On 1/20/2015 10:39 AM, Jim Lux wrote:
Around here the footings have to go below the frost line which IIRC is
still officially 30 inches although its been years since it even made
12". The floor in my 30' by 40' shop is poured over 1" of Styrofoam,
but it still has a large area exposed as the footings are roughly 30'
long and 40' long by 18" wide by about 3' feet deep. Roughly 1050 sq ft
of exposed concrete just from the footings.
BTW the surface area of a 3/4", 8' rod is 1.5 sq tt while the area of
the 1' diameter concrete is a tad over 25 sq ft or the equivalent of
almost 17 8' ground rods. The base pour for my guyed tower is 3' in dia
X 5' deep with a surface area of 47..1 sq ft (area of sides) + 7 (area
of bottom) = 54 sq ft or 36 8' 3/4 inch ground rods. If I made no
mistakes.
When looked at in those numbers the extra ground rods take on a
different significance. Also the concrete will slowly increase the soil
conductivity around it.
73
Roger (K8RI)
On 1/20/15 7:13 AM, Grant Saviers wrote:
If your house slab was installed correctly with a vapor barrier and/or
foam insulation, then it is insulated electrically from earth.
In the middle, yes.. so it forms a big capacitor plate.
But, around the edges, your slab is still in contact with the soil.
My house slab is about 50 feet long and 30 feet wide, so I've got 260
running feet of contact, with at least 1 foot of concrete exposed.
I can't remember exactly how deep the footing edges go, but it's about
12-18" and at least 8" wide, then it pops back up to the 4" thick post
tension slab in the middle. Only the 4" slab part has the vapor barrier.
Tower bases make good Ufers as do perimeter foundations, so my towers
and shop both had the rebar set as Ufers when constructed.
I also noticed that a new service transformer I had installed is set on
a concrete vault that has a ground stub cast into the side. The power
company used it, no ground rods. I'd estimate its surface area in
contact with earth as more than 16 sq ft. Compare that to less than 2
sq feet for a 10' 3/4" ground rod.
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73
Roger (K8RI)
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