Grounding Separately Derivied Systems Article 250.30

[wellke]

In an older existing precast concrete building (no effectively grounded structural steel)with all of the interior piping insulated can the primary equipment grounding conductor (EG) if sized correctly per table 250.66 be used as the grounding electrode conductor (GEC) for the secondary XO connection. This proposed GEC would connect to the service grounding bus bar and the ground bus for the service is not smaller than the required grounding electrode conductor.
What do you all think is this a code compliant system?

[bennie]

I promised to leave this subject alone, but I'm obsessed.

The procedure you describe is the only correct way to ground the X-O, core, and enclosure, of a transformer.

The only reason for grounding a transformer, is to provide a clearing path for primary ground faults, and winding to winding faults. There is no other purpose.

Sticking one conductor in the dirt by itself is nonsense, unless you have lightning in your building. The equipment ground conductor can be sized as an equipment ground conductor not as a ground electrode conductor. A transformer does not need a ground electrode conductor.

This is my opinion, it is substantiated by engineering standards. The NEC text is correct, only the interpretation is wrong.

[pierre]

Bennie

We are sending the 'Grounding Doctor and Couch' right away, it may not be too late.
All kidding aside, would you not also be concerned with voltage stabilization on the secondary side?

Pierre

[bennie]

What destabilizes the voltage?

[roger]

I try to stay away from Bennies posts for the fact that I usually agree, and don't want to appear to be backing him up for the sake of such.

With that said, I agree with Bennie.

Bennie, what destabilizes voltage is unequal loading of the voltage boat. :(


Roger

[ July 02, 2003, 09:27 PM: Message edited by: roger ]

[pierre]

Roger

your response answered my question, the only problem is my ignorance of transformer theory. Time to go 'back to the books'.

Bennie

My intention was not to question you, but to try and see if I understood, and I didn't. The only dumb question is the one not asked, and as you can see I ask alot of questions (sometimes disguised as a statement).

Pierre

[don_resqcapt19]

Actually in this case, I too, agree with Bennie. I see no electrcial or safety reason to have a grounding electrode and grounding electrode conductor for any transformer that is in the same building as primary feeder OCPD. The EGC run with the primary feeder circuit will provide all of the grounding that is required. I submitted a proposal (5-99) on this subjet for the 2005 code.
Don

[dereckbc]

Although I agree with Bennie in theory, it would be safe to do it this way. However I do not think it meets current code. Local AHJ may squeal.

But give it a shot and run the GEC with the primary conductors and size the GEC per the secondary conductors and 250.66. The only concern I would have is that the GEC be bonded to both ends of the conduit if used. You could claim the metalic conduit serves as the EGC for the primary, and the GEC is for the secondary. The local AHJ may approve. I would.

Lets us know how it works out.

[kqresq]

I also agree that this installation should be safe but, 250.30 clearly requires a Grounding Electrode Conductor sized per 250.66 to be connected to the Grounded Conductor. As such, I could not possibly pass that installation as "Code Compliant".

[tom baker]

The grounding electrode for a SDS system is installed for the same purpose as on a serive: to stablize the voltge to ground and to protect against overvoltage (lightning-transients). Primary faults are cleared by the equipment grounding conductor to the primary source.

[bennie]

kqresq: In 250.30 where do you see anything about a ground electrode conductor on the X-O of a transformer?

I know this topic has been around the block many times. I feel it is important to clear up the issue and definition of a separately derived system. I know it has been corrupted through time.

[pierre]

Bennie

250.30(A)(2)
The last sentence before the exception.
"Except as permitted by 250.24(A)(3) or (A)(4), this connection shall be made at the same point on the separately derived system where the bonding jumper is installed."

Isn't that the x/o?

Pierre

[bennie]

pierre: 250.24(A) 2, applies to an outdoor transformer. The NE (neutral earth) connection is required to prevent a fault from appearing on the premises wiring. This specifically is for a transformer, not a separately derived system.

250.30(A)1, Bonding Jumper...Connection of ground electrode shall be at any point from the source(transformer) to the first overcurrent device.(paraphrased). This is the normal connection method, of a service, from the utility.

The reference to grounding a separately derived system applies only to a premises being supplied by an isolated AC source. This can be a generator or transformer. The transformer, or generator, can not be electrically connected to the utility power system to be defined as a separately derived system. Electrically connected means "capable of carrying current".

The origin of Separately Derived System, was "AC systems without external connections".

[wellke]

Thanks for all the input. I will keep looking to see what else others say. I know there is no safety problem this is a general practice for these installations but does it meet the 2002 code?

[kqresq]

"250.30 (A)(2)(a) A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived phase conductors and shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). Except as permitted by 250.24(A)(3) or (A)(4)., this connection shall be made at the same point on the separately derived system where the bonding jumper is installed."

[don_resqcapt19]

wellke,
The installation in your original post is code compliant as long as the power source for your transformer originates in equipment that is listed "suitable for use as service equipment". See the exception to 250.30(A)(4).
Don

[wellke]

Thanks I think I have it now. Well keep watching for any additional comments.

[brian john]

Pierre:

Bennie states that the "XO", core and enclosure, that means the secondary is grounded not floating.
While on occasion Bennie has pulled my string on this he is right, IMHO. Sticking that rod in the ground especially in the middle of a building would buy you nothing, and per the NEC this rod would have to be connected to any other electrodes. Of course it would be through the feeder GEC, not sure if your AHJ would buy that?

[wellke]

Thanks again to all the answer I was overlooking was "Except as permitted by 250.(A)(3) or (A)(4)"
this gets me back to the sevice panel with the GEC. :D

[bennie]

The reason for the X-O of a transformer being permitted to connect to the building ground electrode if the panel feeding the transformer is for service only, is to insure a bond is complete from the equipment ground conductor and the system neutral.

A transformer fed by a feeder panel will make for a longer distance before the equipment ground conductor is shorted(bonded) to the neutral. Anyone thinking the building steel offers a lower impedance than a direct copper conductor to the service, has lost the meat on his sandwich.

[dereckbc]

KQRESQ, what would be wrong with running the GEC to the transformer with the primary phase conductors as long as it is sized per 250.66 for the derived secondary conductors, and terminated to the Xo? You could use the metalic raceway as the EGC for the primary protection.

Provided the GEC is bonded to the metalic raceway at each end, I cannot see how that would be a problem with the code as long as it originated from the service.

[kqresq]

The original post does not say that there is any metallic conduit. For all I know, the transformer is fed by underground PVC. I try never to assume.

[pierre]

Okay here I go again.
I know what a separately derived system is, but... if one were to run the GEC with the primary conductors would that not make this a moot point? Or is it only when the Grounded Conductor is mechanically and electrically not connected that it is a separately derived system?

Bennie,
I am confused and I am going to ask for your help. If my slow response to understanding this is irritating you, just don't respond and I will get the message. But I need to see if I understand this or not.

kqresq: In 250.30 where do you see anything about a ground electrode conductor on the X-O of a transformer?This is from your July 3rd 1:09 am post.
Is not the answer to this, 250.30(A)(2)(a), the last sentence before the exception ", this connection shall be made at the same point on the separately derived sustem where the bonding jumper is installed."?

Thanks for working with me on this

Pierre

[kqresq]

Bennie, 250.30 does not "permit" the XO to connect to the grounding electrode, It requires it to do so.

Does anybody think that the following has any bearing?
250.64(C) Continuous. The grounding electrode conductor shall be installed in one continuous length without a splice or joint, unless spliced only by irreversible compression-type connectors listed for the purpose or by the exothermic welding process.
Exception: Sections of busbars shall be permitted to be connected together to form a grounding electrode conductor.

Would the connection at the busbar be considered a "joint"?
If there is no metallic conduit to use as the EGC, is anyone proposing the conductor sized per 250.66 can double as both the EGC and the GEC?

[gwz2]

Pierre,

Note the last sentence of the first paragraph of 250.30(A)(4).
"The grounding electrode SHALL be the nearest of the following:"

The combination run of primary EGC and the Secondary GEC could possibly be the Feeder EGC.

[bennie]

kqresq: A transformer does no require a ground electrode. An equipment ground conductor is all that is necessary.

A separately derived system requires a ground electrode conductor.

[roger]

kqresq, one question, in your opinion are GEC's and EGC's electrical connections?

Roger

[kqresq]

Bennie, 2002 NEC Art. 100 Definitions
Separately Derived System. A premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
In the several jurisdictions that I have worked in, a common delta/wye voltage transformer has always been considered to be a separately derived system as defined by the NEC.

[kqresq]

Roger,
At the risk of being corrected no matter how I answer; no. In a properly functioning system the GEC and EGC are not electrical connections because they are not intended to normally have any current flow except in a fault and/or surge situation.

By the way guys,
I am not trying to be argumentative. I am just trying to determine what the NEC says, as opposed to anyones opinion about "how it ought to be".

[roger]

kqresq, I know you have been here for awhile but not very active. :(

So, let me say WELCOME to the forum.

I'm not responding to your post yet. :D

Roger

[bennie]

kqresq: Read 200.3

Also...A transformer is the power source for a separately derived system. It is not the separately derived system.

The definition clearly spells out...A separately derived system is a premises wiring system.

[ July 03, 2003, 11:04 PM: Message edited by: bennie ]

[kqresq]

Here is a link to the August 1, 2001 edition of Electrical Construction & Maintenance magazine.

http://www.ecmweb.com/ar/electric_separately_derived_system/


The article describes separately derived systems as follows:

Separately derived systems are power sources with no direct electrical connection between any of the circuit conductors and supply conductors. For example, no direct connection exists between the primary and secondary winding of a delta-wye transformer.

the author of this reads:

By Mike Holt, Mike Holt Enterprises, Inc.

[bennie]

Of all the code experts I respect Mike Holt more than any. I have had occasion to talk to him personally, on the land line.

His statement is the mainstream of the correct interpretation of a separately derived system.

However there is no basis to consider a transformer a separately derived system.

The windings of a delta wye transformer are solidly connected electrically, mechanically, and physically. The definition in 200.3 makes it clear that a conductor is electrically connected if it is capable of carrying current.

The origin of separately derived system was "isolated systems" in 1908, later it became AC systems without external connections, then in 1956 it became separately derived systems. The premises wiring system is the subject, not the power source.

[kqresq]

Don,
I found an article explaining the exception you mentioned earlier. This would seem to negate 250.64(C)in certain applications. (I think this is from the 1999 NEC)

250-30(a)(1)
The rules have been changed for the grounding of a separately derived AC system. The rule states that the bonding of grounding electrode and the grounded conductor shall take place at the same location, either at the source or at the disconnecting means. Also, there shall not be a bonding of the grounded conductor at the source and at the disconnecting means unless it is assured that there will not be parallel paths for current carried by the grounded conductor. The earth is not considered a parallel path.
The grounding electrode is required to be the nearest of 1) the effectively grounded metal structural member, 2) grounded water pipe, or 3) other electrodes meeting 250-50 (old 250-81). The connection to the water pipe shall be within five feet from the point of entrance into the building.
A very important exception has been added at 250-30(a)(3)(c) Exception for separately derived systems that originate from listed service equipment. In these cases, the service grounding electrode is permitted to be the grounding electrode for the separately derived system. The internal ground bus that is not smaller than the required grounding electrode conductor is permitted to be the connection point for the separately derived system.

Author's Comment

The proposal for this change [5-116a] was submitted by Code Making Panel 5 in the rewriting of section 250-26 of the 1996 Code. For years engineers have been using the service equipment grounding electrode as the grounding electrode for separately derived AC systems and have been installing the grounding electrode conductor as a the primary equipment grounding conductor. The two conductors are the same in many installations. The Code now allows this in the new exception to 250-30(a)(c)(3).