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#96522 - 12/05/05 04:03 PM What size GEC?  
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
If I have a 1000a bus bar service - What size GEC do I need to a qualifying water pipe electrode?

George Little

2017 / 2014 NEC & Related Books and Study Guides

#96523 - 12/05/05 04:16 PM Re: What size GEC?  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Assuming that the calculated load is 1000 amps I would say 3/0 CU

See note 2 to Table 250.66


Bob Badger
Construction & Maintenance Electrician

#96524 - 12/13/05 07:20 AM Re: What size GEC?  
cpal  Offline
Joined: May 2004
Posts: 162
Cohasset MA
This may be a little late but you might consider that the intent of 250.66 is to provide a grounding electrode conductor that has a cir mil area based upon the cma of the supply conductors. In the case described the busbar would be considered the service entrance conductors???

The conversion from area in mils sq to cir mil area is Amil sq divided by .7854.

A 1/4" X 2" bar would have a sq in area in mils of 250 X 2000 or 500,000 sq mils.

divide 500,000 by .7854 to obtain 636,618 + or - cir mil area.

reference to 250.66 and select a 2/0 CU or a 3/0 Al GEC .

Although one could consider busbar as not fulfilling the intent of the term conductor it is conducting material.

just a thought!

#96525 - 12/13/05 08:34 AM Re: What size GEC?  
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
There's a question in my mind about what size buss bar is used for a 1000a. Service. I was under the impression that a buss bar .25" x 4" would be used. Using cpal's math the required GEC would be 3/0 cu. or 250 kcmil al.

250 x 4000 =1000000 sq. mils. 1000000/.7854 = 1273236 cir mils. Reference 250.66 would give us a 3/0 cu.

Maybe the question is the physical size of the buss for 1000a. I do agree that it is a conductor.

George Little

#96526 - 12/13/05 09:46 AM Re: What size GEC?  
cpal  Offline
Joined: May 2004
Posts: 162
Cohasset MA
The Chase Shawmut Book of Electrical Information indicates that a 4" X 1/4" Copper Bar can handel 1000A or 1000 A per Sq"

This information was in the NEC around 1971 or so and it indicated that Copper Bus could carry 1000A per Sq" and Aluminum could handel 700A per Sq"

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