Grounding Electrode

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mjcelli

Member
I have a Chicago Electrical Code grounding issue.

If a building contains multiple services due to size of building, say (2)@3,000A and (2)@400A, what size would the grounding electrode conductor be from the main building ground bus to the waterpipe? Would this conductor be sized based on the largest service(1@3,000A), or would it be sized based on combining the sum of the areas of ALL of the service conductor sets?

(NEC is simpler because the largest grounding electrode required is #3/0 per table)

Any input would be greatly appreciated!!
 

bphgravity

Senior Member
Location
Florida
Re: Grounding Electrode

I'm not familiar with the Chicago code, however I feel each service should be grounded based on the largest ungrounded conductor brought to that service. 250.58 requires each service to utilize the same electrode. Since you are using the water pipe, no additional bonding would be required.

I don't know what you mean by main grounding bus? Are you sure you have 4 services, or do you just have one services a four mains.
 

mjcelli

Member
Re: Grounding Electrode

Thanks for the reply!

Just to clarify, usually I provide a "main ground bus" located in the main electrical room in which to land any service grounding electrodes and grounding electrodes (if multiple services) and for separately derived systems (e.g. xfmrs, etc.). Then, from this main ground bus, i'll bring a single conductor to the main water service pipe. Normally this would be 1#500kcmil or so. However, this building I'm doing now contains 3 utility services at 3,000A (sry, i stated 2 before) and 2 at 4,00A, so there's a bunch of parallel service conductor sets to account for.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Grounding Electrode

250.30 A 3 seems to say that the conductor from the ground bus to the water pipe has to be sized for the combined area of the service conductors. If you have the handbook, they have a pretty long example.

I'm not sure that 3/0 is the limit on this "common" bonding jumper.

[ March 04, 2005, 03:48 PM: Message edited by: steve66 ]
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Grounding Electrode

Bryan:

I was thinking each service would be considered a separately derived system. Per the definition:

"A premises wiring system whose power is derived from a....transformer.....and has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system."

It doesn't say the transformer has to be part of the premises wiring system.

Also, it sounds like the "ground bus" connections to the services are taps, so if these aren't separately derived systems, I think a solid unspliced GEC is required from each service to the water pipe.
 

bphgravity

Senior Member
Location
Florida
Re: Grounding Electrode

Originally posted by steve66:
It doesn't say the transformer has to be part of the premises wiring system.
No it doesn't however, I think the revised definition in the 2002 clears this up.

A premises wiring system whose power is derived from a source of electric energy or equipment other than a service.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Grounding Electrode

Yes, that does clear it up :)

I'm not sure where that leaves our poster, though. If the ground bus isn't mounted directly on grounded building steel, these sound like taps to me.

Steve
 

shortcircuit2

Senior Member
Location
South of Bawstin
Re: Grounding Electrode

The NEC doesn't seem to specifically address the use of a common grounding electrode conductor for multiple services to a building or structure.

It does however allow the use of a common GEC for multiple separately derived systems in 250.30(A)(4), and the maximum is a 3/0 copper according to table 250.66.

So if it is allowed for multiple SDS, why not for multiple services to a building?

In reading through the NEC today, I don't see any wording that would prohibit the use of a common GEC for multiple services to a building...

Does anyone see a reference disallowing it?

shortcircuit2
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Grounding Electrode

250.58 Common Grounding Electrode.
Where an ac system is connected to a grounding electrode in or at a building as specified in 250.24 and 250.32, the same electrode shall be used to ground conductor enclosures and equipment in or on that building. Where separate services supply a building and are required to be connected to a grounding electrode, the same grounding electrode shall be used.

Two or more grounding electrodes that are effectively bonded together shall be considered as a single grounding electrode system in this sense.
 

yoopersup

Member
Re: Grounding Electrode

Table 18-27-250-66
Chicago Code Does not stop at 3/0 but goes to 1500 copper Depending on sevice size and grounding Electrode conductors MUST be enclosed in Conduit.
 
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