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Re: [TowerTalk] ground rod spacing

To: Mark Robinson <>
Subject: Re: [TowerTalk] ground rod spacing
From: jimlux <>
Date: Mon, 03 Jan 2011 09:27:32 -0800
List-post: <">>
Mark Robinson wrote:
>> NEC doesn't say anything about multiple rods, other than they have to be
>>  bonded together.   (and, of course, that a rod can't be the sole
>> grounding means in a new installation)
> What do they require now? Several rods. I just beefed up my service 
> ground (an old steel pipe banged in the ground 30 years ago) with 
> another 10 foot copper clad rod right by it and then tied the system 
> into the 30 ground rods in my tower grounding system.
Here's what Art 250.50 of 2005 NEC says:
1) metal underground water pipe at least 10 feet long that is outside 
the building. A well casing will do.
2) metal frame of building, as long as at least 10 feet of a single 
structural member is in contact with the earth or encased in concrete 
which is in contact with the earth. (plus some other restrictions)
3) A Concrete encased grounding electrode (Ufer ground) 20 ft of AWG 4 
bare copper encased in at least 2" of concrete in contact with earth. 
Rebar also works.
4) A ground ring of AWG 2 bare copper at least 20 ft long.
5) Rod and pipe.. at least 8 feet in contact with soil,5/8" diameter
6) A plate of at least 2 square feet of surface area, no thinner than 1/4"

As far as spacing goes:
250.53(B) says that electrodes of types 5 and 6 (rods and plates) must 
be spaced at least 6 feet apart and from other grounding electrodes 
(e.g. for air terminal grounding).

if you want to use #1, 250.53(D)(2) says you have to have another 
electrode as well

  250.56 says if you're using rods/plates, the resistance has to be <25 
ohms and multiple rods have to be at least 6 ft apart.

250.53(G) says that if you bury the rod horizontally (say, you're on a 
rock ledge), it has to be at least 30" deep.

Multiple rods can be used if measured resistance is <25 ohms, but 
there's a bunch of other rules too. (8 feet of rod in contact with soil, 

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