Grounding rod: Why not just use rebar or copper tubing
kyrral
| Posted in Construction Techniques on
It seems to me that the rebar or copper tubing conduct electricity so why not use those? Why does the grounding rod have to go so deep? thanks for enlightenment.
Replies
In the case of Rebar - it is used. I believe this is called a "Ufer", but I may be mistaken. Basicly you are tied into rebar that is then covered with concrete. There are specific rules about how it is tied in, and the rebar has to be a minimum length.
Copper pipe going through the house and out to the street USED to be considered a good enough ground... then we started chopping it up with plastic pipe. My house only had a tie into the coper piping, which is fine unless I get repiped and the old pipes no longer are able to properly ground.
The grounding rod has to go deep so that it has enough conduction with the soil. Usually it's better to drive two grounds a certain minimum distance from one another to make sure you have enough contact. Soil type and moisture can make a big impact on grounding too. A good electrician should be able to test and see if the ground rod is sufficient, or if you need multiple rods driven.
Ufer or a concrete encased electrode is just that. It must be encased in 2" of concrete located in the foundation.
There is some dispute about wheher you can turn up a stub of rebar and make the connection there but it has to be done in a dry location in any case.
Note that there are specific rules for grounding, and they vary somewhat from jurisdiction to jurisdiction, depending on common construction techniques, soil conditions, etc.
Mostly code now calls for a "grounding system" that consists of 2-3 different grounding sources, plus the "bonding" of other parts of the structure. My understanding is that a Ufer (rebar in concrete) ground is now required for new construction (in most areas), but usually there's a requirement for a secondary ground rod. The ground rod must go deep enough to contact damp soil.
Water pipes, and, in some cases, metal siding must be "bonded" to the ground system. That doesn't make these "grounds", but rather assures that they are "grounded" should a "hot" wire come in contact (or an appliance malfunction). (As someone else said, water pipes are too often plastic anymore for them to be considered reliable grounds. Even if the pipe is currently copper, the water company may come along next week and replace it with plastic.)
But, like I said, this all varies with location, so you need to find out what local code calls for.
I used to install grounding systems for the military, We would pound in ten foot rods in a star pattern, if we were close to water, we would throw a rod into it as well
We used copper coated steel rods
You cant usecopper pipe as it will self distruct from the pounding. And hitting rocks
Rebar cant be used as it rusts....i believe that if you tap into a rebar grid that is large thats allowed, but i only heard that
grounds are not something to skimp on...the rods are only a few bucks, i have three pounded in on the side of my house
25 ohms
Rebar Ground
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We are now requred to do a "ufer" ground where we currently build.
One 20' piece of rebar tied into the footing, with the stub extending above the foundation wall in the vicinity of the service panel.
Inspector attaches orange tag after approval so the framers don't cut it off flush with the foundation wall when installing mudsill.
HV
They paint the Ufer green here and put a green stripe on the block wall so they won't pour that cell solid with concrete like the rest of the doweled cells.
http://gfretwell.com/electrical/ufer.jpg
I found this post to be quite informative. Thank you for writing it so brilliantly and sharing it with us.
Practicality
As others have said: A driven ground rod needs to get several feet below the phreatic surface of the soil, which is where the soils pores are filled with moisture, in som areas that is a few inches, in desert areas it can be twenty feet or more. It isn't practicle to use rebar as a driven ground rod, because it rusts in contact with moisture, and looses conductivity. Copper tubing cant be sued because it is impossible to drive.
The Ufer ground, (named for it's inventor, Herbert G. Ufer, ans should always be capitalized), utilizes rebar encased in concrete. The concrete encased rebar must have a minimum of 2-inches of cast-in from the concrete surface, (top, bottom, and side) for a couple of reasons.
The first being that concrete being of high pH "passivates" the steel, meaning that while it does corrode, it can not form the common "red" rust that is flakey and continues to deteriorate over time. The high pH causes the corrosion to form a black oxide, similar to Parkerizing of gun barrels, that is stable and prevents teh formation of red rust even if the encasing concrete is later damaged.
The second is that the high number of free ions in the highly basic material increases electrical conductance, and leaching into the surrounding soils also increases the conductance leading to a loser resistance to ground than untreated soils.
The third reson is that the 2-inch cover provides sufficient strenght to not blow apart in the event of a high ground current, and associated heat generating flash steam within the concrete from the pore water.
Not to be smart, but ... because code requires it. I think often times, many different methods work for many things but that sometimes those other methods fail. For example in many types of soil a steal rebar might just be fine, but in other soils not so much. One ground rod might be great in some places, but to get to teh 25 OHm requirement, you either need to test teh one rod or just put two rods in and be done with it. Therefore, code might be overkill in 90%of applications but not might just be adequate in teh other 10%. Rebar might be fine, but not always.
I find it a great stress relief not to argue with code, just do it, and shut up. LOL.
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