Concrete encased electrodes

[britt]

The home builders in the Texas gulf coast are going ape over the the change in 250.50. I believe that the intent of the present code is to install a concrete encased electrode. However several contractors have different interpretations. Here is a list of questions that we are going to send in for formal interpretation by the NEC. I would appreciate any comments regarding the questions especially if they are not clear. We want to ensure the the review panel understands the question's intent.

Thanks in advance

1. If a concrete slab does not have 20 feet or more of 1/2 inch or greater diameter of rebar in the footer, is the intent of this section to require this amount of steel as the grounding electrode?

2. Does the existence of a plastic vapor barrier between the slab and the earth affect the interpretation of this section?

3. Does the Code permit OR require in lieu of the rebar mentioned above, 20 feet or more of bare copper not smaller that 4 AWG?

4. Is the intent of this section to require 20 feet of 1/2 inch rebar or greater or 20 feet of 4 AWG copper in a pre-stressed post tension slab, where the reinforcing steel steel is plastic coated?

 

[don_resqcapt19]

Re: Concrete encased electrodes

1) No.

2) Yes. The concrete must be in direct contact with the earth. The plastic prevents direct contact, so you don't have an concrete encased electrode. Note, you can't install a concrete electrode in a slab...it is only installed in a foundation. If the there is the required amount of rebar in the footing or foundation, and the footing or foundation has direct contact with the earth, the presence of a vapor barrier under the slab has no effect.

3)The code permits, but does not require, the installation of 20' or more of #4 or larger bare copper to be used as a grounding electrode. If you install the bare copper in a foundation or footing that also has 20' or more feet of #4 or larger rebar, you must also, in my opinion, bond the copper to the rebar.

4) No. The requirement is that if there is 20' or more feet of #4 or larger rebar in the foundation or footing that this must be used as a grounding electrode. There is no code requirement to create a concrete encased electrode.

Don
edited to revise answer #2

[ July 14, 2005, 11:16 AM: Message edited by: don_resqcapt19 ]

 

[physis]

Re: Concrete encased electrodes

I would appreciate any comments regarding the questions especially if they are not clear

I think I can help there.

I don't have the 2005 But I think the change is that "if available" was removed and it has to be made available and used period now.

But that doesn't change the definition of an encased electrode. I would think that if you have something that doesn't meet the definition you wouldn't have to include it in the grounding electrode system.

 

[britt]

Re: Concrete encased electrodes

Don, Thanks for the timely response. For whatever reason the entire foundation footer and slab is separated by a vapor barrier from earth. So would you still say that there is no concrete encased electrode?

Britt

 

[steve66]

Re: Concrete encased electrodes

It would be hard to isolate a footer or foundation. You would have to line the bottom and sides with plastic, and probably even the top.

If that is the case, I guess there isn't any concrete encased electrode.

Steve

 

[jwelectric]

Re: Concrete encased electrodes

This misunderstanding of the concrete encased electrode is very clear if time was taken to read the sections of both code cycles.

2002
If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system.

2005
All grounding electrodes as described in 250.52(A)(1) through (A)(6) that are present at each building or structure served shall be bonded together to form the grounding electrode system.

The only difference that I can see between the two cycles is that the 2005 cycle requires a rebar that is installed to be used where the 2002 cycle permitted the rebar to be used.

In either cycle if no rebar is being installed then no rebar is required to be part of the electrode system.

 

[britt]

Re: Concrete encased electrodes

Steve, That's exactly what they do to the foundation in this part of the country on residential foundations.

 

[physis]

Re: Concrete encased electrodes

Just out of curiousity, are the AHJ's insisting that the foundations be included as an electrode when they don't meet the definition of an encased electrode? Or asking that they be made to meet the definition?

 

[don_resqcapt19]

Re: Concrete encased electrodes

Britt,

Don, Thanks for the timely response. For whatever reason the entire foundation footer and slab is separated by a vapor barrier from earth. So would you still say that there is no concrete encased electrode?

Correct. There must be direct contact between the concrete and the earth to have a concrete encased electrode. The vapor barrier prevents this direct contact, so there is no electrode.
Don

 

[petersonra]

Re: Concrete encased electrodes

I wonder if home builders will now start putting vapor barriers under and around the foundation to avoid using it as a GE?

 

[physis]

Re: Concrete encased electrodes

Sounds a lot more expensive than a bonding jumper.

 

[LarryFine]

Re: Concrete encased electrodes

Originally posted by petersonra:
I wonder if home builders will now start putting vapor barriers under and around the foundation to avoid using it as a GE?

Originally posted by physis:
Sounds a lot more expensive than a bonding jumper.

Not to mention, more difficult to retro-fit later.*

 

[George Stolz]

Re: Concrete encased electrodes

What about that black stuff they spray on the foundation prior to backfill? Is that stuff conductive/insulative? I hadn't thought about that!

Sorry for using such big words, I'm not a concrete guy.

 

[tx2step]

Re: Concrete encased electrodes

Originally posted by georgestolz:
What about that black stuff they spray on the foundation prior to backfill? Is that stuff conductive/insulative? I hadn't thought about that!

Sorry for using such big words, I'm not a concrete guy.

George -- that's probably waterproofing. It's usually an asphalt (bituminous) product. Similar to the stuff you paint on conduit to protect from corrosion underground. It wouldn't be very conductive, but it isn't usually applied to the bottom of the foundation, so it shouldn't keep the foundation from being an effective grounding electrode.