Is grounding rebar in concrete slab required by NEC?

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ROHM

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What is the NEC rule for grounding rebar in a concrete slab?

I have a slab area of 9 feet by 44.5 feet for a total square footage of 400 square feet. The depth of the slab is no more than a foot deep.

I was looking into article 250.50 of the NEC and could not find any specific requirements of the specifications for rebar sizes and rebar distances that should be brought to ground. Is there any statement that says based on the size (1/2 inch) of rebar I have and with lengths (4 feet) greater than this interval of amount of rebar in concrete that I have to have a specific number of grounding points?

If someone could point to be a specific article in the code that talks this, that would be great. I could not find any specific technical documentation on this topic.
 
If the rebar is not in a footing it doesn't qualify as an electrode and is therefore not required to be connected to.
 
ROHM said:
What is the NEC rule for grounding rebar in a concrete slab?

I have a slab area of 9 feet by 44.5 feet for a total square footage of 400 square feet. The depth of the slab is no more than a foot deep.

I was looking into article 250.50 of the NEC and could not find any specific requirements of the specifications for rebar sizes and rebar distances that should be brought to ground. Is there any statement that says based on the size (1/2 inch) of rebar I have and with lengths (4 feet) greater than this interval of amount of rebar in concrete that I have to have a specific number of grounding points?

If someone could point to be a specific article in the code that talks this, that would be great. I could not find any specific technical documentation on this topic.
Click to expand...

From my 2017 NEC:
250.53 (A) (3) Concrete-Encased Electrode. A concrete-encased
electrode shall consist of at least 6.0 m (20 ft) of either (1) or (2):

(1) One or more bare or zinc galvanized or other electrically
conductive coated steel reinforcing bars or rods of not less
than 13 mm (1⁄2 in.) in diameter, installed in one continuous
6.0 m (20 ft) length, or if in multiple pieces connected
together by the usual steel tie wires, exothermic welding,
welding, or other effective means to create a 6.0 m (20 ft)
or greater length; or

(2) Bare copper conductor not smaller than 4 AWG

Metallic components shall be encased by at east 50 mm
(2 in.) of concrete and shall be located horizontally within that
portion of a concrete foundation or footing that is in direct con-
tact with the earth or within vertical foundations or structural
components or members that are in direct contact with the earth.
If multiple concrete-encased electrodes are present at a building
or structure, it shall be permissible to bond only one into the
grounding electrode system.
 
Rebar grids in slabs are not required to be bonded. They are not “likely to become energized”.

A CEE (concrete encased electrode) is a different thing entirely. The NEC now requires 20’ of 1/2” rebar, OR, 20’ of #4 solid copper wire, to be embedded in a *footing or foundation*, if the concrete is in contact with earth, and to be used as the grounding electrode.

“In direct contact with earth” is key here. The building code requires all heated structures to have a 10 mil vapor barrier below slabs. Some residential, and even commercial buildings, will have insulated slabs, especially if they have radiant floor heating. As both vapor barriers and polyisocyanurate foam board insulation, which is used in insulated slabs, break the direct contact with earth, the CEE is virtually never used in new slab-on-grade construction.
 
infinity said:
If the rebar is not in a footing it doesn't qualify as an electrode and is therefore not required to be connected to.
Click to expand...
That's not quite what the code says. If the slab is a 'structural component' then the rebar may still meet the definition. 'Slab' is a pretty broad and ambiguous term, which might also count as a foundation in some construction. That said I would agree that most of the time rebar in a 'slab' does not require use as a CEE.
 
jaggedben said:
That's not quite what the code says. If the slab is a 'structural component' then the rebar may still meet the definition.
Click to expand...
I think you've misparsed part of the code quoted in post #3:

"Metallic components shall be encased by at east 50 mm (2 in.) of concrete and shall be located horizontally (within that portion of a concrete foundation or footing that is in direct contact with the earth) or within vertical (foundations or structural components or members that are in direct contact with the earth)."

Since a slab is not a vertical component, even if it is structural, rebar within it would not qualify as a CEE under the second part of that sentence. If the "slab" is a raft type foundation, then the rebar in it may qualify as a CEE under the first part of that sentence.

I am, of course, relying heavily on the parallelism of horizontal vs vertical in parsing the sentence this way, but as everything after "shall be located" is modifying that verb "located", which refers to position, the horizontal vs vertical parallelism strikes me as clearly the top level division.

Cheers, Wayne
 
jaggedben said:
That's not quite what the code says. If the slab is a 'structural component' then the rebar may still meet the definition.
Click to expand...
I see nothing in the description of the slab in the OP that would make it be required to be part of the GES. You are correct that although I mentioned footing the vertical component was left out of my response as I didn't see it as relevant.
 
You didn't mention the slab location. There are a few cases where bonding would be required. Art 547 & 680 for example.
 
wwhitney said:
"Metallic components shall be encased by at east 50 mm (2 in.) of concrete and shall be located horizontally (within that portion of a concrete foundation or footing that is in direct contact with the earth) or within vertical (foundations or structural components or members that are in direct contact with the earth)."
Click to expand...

I don't see the basis for parsing it the way you do instead of "within (vertical foundations or structural components or members that are in direct contact with the earth)." But in any case I'll fall back to the question of whether there is no case in which a 'slab' is a 'foundation'.

infinity said:
I see nothing in the description of the slab in the OP that would make it be required to be part of the GES. You are correct that although I mentioned footing the vertical component was left out of my response as I didn't see it as relevant.
Click to expand...

If the slab described were the only thing holding up a structure I'd say it'd count as a foundation.

Seems to me I've seen some metal barns that were built on top of only a slab, or at any rate any difference between slab and footing was invisible from the top side.

Note the part of the section that says if there are multiple CEEs you only have to use one. So if you have footings and a slab, you don't need to connect the slab, I agree. But if you only have a >4" thick slab with enough rebar in direct contact with the earth, I think the intent of the code is to use it.
 
jaggedben said:
I don't see the basis for parsing it the way you do instead of "within (vertical foundations or structural components or members that are in direct contact with the earth)."
Click to expand...
Because with your reading the entire phrase can be collapsed as "Metallic components shall be encased by at east 50 mm (2 in.) of concrete and shall be located . . . within . . . structural components . . . that are in direct contact with the earth)." There would be no reason to discuss horizontally or vertically.

So qualifying horizontal metal components may be located in foundations or footings; other qualifying metal components may be located within vertical foundations/components/members; and in both cases the members must be in direct contact with earth.

Admittedly, if the intended meaning was as simple as the version in the first paragraph of this post, the CMPs are perfectly capable of making it less clear by being overly verbose as in the language we have. : - )

Cheers, Wayne
 
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