[TowerTalk] Ground Rods in rocky soil

[Jim Lux]

At 10:46 AM 1/18/2007, you wrote:
>In a message dated 1/18/2007 6:42:41 P.M. Greenwich Standard Time, 
>jimlux at earthlink.net writes:
>OTOH, if
>you're already pouring a building foundation, laying the wire into
>the footing is easy, and the cost is MUCH lower (just the wretchedly
>expensive copper wire.. but hey the price of that is coming down, too)
>
>Footers for the shear walls will be going in so I don't have to 
>worry about getting a small load of concrete.  In fact I can use 
>these for the UFER.  How much wire is necessary for the these 
>things...more info site?

They're called "concrete encased grounding electrodes", and googling 
turns up a bunch of hits.

Generally, 20 ft of conductor encased by at least 2" of concrete (on 
all sides). either 1/2" rebar  (galvanized or not) or AWG4 bare copper wire.

Electrical code NEC250-50(c) is probably your best bet.. The Mike 
Holt low voltage handbook (http://www.mikeholt.com/ has a link) covers it.

http://www.iaei.org/subscriber/maagazine/01_d/johnston.htm has a 
summary, too, along with some history (Ufer did 24 grounds in Arizona 
in 1942, all had resistances <5 ohms at the time, in 1960, the max 
reading was 4.8 and min was 2.1.. I'm sure that's resistance at low 
frequencies)

One warning on Ufer grounds is that you have to make sure there's no 
vapor barrier or liner between the concrete and the soil. (for 
instance, the slab under my house has a vapor barrier under the slab, 
but not under the footings around the edge, so it works ok.)

I can look up the data in the IEEE-141 spec for grounding and get the 
equation if you want more info.



Jim