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#80186 - 03/14/02 10:09 AM # of Ground rods
AndrusT Offline
Member

Registered: 03/14/02
Posts: 24
Whats the rule, on when you need to have 2 ground rods?
I have heard 2 reasons.
1. When the service is over 200 amps.
2. When your water pipe has a plastic main.

I have also heard the reason for this is cause you need 10 feet of ground (rod, or copper main) going into the earth. I can't seem to find this stuff myself. Thanks

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2014 / 2011 NEC & Related Books and Study Guides
#80187 - 03/14/02 01:06 PM Re: # of Ground rods
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
Unless you've got the $$$ for the tester & time to waste, you'll need 2 rods at all times.

See 250-56 1999 NEC
_________________________
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#80188 - 03/14/02 01:54 PM Re: # of Ground rods
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
2002 NEC 250.56

A little more explanation:

Unless you have the equipment and time to prove that your one grounding electrode has a resistance of 25 Ohms or less (and the Power Company doesn't mind only one) then the NEC requires no less than two.

Ground Meggers are expensive...

Also, 250.52(A)(5) requires 8 ft to be in contact with the earth. 10 ft. could be your local PoCo requirement.

[This message has been edited by sparky66wv (edited 03-14-2002).]
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#80189 - 03/14/02 04:34 PM Re: # of Ground rods
motor-T Offline
Member

Registered: 11/10/01
Posts: 293
Loc: Girard, Ohio, USA
AndrusT:
As it was explained to me by our inspector, he said, " Show me 25 ohms to ground, or drive two ground rods, its that simple."
-Mark-

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#80190 - 03/14/02 07:09 PM Re: # of Ground rods
spkjpr Offline
Member

Registered: 12/01/00
Posts: 226
Loc: Sedalia,MO, USA
Motor t, my inspector is even easier than that. He just says two ground rods no matter what.

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#80191 - 03/22/02 02:48 AM Re: # of Ground rods
joeh20 Offline
Member

Registered: 03/21/02
Posts: 95
Loc: Tullahoma, TN, 37388
Our local PoCo went behind their own office and drove 10 8' ground rods and never got 25 ohms. So they let us get by with two now. I worked on a 1200 amp 480v with 5 rods 10' apart cadwelded to 4/0 copper in addition to a clamp on a 4" galvanize water line thirty feet in direct contact with the earth and never got 25 ohms. But the key here was undisturbed earth, seems any fill really messes thing up.
_________________________
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#80192 - 03/24/02 05:57 PM Re: # of Ground rods
NonLinearLoad Offline
Member

Registered: 03/23/02
Posts: 56
Loc: McHenry, IL. USA
Illinois.
One rod as supplemental to primary ground as in city supplied water, attachment per code distances, service size to AWG sizing.
Got a well, TWO rods 6' apart all continous wire. Still it gets attached to the first copper accessible to tank as the backup water in the hole deal, got water it gets to the earth.
Wisconsin... TWO rods, due to soil.

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#80193 - 04/02/02 03:56 PM Re: # of Ground rods
electricdiver Offline
Junior Member

Registered: 04/02/02
Posts: 2
Loc: howell,nj,usa
read 250-81 carefully. you don't need any grd rods if you meet the first several conditions

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#80194 - 04/02/02 05:25 PM Re: # of Ground rods
electure Offline

Member

Registered: 12/24/00
Posts: 4226
Loc: Fullerton, CA USA
All you really have to do is have less than 25 ohms to ground on your grounding electrode connection...if it takes 1 rod, or 100.
(rods are kind of a junky ground path. If you're in trouble, you can pour a concrete encased electrode aka:Ufer, by just putting 20' of GEC sized Cu wire in a trench filled with concrete. Unless you have some kind of "resistance dirt", it'll pass)

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