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#88815 07/30/04 07:56 PM
Joined: Aug 2001
Posts: 599
Nick Offline OP
I have a scenario that I want to run buy you guys. The job is under the '99 code and I believe the building was done under the '93.
There is a main building which has a utility fed main service (480/277V 1600A)
Right next to it is a central plant building (physically about 15ft) that also contains a utility fed main service (480/277V 1600A) from the same utility transformer as the main building. The two buildings are connected by a "utility bridge", for lack of a better term, that was used to conceal mechanical piping that runs from the central plant to the main building. There is an expansion joint at the connection of the bridge to the main building and the cold water for the site enters the central plant, feeds equipment in there and runs overhead through the "utility bridge" to the main building.
The central plant cold water ground is properly connected within 5ft of where the water service enters the central plant. The main buildings cold water ground is connected about 20ft from where the water line enters the main building which is about 100ft away from where the water line emerges from the ground in the central plant. (the UG is metalic) I believe since the buildings are connected it should be treated as one building with two services and the main buildings cold water ground should be connected at the same place the central plant cold water ground is.
There are a couple of things to factor in here though. The building is part of a hospital campus so it could possibly meet the exception of having only qualified personnel service the installation. It also could be considered two separate buildings in which case the main buildings CW ground would just have to move about 15ft to become compliant. The problem with this is the CW entrance to the building is overhead from another building. This would also open a can of worms with the emergency system as all the feeds to the main building from the central plant would need to have service rated disconnects and additional grounding.
I don't really have a specific question, I am just looking for your thoughts on this.
[Linked Image from]

Edited to add picture. It shows how the buildings are connected. (Sorry for the fence thats in the way)

[This message has been edited by Nick (edited 07-30-2004).]

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#88816 07/31/04 12:02 PM
Joined: May 2004
Posts: 162
Is the transformer supplying both services utility or customer owned?

Does te local building code view this structure as one or two buildings.

does the water pipe to the main bld qualify per 250.52 (A) (1) Excption?

#88817 08/11/04 04:37 PM
Joined: Mar 2001
Posts: 345
Let me say that I have not been responsible for the arrangement of services as large as yours are likely to be. Having said that are not the basic principals the same regardless of the size of the two services involved?

It would seem to be completely impractical to isolate these buildings from each other electrically.

Since the water supply to the building you are concerned about is not underground the connection in that building is a bonding connection rather than a grounding electrode connection so it need not be within five feet of were the piping enters the structure.

If a single emergency electrical supply is used for both buildings then I suspect that four pole transfer switches are in order to avoid grounding the neutral on the load side of the service disconnecting means during normal operation from the utility supply.

I hope that the reinforcing steel in the footer of that building was made available as a Grounding Electrode. If the footer has yet to be back filled you might consider a ground ring. My concern with the electrode quality is that having markedly higher impedance to ground in one building relative to another building that shares the same metallic water supply is not a great idea. Maybe I'm just a nervous Nellie but I would test the grounding in that second building to make sure that it is not wildly different than the one were the water pipe comes out of the earth.

Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison
#88818 08/11/04 07:30 PM
Joined: Feb 2003
Posts: 4
Junior Member
Like others here i haven't had a great deal of experience dealing with this sort of set-up,but seems to me your dealing with two seperate structures with two seperate services that just happen to be electrically bonded through the water piping.
Each building needs to be treated as completely seperate.Of course if this is a supervised facility with only trainded personal working on it there may be some exception.except for grounding and bonding purposes i can't think of any exceptions that would apply

#88819 08/13/04 04:13 PM
Joined: Dec 2003
Posts: 751
The rules on when two separate structures are to be classified as separate buildings centers on two points:
1) The construction of the wall separating the structures
2) The distance between the structures

The wall separating the structures are called "fire walls" if they meet certain complex rules based on distance between and occupancy.

The maximum distance to consider is 30 feet. That is, if two buildings are separated totally by 30 feet, then they require no fire rating to protect them from each other, and they can be classified as two buildings.

If, on the other hand, two structures are closer to one another, and there is no (or inadequate) fire rating of the walls that separate them, then they are classified as one building, even if you can drive a car between them!

Be careful when assuming that two structures are two buildings. The building code defines terms carefully, just as the NEC does, sometimes it seems counter-intuitive, until you study the definitions.


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