slab grounding

[wireperson]

does a slab need to be grounded for a residential addition? can i get a code reference?the existing service is already grounded and far from the new addition.

 

[raider1]

A concrete slab can't be used as a grounding electrode so there is no need to connect to any steel in that slab.

Chris

 

[wireperson]

the GC had a slab inspection today , He called me , saying that the inspector said that the slab needs to be grounded ....?? is it in the NEC ? or what code?

 

[Dennis Alwon]

He is mistaken. He is probably talking about a CEE (concrete encased electrode) which has to be in the footer not the slab itself.

 

[renosteinke]

I beg to differ.

My first intimation that I might disagree with the above opinions comes from local Northern Nevada experience, where they've been requiring ALL slab home foundations - even those without footers - to have the "Ufer" ground.

Reviewing 250.52(A)(3), I do not see any depth requirement for the slab at all, or even and requirement that the slab have rebar in it. (IRC does not require rebar in house slabs) You're only required to have 2" of concrete around the 'electrode.'

From that, I conclude that yes, even a surface-laid slab can qualify as a grounding electrode. (This only makes sense, as that it exactly the sort of foundation that Mr. Ufer was dealing with during the war, when he invented this sort of electrode).

Which returns us to the original question: Does the NEC require such? Well, if we back up to 250.50, the 'exception' tells us that we need not tie in the slab if the rebar is not accessible.

Mind you, my answer is referencing the 2011 code, where this section (250.52) has been extensively edited. I strongly recommend review of this section, as the new text is quite at variance with many opinions that I have heard over the years.

 

[Cavie]

the GC had a slab inspection today , He called me , saying that the inspector said that the slab needs to be grounded ....?? is it in the NEC ? or what code?

It is not the slabs or footers that are being grounded. It is the services that are being grounded using the UFER method. The old service is already grounded and needs nothing more. No UFER grounding is required for the addition. A new service being built on an old house also does not require UFER grounding.

 

[raberding]

what is the rest of the story?

what is the rest of the story?

perhaps there is more to the story...slab for a hot tub? whirlpool?

 

[iwire]

It is not the slabs or footers that are being grounded. It is the services that are being grounded using the UFER method. The old service is already grounded and needs nothing more. No UFER grounding is required for the addition. A new service being built on an old house also does not require UFER grounding.

Slabs never qualify as a uffer.

However, if the footings of the addition contain 20' or more of 1/2" or larger rebar connected together than you must connect it to the service.

250.50 Grounding Electrode System. All grounding electrodes
as described in 250.52(A)(1) through (A)(7) that are
present at each building or structure served shall be bonded
together to form the grounding electrode system......

 

[renosteinke]

Iwire, that's why I stressed checking the latest code edit.

250.50 exception says there's no need to connect the rebar if the rebar is not accessible.

The 250.52 section I cited sure looks to me as if every slab that's in contact with the ground can be used- there's no depth requirement at all. Just give me a 4-1/2" chunk of concrete that has either rebar, or a copper wire in it, and you're good to go.

I think we need to re-open the entire topic of 'what is a Ufer.'

(BTW, I agree completely with you as to service changes, etc.)

 

[ksmith846]

Slabs never qualify as a uffer.

However, if the footings of the addition contain 20' or more of 1/2" or larger rebar connected together than you must connect it to the service.

I wanted add that any electrode available is supposed to be connected to the service. So if they create one (I.E. the example above) you must use it.

 

[infinity]

Since when does a slab with rebar qualify as a CEE?

250.52(A)(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm (2 in.) of concrete, located horizontally near the bottom or vertically, and within that portion of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 6.0 m (20 ft) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm (? in.) in diameter, or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG. Reinforcing bars shall be permitted to be bonded together by the usual steel tie wires or other effective means. Where multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.

 

[Cavie]

Slabs never qualify as a uffer.

However, if the footings of the addition contain 20' or more of 1/2" or larger rebar connected together than you must connect it to the service.

Not in my neck of the woods.

 

[LarryFine]

It is not the slabs or footers that are being grounded. It is the services that are being grounded using the UFER method. The old service is already grounded and needs nothing more. No UFER grounding is required for the addition. A new service being built on an old house also does not require UFER grounding.

I tend to agree with this, except for a little shakiness on the last point, depending on the code cycle in force. I agree with you, and haven't had an issue with '05 and prior cycles here.

However, if the footings of the addition contain 20' or more of 1/2" or larger rebar connected together than you must connect it to the service.

The requirement is for service installations, not concrete installations. It's illogical to say that an existing service needs a new electrode just because something that qualifies as one is being built somewhere else on the property.

 

[iwire]

Iwire, that's why I stressed checking the latest code edit.

250.50 exception says there's no need to connect the rebar if the rebar is not accessible.

I agree, but if you are pouring a new footer it is available.

The 250.52 section I cited sure looks to me as if every slab that's in contact with the ground can be used- there's no depth requirement at all. Just give me a 4-1/2" chunk of concrete that has either rebar, or a copper wire in it, and you're good to go.

Well, you just taught me something, it has been rewritten for the 2011 NEC, it looks like it no longer has to be a footing, or foundation wall OR EVEN IN CONTACT WITH THE EARTH! :roll::roll: What the heck did they do?

I think we need to re-open the entire topic of 'what is a Ufer.'

It looks like the CMP did just that with the 2011.

I wanted add that any electrode available is supposed to be connected to the service. So if they create one (I.E. the example above) you must use it.

Thanks for the support.

Since when does a slab with rebar qualify as a CEE?

It did not used to, now I think it might. 2011 250.52(A)(3(1).

Not in my neck of the woods.

Well, I think we have established that your area makes its own rules.

The requirement is for service installations, not concrete installations. It's illogical to say that an existing service needs a new electrode just because something that qualifies as one is being built somewhere else on the property.

Larry you will have to show me the code section that limits 250.52 to new services.

Article 250s scope has nothing at all to do with services.

250.1 Scope. This article covers general requirements for
grounding and bonding of electrical installations, and the
specific requirements in (1) through (6).

(1) Systems, circuits, and equipment required, permitted,
or not permitted to be grounded

(2) Circuit conductor to be grounded on grounded systems

(3) Location of grounding connections

(4) Types and sizes of grounding and bonding conductors
and electrodes

(5) Methods of grounding and bonding

(6) Conditions under which guards, isolation, or insulation
may be substituted for grounding

If you add electrodes they must be connected.


Of course maybe the the EI might let you call it an 'Auxiliary Grounding Electrode' and connect it too an EGC. <JK>

 

[realolman]

im usually all wet

im usually all wet

but I got the idea that the OP was talking about grounding the slab... like you would a washing machine or something... not using the slab for a ufer.

somebody mentioned a hot tub

 

[tom baker]

does a slab need to be grounded for a residential addition? can i get a code reference?the existing service is already grounded and far from the new addition.

A concrete encased electrode has been required since the 2005 NEC see 250.52(A)(3). The language is for reinforcing located in a footing. A slab does not have a footing. Having said that, using the reinforcing steel in a footing would be nearly the same (for a ground) as in a footing.
Read the language and discuss with your AHJ.

 

[taylorp]

.



Larry you will have to show me the code section that limits 250.52 to new services.

Article 250s scope has nothing at all to do with services.

If you add electrodes they must be connected.


Of course maybe the the EI might let you call it an 'Auxiliary Grounding Electrode' and connect it too an EGC. <JK>

I agree with iwire.

 

[Cavie]

A concrete encased electrode has been required since the 2005 NEC see 250.52(A)(3). The language is for reinforcing located in a footing. A slab does not have a footing. Having said that, using the reinforcing steel in a footing would be nearly the same (for a ground) as in a footing.
Read the language and discuss with your AHJ.

Well here in Florida we don't have basements. We have monolithic slabs. Thats a slab and a 16" x 24" footer (in most cases) all in one. The footer has rebar in it. The slab has wire mesh or fiber reinforcing.

 

[OregonSE]

does a slab need to be grounded for a residential addition? can i get a code reference?the existing service is already grounded and far from the new addition.

We just had a similar case where grounding was required and no reinforcing steel was present. 680.26(B)(2) using the alternate means.

(2) Perimeter Surfaces. An equipotential bonding grid shall extend 3 ft horizontally beyond the inside walls of a pool, outdoor spa, or outdoor hot tub, including unpaved, paved, and poured concrete surfaces. The bonding grid shall comply with (a) or (b) and be attached to the conductive pool reinforcing steel at a minimum of four points uniformly spaced around the perimeter of the walls of a pool, outdoor spa, or outdoor hot tub.
(a) Structural Reinforcing Steel. Structural reinforcing steel [680.26(B)(1)(a)]. Author?s Comment: The 2008 NEC does not provide any guidance on the installation requirements for structural reinforcing steel when used as a perimeter equipotential bonding grid.
(b) Alternate Means. Equipotential bonding conductor meeting the following:
(1) 8 AWG bare solid copper bonding conductor.
(2) The bonding conductor shall follow the contour of the perimeter surface.
(3) Listed splicing devices.
(4) Bonding conductor shall be 18 to 24 in. from the inside walls of the pool.
(5) Bonding conductor shall be secured within or under the perimeter surface 4 to 6 in. below the subgrade.

 

[Dennis Alwon]

The bonding of the perimeter surface is not the same as grounding the slab(whatever that means) or the same as a ufer.