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# of Ground rods #80186 03/14/02 02:09 PM
Joined: Mar 2002
Posts: 22
A
AndrusT Offline OP
Member
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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Re: # of Ground rods #80187 03/14/02 05:06 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline
Member
Unless you've got the $$$ for the tester & time to waste, you'll need 2 rods at all times.

See 250-56 1999 NEC


Few things are harder to put up with than the annoyance of a good example.
Re: # of Ground rods #80188 03/14/02 05:54 PM
Joined: Nov 2000
Posts: 2,236
sparky66wv Offline
Member
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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Re: # of Ground rods #80189 03/14/02 08:34 PM
Joined: Nov 2001
Posts: 280
M
motor-T Offline
Member
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-

Re: # of Ground rods #80190 03/14/02 11:09 PM
Joined: Dec 2000
Posts: 218
S
spkjpr Offline
Member
Motor t, my inspector is even easier than that. He just says two ground rods no matter what.

Re: # of Ground rods #80191 03/22/02 06:48 AM
Joined: Mar 2002
Posts: 95
J
joeh20 Offline
Member
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.


Lighting the way
Re: # of Ground rods #80192 03/24/02 09:57 PM
Joined: Mar 2002
Posts: 53
N
NonLinearLoad Offline
Member
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.

Re: # of Ground rods #80193 04/02/02 07:56 PM
Joined: Apr 2002
Posts: 2
E
electricdiver Offline
Junior Member
read 250-81 carefully. you don't need any grd rods if you meet the first several conditions

Re: # of Ground rods #80194 04/02/02 09:25 PM
Joined: Dec 2000
Posts: 4,283
electure Offline
Member
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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