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,
etc.)
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