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#86263 - 09/26/03 08:49 PM GEC sizing
Ryan_J Offline
Moderator

Registered: 08/19/03
Posts: 1355
Loc: West Jordan, Utah, USA
I would like to hear your opinions on what the minimum size of grounding electrode conductor would be for this installation, if using the water pipe as an electrode.

This panel is rated for 400 amps. The two breakers towards the bottom are 150 amps and each feed a subpanel, the two breakers at the top are 40 amps and each feed an A/C unit.




Please try to justify your answer with a code reference.

Thanks to Joe Tedesco and Dave Nix. Please visit their sites at www.joetedesco.com and also www.grafixbynix.com
_________________________
Ryan Jackson,
Salt Lake City

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#86264 - 09/26/03 09:36 PM Re: GEC sizing
caselec Offline
Member

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
Ryan

This is a 400 amp combination service entrance device, Square D CU12L400B to be exact. The size of the grounding electrode is determined by the size of the service entrance conductors. In this case you don't actually have any service entrance conductors but do have service lateral conductors. If the service lateral conductors are going to be installed by the EC the grounding electrode conductor should be based on the size of these conductors. If the service lateral conductors are going to be installed by the POCO I feel the grounding electrode conductor should sized as if they used 400 amp conductors. If this is a residential service it would require 400MCM CU or 600MCM Al service conductors so the grounding electrode conductor should be 1/0 CU.

Curt
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Curt Swartz

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#86265 - 09/27/03 01:02 AM Re: GEC sizing
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Depending on the Load Calcs, they may use 4/0 AL XHHW (seems to be the commonly used size so far...), or even smaller!

I guess one could just install 3/0 cu for the GEC and be covered in almost any situation!

As mentioned, need to have the Service Entrance feeder size first, then can figure minimum GEC size.

Not really adding anything helpful again huh?

Scott35
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Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#86266 - 09/27/03 07:00 AM Re: GEC sizing
Ryan_J Offline
Moderator

Registered: 08/19/03
Posts: 1355
Loc: West Jordan, Utah, USA
Thanks for your replies. The reason I poseted this is to pose a question...Is this the installation that added footnote 2 to table 250.66? Bear in mind the definition of service entrance conductors. In this application there are either no service entrance conductors or they are the bus bars that come off of the load side of the meter.

I am of the opinion that that the bus bars are the service entrance conductors and therefore a 1/0 GEC is required. I don't think you are ever permitted to use the service lateral conductors to size with.
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Ryan Jackson,
Salt Lake City

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#86267 - 09/27/03 09:43 AM Re: GEC sizing
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
Given that this is a service with four disconnects. . .I don't think the size is tied to the maximum possible service size (limited by the equipment), but rather by the calculated service size, determined by the occupancy's load where this is installed, plus a margin for growth that is the calculator's option. . .not requirement.

That is: What does the installers service calculation say the service is? The service conductor size used to determine the GEC size would be based on that "equivalent" conductor.
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#86268 - 09/27/03 12:13 PM Re: GEC sizing
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
 Quote:
I am of the opinion that that the bus bars are the service entrance conductors
If we're taking a poll I would tend to agree with this (assuming it's an UG Service) because of the definition of Service Entrance Conductors in Art. 100.

In other situations the GEC is sized according to the Service Entrance Conductors and not the load. It wouldn't seem consistant to start looking at the load now.

Say this house has all Gas appliances and therefore a low calculated load. It's 200A OH service requires a #4 GEC. I change it to a UG service and with no change in loads it wouldn't make sense to me that I can now use a much smaller GEC.

JMO
Bill

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#86269 - 09/27/03 12:28 PM Re: GEC sizing
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I tend to agree with ElectricAl.


Lets say we had to provide the service entrance conductors to this service in Ryan's picture.

Would the NEC require 400 amp conductors, the panel rating or 380 amps, the sum of the breakers or will the NEC allow the service entrance conductors sized to the calculated load?

FWIW I think it would be easer to size the conductors to the panel size but I do not think that is what the NEC requires.

JMO Bob
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Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#86270 - 09/27/03 12:47 PM Re: GEC sizing
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
Table 250.66 Note 2.
 Quote:
Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
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Al Hildenbrand

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#86271 - 09/27/03 12:56 PM Re: GEC sizing
Joe Tedesco Offline
Member

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA
I would feel comfortable if the size of the GEC was based upon the size of the MBJ since the service size doesn't take us beyond the Table 250.66 sizes.

If the MBJ is factory installed, and it looks like it is, the only item would be to determine its size so that a properly sized GEC could be installed.

The images are posted on my site also and I asked Ryan to send them in so that we could see what the service looked like.
_________________________
Joe Tedesco, NEC Consultant

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#86272 - 09/27/03 12:59 PM Re: GEC sizing
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
And I will throw this one out there as well.

2002 NEC
 Quote:
230.90(A)
Exception No. 3: Two to six circuit breakers or sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load does not exceed the ampacity of the service conductors.


It seems clear to me that for services it is all about the load and in the absence of service conductors the GEC is based on the calculated load not the potential load.

JMO
Bob

[This message has been edited by iwire (edited 09-27-2003).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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