transformer location

.... I actually like your idea of just not dealing with the extra system bonding jumper. Also re-reading 250.30(A)(1) it says that the two bonding jumpers shall be permitted; so that means the two bonding jumpers are not required but allowed. As for the supply side bonding jumper, I think that 250.30 (2) Exception means I don't need a supply side bonding jumper between the outdoor transformer and the existing 208V switchboard.

Exception No. 2: 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. If a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

I updated my diagram, but I think that we comply with code and the ground system makes sense.

 

[david luchini]

I think that 250.30 (2) Exception means I don't need a supply side bonding jumper between the outdoor transformer and the existing 208V switchboard.

You would need the system bonding jumpers at both locations to not have the supply side bonding jumper.

 

[david]

.... I actually like your idea of just not dealing with the extra system bonding jumper. Also re-reading 250.30(A)(1) it says that the two bonding jumpers shall be permitted; so that means the two bonding jumpers are not required but allowed. As for the supply side bonding jumper, I think that 250.30 (2) Exception means I don't need a supply side bonding jumper between the outdoor transformer and the existing 208V switchboard.

Exception No. 2: 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. If a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

I updated my diagram, but I think that we comply with code and the ground system makes sense.

Your derived neutral must bond to the transformer enclosure. That would be the ssbj coming back from the neutral @ the 208 volt switchgear.

The Ssbj is allowed to be metal conduit if that helps.

Call the bond to the grounding electrode @ the transformer a sbj @ the transformer location

 

[Revous]

You would need the system bonding jumpers at both locations to not have the supply side bonding jumper.

250.30(A)(2) says as long as I install it in compliance with 250.30(A)(1) Exception #2 then I can omit the supply side bonding jumper. Exception #2 does not require me to install (2) system bonding jumpers. I just permits me to have 2 system bonding jumpers. So I think I'm still in compliance with Exception #2

 

[david]

250.30(A)(2) says as long as I install it in compliance with 250.30(A)(1) Exception #2 then I can omit the supply side bonding jumper. Exception #2 does not require me to install (2) system bonding jumpers. I just permits me to have 2 system bonding jumpers. So I think I'm still in compliance with Exception #2

Are you sure the one not being required would be at the building if a ssbj is run from the transformer to the building separating your equipment grounding and neutrals @ the building

 

[david luchini]

250.30(A)(2) says as long as I install it in compliance with 250.30(A)(1) Exception #2 then I can omit the supply side bonding jumper. Exception #2 does not require me to install (2) system bonding jumpers. I just permits me to have 2 system bonding jumpers. So I think I'm still in compliance with Exception #2

In your revised sketch, your System Bonding Jumper is complying with the Main Text of 250.30(A)(1). The exception doesn't come into play.

You have AN unspliced system bonding jumper made at a single point (the first disconnecting means.)

 

[david luchini]

Or to put it another way.

The main section of 250.30(A)(1) REQUIRES you to have a single system bonding jumper.

The exception #2 to 250.30(A)(1) PERMITS you to have TWO system bonding jumpers, one at each end.

250.30(A)(2) REQUIRES you to have a supply side bonding jumper between the source enclosure and disconnect enclosure.

The exception to 250.30(A)(2) says the SSBJ is not required when you have installed the TWO system bonding jumpers permitted by 250.30(A)(1) exception #2.

 

[Revous]

Your derived neutral must bond to the transformer enclosure. That would be the ssbj coming back from the neutral @ the 208 volt switchgear.

The Ssbj is allowed to be metal conduit if that helps.

Call the bond to the grounding electrode @ the transformer a sbj @ the transformer location

Are you sure the one not being required would be at the building if a ssbj is run from the transformer to the building separating your equipment grounding and neutrals @ the building

What this boils down to is the two ways you can ground a regular indoor transformer. You can either have the system bonding jumper at the transformer enclosure or at the first disconnecting means. Then you would have your SSBJ connect the enclosures, but because we are an outdoor transformer feeding an indoor switchboard we can use 250.30(A)(2) Exception to omit the SSBJ.

We need a system bonding jumper at either the transformer or the disconnecting but it doesn't matter which one gets it.

 

[david luchini]

but because we are an outdoor transformer feeding an indoor switchboard we can use 250.30(A)(2) Exception to omit the SSBJ.

We need a system bonding jumper at either the transformer or the disconnecting but it doesn't matter which one gets it.

This is incorrect. You either need TWO system bonding jumpers, one at each enclosure, or you need a supply side bonding jumper between the enclosures.