Spacing Between 2 Earthing Rods
- Thread starter TMMB
- Start date
- Status
- Location
- New Jersey
- Occupation
- Journeyman Electrician
6 Feet.
Thank you for reply, but if we have not enough spacing: can we install the rods with spacing 4.5 - 5 feet is it accepted?? but will not give us the required improving and we have to increase the nos. of rods to reach the required earth resistance , or we can not install rods with spacing not less than 6 feet
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You cannot meet the NEC requirements by putting more rods but having them closer together. You would have to go to some different type of electrode.
Two rods are better than one, three are better than two, but that is not all that the NEC is asking for.
As a practical matter, you can decrease the GES to earth resistance by adding rods, but at some point each additional rod will do almost nothing to decrease the resistance as long as it is anywhere within the same 5 foot circle. Putting a third rod halfway between two rods separated by 5 feet will not be a good as putting the three rods at the points of a triangle that just fits inside the circle.
Are you concerned with measuring the resistance and getting it below some level because of local regulation or contract specification, or are you just trying to satisfy the NEC?
we have a building and we have one side only available to install the rods, as per design we have to install 100 rods in area 76m x 3m (ROCK) to reach 2 ohm or less , then we have not any choice , the spacing will be 1.5m (4.9 feet) , , or we have to think about another solution by using PLATE/LATTICE ??
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Wow!
Looks like a job for an EE who can actually look at the site. Somewhere along the way there has to be conductivity testing done on the rock or rocky soil.
If the resistance of the rock is high enough, then there may be no type of electrode that will fit into that space and give you the resistance you need.
Wet limestone is pretty good. Wet or dry quartz is pretty hopeless.
Is the rock solid enough that you will have to be drilling the holes for the ground rods? That would be a bad sign.
A poured UFER ground (not necessarily meeting the NEC definition of a CEE, but still quite usable for meeting your specification) may be the best way to go.
If you are limited in the depth to which you can go, a plate or lattice will not be a lot better (factor of two maybe?) than a network of rods that covers the same area and goes to the same depth.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I cannot say for sure without getting the actual conductivity measurements, doing the calculations, and being a P.E. :angel:
But I would not count on a big improvement, if any, from using a plate or lattice which is above the rock level.
You may be better off with plate or lattice, you may not. It depends.
You want to maximize the surface area of rock that you are in close contact with and if possible get below the surface water table.
One deep well might be better than 100 short electrodes.
kwired
Electron manager
- Location
- NE Nebraska
NEC does not specify this. If the building foundation also has a CEE in it, then that rod probably is not as effective as it would be further away though.
mwm1752
Senior Member
- Location
- Aspen, Colo
The EE is requiring above & beyond NEC code. All direction with details should come from him. Your min requirement would stop at the 3rd rod based on the info given. at least 2 of the three would be 6' apart. a UFUR is stand alone GE and would not require a supplemental.
kwired
Electron manager
- Location
- NE Nebraska
I agree and would think if the EE asked for 100 rods that there probably was some effort involved in coming up with that figure. Once two rods have been installed, the rest are beyond the NEC requirements and there probably is more than six feet between at least two rods somewhere in those 100 rods
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