250.30(C) Question

[Bcornell377]

Hello All,
I am located in New York and we just switched to the 2014 NEC. After watching the Webinar Mike and is panel held this past weekend on Bonding and Grounding, I have a question on 250.30(C).
1. Why is the separately derived system now needed to be bonded at the source when outside I looked in the 2014 NEC handbook but there was no no explanation on the reason. I have been racking my brain on this one thanks for the help.

 

[Smart $]

Someone had the bright idea that outdoor SDS transformers should be grounded in a similar manner as service transformers [250.24(A)(2)].

A ground rod with GEC will suffice.

PS: This requirement was added in 2011.

 

[electrofelon]

Someone had the bright idea that outdoor SDS transformers should be grounded in a similar manner as service transformers [250.24(A)(2)].

A ground rod with GEC will suffice.

PS: This requirement was added in 2011.

I guess it makes sense to align the sds and service requirements, but I don't understand 250.24 (A)(2) at all and have been meaning to ask about it for some time. Do we just rely on the utility electrode to meet this?

 

[Smart $]

I guess it makes sense to align the sds and service requirements, but I don't understand 250.24 (A)(2) at all and have been meaning to ask about it for some time. Do we just rely on the utility electrode to meet this?

On occasions where the service point is the secondary terminals of the service transformer(s), the requirement applies. Every instance where I was involved in running a service lateral from a pad mount service transformer, a ground rod and GEC were installed at the transformer (also at outdoor SDS transformers, even before the requirement to do so). I've not been involved with installing a service drop, nor running a lateral up a pole to the transformer(s).

 

[Bcornell377]

Someone had the bright idea that outdoor SDS transformers should be grounded in a similar manner as service transformers [250.24(A)(2)].

A ground rod with GEC will suffice.

PS: This requirement was added in 2011.

Okay thank you that answers my question and we jumped from the 2008 right to the 2014 here in New York so there is a lot to get caught up on thanks again for the help everyone.

 

[Jim1959]

SDS Transformer?

SDS Transformer?

What is an SDS transformer?

 

[Smart $]

What is an SDS transformer?

Separately Derived System. An electrical source, other
than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those
established by grounding and bonding connections.

Essentially all isolation transformers on the load side of the service disconnecting means.

 

[texie]

I guess it makes sense to align the sds and service requirements, but I don't understand 250.24 (A)(2) at all and have been meaning to ask about it for some time. Do we just rely on the utility electrode to meet this?

But they are not aligned. Service supplied and SDS are different in that with a service anything on the line side of the main disconnect can be bonded to the grounded conductor. With an SDS that is not the case. If you have an outside SDS would have to do the GES and system bonding jumper at the transformer and use a supply side bonding jumper.

 

[Jim1959]

SDS Transformers

SDS Transformers

OK, thanks, I guess not all transformers are, such as buck boost transformers. I have been familiar with the term, but not initialized like that.

 

[GoldDigger]

OK, thanks, I guess not all transformers are, such as buck boost transformers. I have been familiar with the term, but not initialized like that.

Technically, if you have an isolation transformer but you connect a tap on both windings to the neutral (except indirectly through the grounding system) you do not have an SDS. So in many cases you get to make the choice of whether you want to create an SDS or not.

 

[Smart $]

But they are not aligned. Service supplied and SDS are different in that with a service anything on the line side of the main disconnect can be bonded to the grounded conductor. With an SDS that is not the case. If you have an outside SDS would have to do the GES and system bonding jumper at the transformer and use a supply side bonding jumper.

Under an exception and conditions thereunder, you can install an SBJ at both transformer and disconnect and it'll be exactly like bonding a service. No SSBJ.

 

[Smart $]

Technically, if you have an isolation transformer but you connect a tap on both windings to the neutral (except indirectly through the grounding system) you do not have an SDS. So in many cases you get to make the choice of whether you want to create an SDS or not.

You are correct in there is no prohibition for wiring an transformer as non-SDS. However, on a step-down transformer, you may end up with an undersized primary neutral conductor and EGC... and it won't technically be a violation. Code simply does not cover non-SDS transformers because it assumes all will be installed as SDS's.

 

[electrofelon]

Under an exception and conditions thereunder, you can install an SBJ at both transformer and disconnect and it'll be exactly like bonding a service. No SSBJ.

Although in the case of a service, we can probably have a parallel path.....

 

[Smart $]

Although in the case of a service, we can probably have a parallel path.....

CATV shield on coax...?

 

[electrofelon]

CATV shield on coax...?

what I mean is a parallel path is specifically prohibited for an SDS, while it is not for a service. With a service one could have neutral current on a metal raceway.

 

[Smart $]

what I mean is a parallel path is specifically prohibited for an SDS, while it is not for a service. With a service one could have neutral current on a metal raceway.

True... but not often nowadays is there a solid metal raceway pathway from the main bonding jumper at the service disconnecting means to service transformer neutral-ground bond. That's why I mentioned CATV, because it typically uses the POCO MGN along the trunk and each service GES to bond the coaxial shield, thus a common parallel path, but not often thought of as one.