grounding rod placement

I need advice in placing grounding rods when upgrading electrical service from 100 amp to 200 amp.
Code requires at least 2 5/8" x 8' rods placed no closer than 6'.
Soil conditions: sandy loam. Water table is about 15-18' below grade.
I am considering stacking rods so I will end up with 2 16' rods.
Question is how close can these be without compromising effectiveness of grounding system?
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wrote:

Best performance is somewhere around the length of the rod. They create a "well of influence" that is about as wide as it is deep. I would do about 16'
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No less than 6'. There are rods made that have the ends threaded to add length. I think that they are available from McMaster-Carr; http://www.mcmaster.com
You can also buy 3/4" x 10' ground rods at some electrical supply companies.
As another posted stated a distance of 16' or more from each other is optimal for 8' rods. If the rods are longer they should be further apart. If you are very concerned about soil resistively you could always install 3 or 4 rods.
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You may want to try stacking two 3/4" x 10' instead as previously suggested. Hard to tell best placement with just two locations as soil resistivity varies so much from just a few feet away and from time to time. For sandy loam, best to install couple of ground wells and keep rods wet year around.
If you have access to the foundation rebar(continuous 20' with no vapor barrier) known a Ufer ground, its better than ground rods.
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Why don't you just drive another 8' rod (IAW code) and be done with it. You will be perfectly legal. If the earth doesn't really conduct electricity then your "perfect groups" to the water table will not make any difference.

What are you trying to protect yourself against? Why sort of fault or event will your "perfect" ground protect you against? The only thing I can think of is if your "service neutral" opens. In that case, your ground system could get charged (depending upon what is on and what is off in your home) with up to 120 volts. But since your soil is a good insulator, it really would not make any difference.
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