Ground Rods & Concrete Encased Electrodes (250.52 of NEC)

[msimms]

A job I'm working on currently takes into account the new NEC code changes involving concrete encased electrodes found in 250.52(A)(3) (This also seems to affect NFPA780 for Lighting Protection as well). As stated in 250.52(A)(2), you can connect the structural metal frame to reinforcing bars of a concrete encased electrode as provided in the section just stated. In the design, we have for the foundation of the building, a drilled shaft that extends at times as low as 30'. Within this structure are vertical bars equally spaced with standard anchor bolts and a cap laid on top at grade, where the steel structure is then connected. More of these structures are then built, some within 20 ft. of each other.

One of the questions I have is can this structure, if connected with an already constructed ground grid, be used in place of a standard ground rod? If so, isnt it well maintained in the industry that the minimum distance between ground rods is 2 times the length of the ground rod itself, or is that an arbitrary sentiment? I know that this design helps improve the contact resistance with the earth. Is it necesarilly bad if they're closer?

Also referencing 250.52(A)(3), what constitutes 20ft of metal rods? The length that it extends from top to bottom of the concrete?, or what if the concrete extends only 5 ft and metal rod is twisted throughout the concrete, eventually obtaining 20 ft of total length?

A long drawn out question, I know. Any help is appreciated.

 

[iwire]

A little confused, but no need for ground rods at all if you have a concrete encased electrode.

 

[barbeer]

One of the questions I have is can this structure, if connected with an already constructed ground grid, be used in place of a standard ground rod?

Agree with iwire, no need for rods.

Also referencing 250.52(A)(3), what constitutes 20ft of metal rods? The length that it extends from top to bottom of the concrete?, or what if the concrete extends only 5 ft and metal rod is twisted throughout the concrete, eventually obtaining 20 ft of total length?

IMHO, and the opinion of most here I believe, the referenced article allows connection of various steel rods to form your 20'.

 

[gndrod]

rebar UFER

rebar UFER

Location of the 20' x #4 is allowed to be wired tied to the foundation rebar in the footer that is in direct contact with earth. [250.52(A)(3)] CEE. Note that the vertical rebar in foundation stemwall construction is #3 (3/8") and is not permitted for GEC bonding. A good reason for the bottom of the trench location is the soil hardpan contact integrity for lightning dissipation to earth.

I have observed a stemwall 'blowout' where the UFER was used above the soil and the stroke disinegrated a portion of the concrete wall on its way to the interior panel electrical gear. Ligtning does what it wants, but in this case, a deep rebar in the footer most likely would have had better protective dissipation. JMO, rbj

 

[msimms]

sorry for the confusion

A little confused, but no need for ground rods at all if you have a concrete encased electrode.

so at this point the whole 2 times the length of the ground rod becomes obsolete then?

IMHO, and the opinion of most here I believe, the referenced article allows connection of various steel rods to form your 20'.

so theoretically, i can bend the rebar so that vertically downward it streteches 1' but at a length horizantally until i reach 20 total feet and still meet the standard? shouldn't there be some type of depth the rebar should be lengthened to, to become a solid CEE?

 

[gndrod]

sorry for the confusion



so at this point the whole 2 times the length of the ground rod becomes obsolete then?



so theoretically, i can bend the rebar so that vertically downward it streteches 1' but at a length horizantally until i reach 20 total feet and still meet the standard? shouldn't there be some type of depth the rebar should be lengthened to, to become a solid CEE?

Hi msimms,

I am not sure what you mean by being stretched. Using a 20'x #4 rebar in the bottom trench footer can be wire tied to another vertical #4 riser for a stub-up through the stemwall for an accessible connection point to the GEC.


" isnt it well maintained in the industry that the minimum distance between ground rods is 2 times the length of the ground rod itself, or is that an arbitrary sentiment?"

The distance is not arbitrary, but based on soil conductive characteristics contact with the rods. Check with the local authority who usually sets the min-max requirements for the project. Some areas can require up to 18' horizontal rod distance separation as compared to the 6' minimum. [250.53(B)] I hope this helps. rbj

 

[winnie]

If you are talking about electrodes that are part of the same grounding electrode system, then the issue of distance between grounding electrodes is one of making best use of material. Adding additional grounding electrodes closer together than suggested will not _reduce_ the performance of the grounding electrode system.

The resistance of a grounding electrode to 'earth' is set by the resistance of the soil surrounding the rod. Two ground rods right next to each other will have essentially the same total resistance to earth as a single ground rod, because most of the resistance is set by the _same_ soil that surrounds both of the rods. As you move the rods further and further apart, each has more of its own soil, forming a _separate_ path to 'earth', so that the total resistance of the two rods together goes down.

If your job is to get some minimum grounding electrode resistance with the minimum amount of material, then you will want to space the electrodes apart. In this way each grounding electrode has the maximum of its own soil forming its own path to earth. However if the location of the grounding electrodes is set by some other factor, say _structural_ requirements of the rebar in a concrete encased electrode, then the close spacing won't harm anything. T

-Jon