SDS Grounding

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dereckbc

Moderator
Staff member
Location
Plano, TX
I want to start discussion concerning proper SDS grounding, specifically transformers. I hear a lot of different opinions from PE?s, electrical engineers, inspectors, and electricians about the proper terminating point for the GEC in step-down/isolation transformers.

The differences of opinion arise in the interpretation of 250.30 (A)(2) where it says except as permitted in 250.24(A)(4). Some say you can land the GEC on either the Xo terminal or ground bus in the transformer, or at the neutral or ground bus in the first disconnect.

I will stick my neck out first and say my interpretation is the GEC should be landed on the Xo terminal at the transformer, or on the neutral buss in the first disconnect. If the transformer is part of the ?service equipment? then the exception of 250.24(A)(4) would permit the GEC to terminate to either the Xo or ground bus provided the MBJ is a wire or bus-bar and is installed from the neutral bus to the equipment grounding terminal.

Don?t be shy. I am not looking to prove anybody right or wrong, just wanting to know how you interpret the code.
 

tom baker

First Chief Moderator
Staff member
Re: SDS Grounding

The first thing I would do is to call Mike Holts office and get his: Grounding and Bonding text and video. The text has got great graphics and you can take on the job with you to show others.

Is this transformer a service? Or a SDS downstream of a service?
 

kevin

Member
Location
Post Falls, ID
Re: SDS Grounding

Section 250.30(A)(2) requires that the GEC for an SDS be connected to the grounded conductor of the SDS AT THE SAME POINT where the EGC's are connected to the grounded conductor by the bonding jumper required by Section 230.30(A)(1).

The bonding jumper may be connected at the source (X0 to transformer case) OR at the downstream disconnect (neutral bus to enclosure) OR at any point between. The Code permits the installer to use discretion to meet these requirements. If there is a preference voiced by anyone, it is merely an opinion and nothing more. The Code itself is mute as far as preferences are concerned.

My own opinion is that it is probably easier to make the point of connection in the downstream disconnect in most cases, since they are generally always furnished with the bonding jumper (green bonding screw) AND with sufficient terminals to accomodate all the various conductors. Many, if not most, transformers would require the purchase of additional lugs, etc.
 

rb

Member
Location
Tennessee
Re: SDS Grounding

Don,

Check out the exception following 250.30(a)(4). I think this is intended to apply where equipment suitable for use as service equipment also contains a transformer and distribution section at a voltage other than the service voltage, such as a minipower zone.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

Kevin,
I don't read 250.30(A)(2) the way that you do. The GEC connection is to be made at the same point as the bonding jumper is made at, not where the EGCs are connected. This may not tbe the same as where the EGCs are connnected. If the GEC is connected to the grounded conductor at the transformer, then a bonding jumper is run from the transformer to the panel and connected to the EGCs at the panel.

RB,
Just because equipment is suitable for use as service equipment doesn't mean that it is service equipment. My point it that I don't think a transformer that is covered by the rules in the NEC is ever service equipment. It is a SDS. Transformers that are utility owned are not covered by the NEC rules.
The intent of the exception that you cited, is to permit the use of the service grounding electode as the SDS grounding electrode when the SDS feeder OCPD is in the service equipment.
Don
 

kevin

Member
Location
Post Falls, ID
Re: SDS Grounding

Don,
I don't believe we really have a disagreement. We both concur that the GEC's are required to connect at the same location where the BJ is installed. The purpose of the BJ, per the first sentence in 250.30(A)(1), is to connect the EGC's to the grounded conductor. If the BJ is installed at X0 then an EGC is required to connect the transformer enclosure to the downstream disconnect enclosure, and it can consist of either a metal raceway or a wiring conductor. Either way, it seems to me that the EGC's at the downstream disconnect are connected to the BJ at X0 by the required EGC between the two points. I did not mean to leave the impression that all of the EGC's downstream of the downstream disconnect needed to be individually extended back to the BJ.
Thanks for giving me the opportunity to clarify this matter.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Re: SDS Grounding

Kevin,
I think you're misreading the definition of "bonding jumper" as spelled out in 250-30(A)(1). It is not the connection from the XO to the transformer case at the transformer, or the connection from the neutral bus to enclosure at the downstream disconnect. Instead, it should be the connection from the XO to the downstream ground bus (if GEC connection at transformer,) or the connection between the ground bus and neutral bus at disconnect (if GEC connection is at disconnect.)

Dave
 

scott

Member
Location
Colorado
Re: SDS Grounding

Statements of belief...

All XFMRS are not an SDS.

The only time an SDS is part of service equipment is as described by 250.30(A)(4)exception. This requires a complete listed assembly.

All services have a XFMR upstream of them somewhere, and that XFMR may or may not be covered by the NEC.

Most XFMRS upstream of services are not covered by the NEC.

To the original question...

I would love to take my EGC's to the first disconnecting means as allowed by code. It is easier, cheaper, and code compliant.

Most AHJ's that I deal with feel this is unsafe do to various reasons, and desire the EGC to be taken to the XFMR of an SDS. I have not been able to change the opinion of any AHJ yet on this, so I don't even bother trying anymore.

Enjoy your day!
 

rb

Member
Location
Tennessee
Re: SDS Grounding

Don,

I agree that the transformer in the equipment referenced by 250.34(a)(4) is not service equipment. It is part of an assembly that is suitable for use as service equipment.

I agree that a separately derived system could not be service equipment. I didn't recognize your first post as a theoretical question.
 

svaurez

Member
Location
California
Re: SDS Grounding

What is the difference between an EGC and an equipment bonding jumper and what are their functions on the secondary side of an SDS?
 

gwz2

Senior Member
Location
Indiana
Re: SDS Grounding

Let's hope the 2005 NEC will be more specific on more definitions.

Appears Don has several proposals to aid in understanding the Grounding and Bonding issues.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

Scott,
Can you give an example of a transformer that is not SDS other than an autotransformer? Can you give an example of a transfomer on the line side of the service equipment that is covered by the NEC?
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

svaurez,
The bonding jumper is used to provide the connection between the grounded conductor and the equipment grounding conductors. The bonding jumper on a SDS has the same functions as the main bonding jumper on a service. For a SDS with the grounding electrode conductor connected to XO at the transfomrer, the bonding jumper will be installed from XO to the equipment grounding bus in the SDS OCPD enclosure.
Don
 

svaurez

Member
Location
California
Re: SDS Grounding

Would there be two bonding jumpers in the transformer?One from XO to the transformer case and one from XO to disconnect?
 

Nick

Senior Member
Re: SDS Grounding

I don't know what happened here but I will re post it. :confused:

Don,
I have a question similar to Svaurez. Are you saying that the conductor from the transformer to the first disconnecting means is in fact a bonding jumper sized per 250.66 as opposed to an equipment grounding conductor sized per 250.122?

[ March 02, 2003, 07:46 PM: Message edited by: Nick ]
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

Nick,
That is correct. 250.30(A)(1) requires a bonding jumper from XO to the equipment grounding bus. 250.30(A)(1) requires that this bonding jumper be installed per 250.28(A) through (D). 250.28(D) requires the bonding jumper to be sized based on the largest phase conductor and Table 250.66.
Don
 

scott

Member
Location
Colorado
Re: SDS Grounding

Don-

I should clarify what I believe SDS's to be. This is my belief, backed up by my understanding of the code, not actual code words.

An SDS is a device that changes existing building power to another power, within the same structure. The rules for SDS's in 250.30 are geared for this purpose, even though they do not specifically say so.

Non SDS transformers...

-A transformer used as in 250.24(A)(2)
-Autotransformers (as mentioned)
-control or metering transformers

A transformer on the line side of service equipment not covered by the NEC...

one used as in 250.24(A)(2) may or may not be covered by the NEC, depending on the utilities location of the service point.

Almost every residence that I have ever seen, has a utility provided transformer on the line side of the service disconnect, none of these are SDS's. Most commercial and some industrial properties would also fit into this same statement.

Enjoy your day!
 
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