ground rod - Electrician Talk - Professional Electrical Contractors Forum

antonio · 12-05-2008, 12:25 AM

if i double the depth of a ground rod how much difference will it make in terms of total earth resistance

Speedy Petey · 12-05-2008, 06:25 AM

I'd have to say none. All you are changing is the amount is GEC that is in the soil.

KayJay · 12-05-2008, 07:58 AM

Quote:

Originally Posted by antonio

if i double the depth of a ground rod how much difference will it make in terms of total earth resistance

When a ground rod is driven down deeper into the earth the resistance is greatly reduced.
Generally, a doubling of the rod length would reduce resistance around 40%.
On the other hand, increasing the rods diameter would not significantly reduce its resistance, as a doubling of the diameter would reduce resistance less than 10%.

Similar statements can be found in most earth/ground resistance test equipment manuals and in the Soares book on grounding.

Speedy Petey · 12-05-2008, 01:20 PM

Quote:

Originally Posted by KayJay

When a ground rod is driven down deeper into the earth the resistance is greatly reduced.
Generally, a doubling of the rod length would reduce resistance around 40%.

Doubling it's depth, and doubling it's length are two vastly different things.

What are we talking about here?

480sparky · 12-05-2008, 01:40 PM

Why would you want ot bury any deeper? That would gain absolutely nothing except more work on your part. If the top is below grade, that's as good as it gets.

brian john · 12-05-2008, 01:58 PM

Quote:

Originally Posted by 480sparky

Why would you want ot bury any deeper? That would gain absolutely nothing except more work on your part. If the top is below grade, that's as good as it gets.

It depends on soil conditions, specs that may call for a lower ground resistance with limited space for a ground grid therefore the best option is drive deeper.

As noted driving deeper can in some conditions get you into better soil more constant moisture. Then again you can hit rock.

480sparky · 12-05-2008, 02:44 PM

Quote:

Originally Posted by brian john

.....Then again you can hit rock.

Isn't hitting a rock (one the size of a microwave or larger) pretty much a given?

Speedy Petey · 12-05-2008, 02:54 PM

Quote:

Originally Posted by 480sparky

Isn't hitting a rock (one the size of a microwave or larger) pretty much a given?

'Round here it is.

480sparky · 12-05-2008, 03:15 PM

Quote:

Originally Posted by Speedy Petey

'Round here it is.

I hit one every time. I'm trying to find the requirement for it in Art. 250, but just can't seem to find it.

Maybe I should write a proposal for the '14.

brian john · 12-05-2008, 03:26 PM

Hitting a rock is one thing hitting one the size of a Volkswagon is another.

When we dug the basement for my house we were suppose to go down 6 feet at 6 feet we hit solid granite. MY neighbor hit granite at 2 feet and had to blast. Needless to say driving ground rods is a hassle.

480sparky · 12-05-2008, 03:32 PM

Quote:

Originally Posted by brian john

Hitting a rock is one thing hitting one the size of a Volkswagon is another.

When we dug the basement for my house we were suppose to go down 6 feet at 6 feet we hit solid granite. MY neighbor hit granite at 2 feet and had to blast. Needless to say driving ground rods is a hassle.

If you hit one square, it doesn't matter what size it is. The rod stops.

I installed one last spring that hit a rock and curved the rod so much it started to come back up out of the ground 3 feet away.

Speedy Petey · 12-05-2008, 03:36 PM

Quote:

Originally Posted by 480sparky

I installed one last spring that hit a rock and curved the rod so much it started to come back up out of the ground 3 feet away.

I've seen that happen! This is the only other time I have heard of it.

jrclen · 12-27-2008, 07:35 PM

I'm lucky. In my location I can usually get the rod down 6 feet just by raising and then pushing it down. Light tapping will take care of the other two feet. One out of 20 will hit a rock and force me to pull it out and move it over a foot. Our ground is like a beach, pure sand.

acrwc10 · 12-27-2008, 10:18 PM

Quote:

Originally Posted by 480sparky

If you hit one square, it doesn't matter what size it is. The rod stops.

I installed one last spring that hit a rock and curved the rod so much it started to come back up out of the ground 3 feet away.

As long as you already had 5' of it in the ground before it started to come out 3' away you should have been good

Or maybe not.
You can get a coal miners bit for the roto hammer that is 8' or more long. Just up one size on the rod 1/2" bit 5/8" rod

InPhase277 · 12-27-2008, 10:55 PM

Quote:

Originally Posted by acrwc10

As long as you already had 5' of it in the ground before it started to come out 3' away you should have been good

Or maybe not.
You can get a coal miners bit for the roto hammer that is 8' or more long. Just up one size on the rod 1/2" bit 5/8" rod

I have often thought it would be nice if the rods all came with a tip and a couple inches of twist. That way, it could be chucked into a hammer drill.

leland · 12-28-2008, 12:49 AM

Quote:

Originally Posted by 480sparky

I hit one every time. I'm trying to find the requirement for it in Art. 250, but just can't seem to find it.

Maybe I should write a proposal for the '14.

Go for it!

nick · 12-28-2008, 07:20 AM

Quote:

Originally Posted by antonio

if i double the depth of a ground rod how much difference will it make in terms of total earth resistance

Well what they are saying is ground rods do come with threaded tops and can be screwed together one to the other with a ground rod coupling ,and they make a ground rod driver to drive them down increasing the length of the rod and this lowers the resistance to earth . I drive lots of rods and by putting one rod in say its tested at 10 ohms resistance and then screw on another it can cut resistance greatly meaning more than half the resistance not all soil will do that . in florida there is no rocks but on the coast there is this coral stone stuff which is like rock we drive at a angle and in the code you can lay a ground plate in the soil flat if you need too. check it out ? best to yas

olduser55 · 01-19-2009, 04:06 PM

We don't have to much trouble with rocks here. But in Tech school we were doing a service change, had to drive a new ground rod. Drove the 5/8'' rod right through the 1" water main feeding the house. 1 in a billion shot.

volty · 01-19-2009, 08:04 PM

Quote:

Originally Posted by jsnbyrn

We don't have to much trouble with rocks here. But in Tech school we were doing a service change, had to drive a new ground rod. Drove the 5/8'' rod right through the 1" water main feeding the house. 1 in a billion shot.

Bummer! I call before I dig, keep meaning to call before I pound.

Sometimes dig down to find the gas line if it is close to be sure we miss it.
811 is the new national Utility Protection Service / call before you dig number.

Quote:

Originally Posted by InPhase277

I have often thought it would be nice if the rods all came with a tip and a couple inches of twist. That way, it could be chucked into a hammer drill.

I always use a 3/4" spline rotory hammer, much easier than a sledge.

76nemo · 01-20-2009, 09:27 AM

Quote:

Originally Posted by Speedy Petey

I'd have to say none. All you are changing is the amount is GEC that is in the soil.

I thought I remember Mike Holt having two 50' rods driven at his own home and proved it didn't make a darn bit of difference.

That was just one location, obviously better soil would make a difference, but we ARE talking about 8' down here. Wouldn't think doubling it would make any difference in most locations.