250.30(A)(2) Supply side bonding jumper

[jap]

To me in figure 5 you would need to extend the equipment grounding conductor to the disconnect and remove the neutral to case bond and seperate the neutral and ground in the disconnect.

The way figure 5 is drawn now the neutral return current would be traveling over the noncurrent carrying parts since they are bonded together. Worse yet, if you lost the neutral between the transformer and the disconnect all of the return neutral current would be resting on the noncurrent carrying parts just waiting on path back to its source.


I've already probably said more than I know.

 

[ActionDave]

I understand that only one SBJ (One MBJ in a service-supplied system also) in a SDS according to the code. But I am trying to learn why. The reason for only one SBJ is to prevent a parallel fault current path. However, the figure 5 has no parallel current path either. So I am trying to learn why I cannot make a SDS like figure 5 when doing so does not create a parallel fault current path.

The reason for only one SBJ is to prevent a parallel current path under normal conditions.

Your figure 5 is the essentially the same set up as a remote meter pedestal feeding a service disconnect on a building. The one distinction between the two is the fact that if a MBJ is used the power company owns the transformer. If a SBJ is used the transformer is owned by someone else.

 

[unsaint34]

The reason for only one SBJ is to prevent a parallel current path under normal conditions.

You are right. I made a mistake by saying "parallel fault current path."

Your figure 5 is the essentially the same set up as a remote meter pedestal feeding a service disconnect on a building. The one distinction between the two is the fact that if a MBJ is used the power company owns the transformer. If a SBJ is used the transformer is owned by someone else.

My figure 5 is also the same setup of an outside transformer where the service point is on the secondary side. I am attaching a photo from a Stallcup illustration book-see the figure C "correct method". Disregard the figure A&B.

I think you are agreeing that my figure 5 will work, meaning the set up will clear the X2 fault at the transformer. So, again, if the figure 5 works, in other words, if it works without the SSBJ (supply side bonding jumper), why does NEC require it for a SDS, but not in a service-supplied system? Who owns the transformer changes how to ground the transformer?!!?

 

[augie47]

Take note to Exception 2 in 250.30(A)(1).
In many installations where the SDS transformer is outside and the conduits are PVC, this Exception essentially allows the SDS system to mimic a utility system

(That said, the '11 Code added 250.30(A)(2) which, to me, seems to conflict with the Exception)

 

[unsaint34]

Take note to Exception 2 in 250.30(A)(1).
In many installations where the SDS transformer is outside and the conduits are PVC, this Exception essentially allows the SDS system to mimic a utility system

(That said, the '11 Code added 250.30(A)(2) which, to me, seems to conflict with the Exception)

Right, that very conflict you mentioned was another reason for my original question. SSBJ is required PER 250.30(A)(2). Making two SBJs is allowed per Exception 2 in 250.30(A)(1). However, those two cannot simultaneously happen without creating the parallel current path.

So, ignore the exception and just run the SSBJ? Yes, that works. But again, I am trying to UNDERSTAND the reason NEC requires SSBJ in a SDS, when following the Exception 2 in 250.30(A)(1) without using SSBJ will work.

 

[roger]

However, those two cannot simultaneously happen without creating the parallel current path.

Sure it can and if you can be sure there will never be anything that will change it you can use the exception.

Otherwise you bond at one point.



Roger

 

[Smart $]

Right, that very conflict you mentioned was another reason for my original question. SSBJ is required PER 250.30(A)(2). Making two SBJs is allowed per Exception 2 in 250.30(A)(1). However, those two cannot simultaneously happen without creating the parallel current path.

So, ignore the exception and just run the SSBJ? Yes, that works. But again, I am trying to UNDERSTAND the reason NEC requires SSBJ in a SDS, when following the Exception 2 in 250.30(A)(1) without using SSBJ will work.

Because the CMP's don't always think these requirements through thoroughly before implementing... :happyyes:

FWIW, you can use 250.30(A)(1) Exception No. 2 and "ignore" 250.30(A)(2) under permission of 250.6... but the AHJ may not see it that way.

 

[Smart $]

Sure it can ...

No they can't. Dual SBJ and SSBJ creates a parallel neutral current pathway.

 

[roger]

No they can't. Dual SBJ and SSBJ creates a parallel neutral current pathway.

That's not what was said, what was said was "Making two SBJs is allowed per Exception 2 in 250.30(A)(1). However, those two cannot simultaneously happen without creating the parallel current path."


Roger

 

[Smart $]

That's not what was said, what was said was "Making two SBJs is allowed per Exception 2 in 250.30(A)(1). However, those two cannot simultaneously happen without creating the parallel current path."


Roger

I see... but I believe he meant implementing both 250.30(A)(1) Exception no. 2 AND 250.30(A)(2).

 

[roger]

I see... but I believe he meant implementing both 250.30(A)(1) Exception no. 2 AND 250.30(A)(2).

And I think you may be right.

Roger

 

[unsaint34]

I see... but I believe he meant implementing both 250.30(A)(1) Exception no. 2 AND 250.30(A)(2).

Yes that is what I meant; implementing both 250.30(A)(1) exception2 & 250.30(A)(2). Sorry about the confusion. I should have said "those two rules..."