I'd very much like to install a ground rod for my garage's electrical system (since the previous owner didn't have it done) but I have a concern. I've been told that the ground rod should be driven into the ground to a depth of about 8 feet. Is this basically the minimum depth?
My concern is that the entire area I live in is sitting on fairly shallow limestone, and I very well may hit limestone before getting anywhere near 8 feet. When the PO had a septic tank installed they had to put a small mound of earth over it because they couldn't get it any deeper into the ground.
If I strike rock at, say, 4 or 5 feet will be that be deep enough to be effective, or am I just wasting my time? Is there a standard procedure for a situation of this kind?
Any input appreciated!!
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I think 8' is standard. You can try using an electric roto hammer that will also just hammer. Just stick it over the rod. It might work.
I can't say if 4' is enough. I am not an electrician. If you hooked it to a water pipe, it would probably be no more than 4' down. Maybe it is enough. I'm sure someone else will come along who knows. Good luck with it.
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As a rule, the electrical code only allows ONE ground point in a distribution system and that is located at the main service entrance panel. Even if your garage is detatched from the main building, it relies on the main service entrance ground. Aside from safety issues, multiple ground locations can cause ground loop currents to flow on the grounding conductors in the building. Some electronic equipment will go nuts under those conditions.
Having said that, if you can only get down 4 feet then use multiple rods in a "ground field", put ground rod clamps on the top of the rods and tie them all together with some #4 bare copper wire. It's not how deep you go but rather how much surface area of the grounding rods are in contact with the earth.
But I'd still check your local electrical code first.
An extra ground won't hurt. If there is a damp area, put it there. 8' is the norm, sometimes driving at an angle will help. A post driver helps (used for driving metal posts) hint: put the ground wire clamp on first before driving the rod it is kind of hard to get on if the rod gets "mushroomed".
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The NEC requires EACH detached building to have a grounding electrode system. The code encourages the connection of the grounded conductor (the neutral) to the grounding electrode system at only one point (the service disconnect), but there is an exception that allows for a 3 wire connection between buildings in some cases. (It is that arrangement that leads to questions of ground loops, not the presense of multiple grounding electrodes at multiple buildings).
Ground rods are typically driven at an angle if a straight down connection cannot be accomplished. Most of the time you will have to use the "two electrodes 8 feet apart" rule when you do this, because neither ground rod will acheive the 25 ohm resistance requirement. (And even the pair in parallel probably won't, if the soil is dry and the underlying limestone is porous).
Rebar in a footer works a lot better than ground rods, at least in terms of resistance to ground.
Water pipe grounds are not really any good any more.
Most underground water pipes are now plastic which does not conduct electricity.
If you have copper pipe inside the building and use the water piping for a ground, during electrical fault conditions the entire water system becomes electrically hot.
We bond the interior water piping to the ground rods to prevent this from happening which is required in NEC 250-80 (a)
Also any other metal piping that may become energized shall be bonded to ground. NEC 250-80 (b)
Also structural steel buildings shall be bonded to ground. NEC 250-80 (c)
The NEC does require a ground rod for seperate buildings unless the 4 wire system is used to supply the building or if there is only one branch circuit supply to the building.
But the neutral and ground shall not be bonded together. NEC 250-24 (a)
The NEC also allows for the ground rods to be driven at a max angle of 45 degrees.
But it also allows for the ground rods to be installed flat in the bottom of a ditch that is a minimum od 2.5 feet deep.(NEC 250-83 (c)(3) and does call for the upper end of the electrode to be flush or below ground level unless provisions for protection are met.
There is also a grounding ring electrode that can be burried at a depth of not less than 2.5 feet and a minimum of at least 20 feet of #2 bare copper wire to be burried in a ditch. (NEC 250-81(d))
Here in WV we have a lot of rock that makes ground rods inpossible to install in many occasions.
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The basic rule as already posted is to drive the rod straight down. If it won't go straight down, drive at 45 deg. If you can't do that, bury horizontal.
I once spent an hour driving a rod down and it seemed to be going fine, other than really slow. I got done and turned around, then tripped on the point of the rod sticking straight up behind me. The rod went down a foot and ended up doing a 180, after hitting rock.
Too many times ground rods are cut at 4', (or less), because someone didn't work at it hard enough. (I'm sure I'm not the only one that's pulled up a two foot ground rod.) Sometimes they just won't go, granite and sandstone are two examples I can think of where they won't. Sometimes the ground just isn't very conductive.
One option is to use a chemical ground. You can buy them, but I've probably built a half dozon of them over the years. Pretty straight forward. I get the cad-weld/wire at the electrical supply and the pipe and bentonite from a pipe supply. I've used rock salt to fill. http://www.erico.com/products/ChemRod.asp
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