Footing Bond

[wboz]

The project is a new school with a poured footing and foundation. We installed the standard 20' of
#4 with the approved clamp to the rebar.
The footing was poured in approx 12 seperate pours
with rebar extending from section to section.
The inspector states it's the electrical contractors responsability to install a bond jumper from section to section and provide
a bonding certificate for all 12 sections. Also,
each section of footing was inspected by the engineer and the local building official.

 

[ryan_618]

Re: Footing Bond

Ask the inspector for a code reference.

 

[eprice]

Re: Footing Bond

Has the 2005 NEC been adopted yet in your area?

When the footing was formed for each pour, was rebar left sticking out so that bar for the next pour could be tied to it? Or were dowels epoxied into the previous pour after the fact?

If the 2005 has been adopted, and rebar in each pour was not tied to rebar in the next pour using "the usual steel tie wires or other effective means" then some jurisdictions could view this as multiple concrete-encased electrodes that all need to be used.

Just a thought...

 

[infinity]

Re: Footing Bond

According to 250.52(3) only 20 feet of 1/2" or larger rebar is required to be bonded. Once you've reached 20 feet you're done. No need to go any further. Also once you connect to the 20 feet of rebar no bare #4 AWG is required in the footing. So for my dollar I'd connect to the 20 foot section of rebar nearest to the service with a #4 AWG copper conductor with an approved clamp and this would satisfy the requirement.

 

[eprice]

Re: Footing Bond

infinity,

The way this is looked at may change under the 2005 NEC. I don't have a copy of that document, since we have not adopted it here yet, but I believe the words "If available" in section 250.50 have been changed to "If present". Yes, 20' of rebar satisfies the definition of a concrete-encased electrode, but if there are more than one section of rebar at least 20' long which are not bonded to each other, would we not have more than one concrete-encased electrod present at the building?

As I was writing this, it just dawned on me that the typical residential footing in many areas consists of two parallel rows of rebar. Would this not be two concrete-encased electrodes? Will the chang in language in 250.50 not require a connection to each of these?

 

[infinity]

Re: Footing Bond

I think of it more as an established standard that 20' of rebar, or 20' of bare #4 AWG embedded within the concrete and in contact with the earth is all that is needed or required by the NEC. This IMO is along the lines of have a #6 AWG copper conductor to a ground rod regardless of the size of the service where you reach a point of not improving the quality of the electrode.

 

[pierre]

Re: Footing Bond

Eprice

"As I was writing this, it just dawned on me that the typical residential footing in many areas consists of two parallel rows of rebar. Would this not be two concrete-encased electrodes? Will the chang in language in 250.50 not require a connection to each of these?"

That is a scary thought

I do not think that is the intent of that section.

 

[George Stolz]

Re: Footing Bond

At the risk of exposing my ignorance (which I will flash defiantly from time to time): How conductive is concrete? There has been mention in other threads dancing around this, and I didn't have the courage to ask.

 

[pierre]

Re: Footing Bond

Concrete and the chemicals in concrete actually are very good conductors.
Next time you have a chance, test from a concrete floor, to a live conductor and you will get a voltage, usually very close to the voltage of the system/circuit.
I believe the term used is "Hydrostatic".

 

[ga77]

Re: Footing Bond

Originally posted by georgestolz:
At the risk of exposing my ignorance (which I will flash defiantly from time to time): How conductive is concrete? There has been mention in other threads dancing around this, and I didn't have the courage to ask.

Search the internet for "Ufer" and you will find a lot of interesting articles. Concrete encased rebar is one of the most effective grounds.

 

[George Stolz]

Re: Footing Bond

ga77, I'm aware that Ufer's are effective.

I was curious about straight-up concrete, no rebar.

Is the conductivity good enough that connecting to just one set is effectively using the entire portion of concrete, is the question I'm creeping up on.

 

[ga77]

Re: Footing Bond

The concrete is actually the connection to earth, not the rebar. The rebar establishes the connection to the concrete, therefore the one 20' section is sufficient. There will probably be a clarification of this in 2008. A literal reading of the Code would appear to require all 20' lengths to be connected, but as I understand it that was not the intent.

 

[don_resqcapt19]

Re: Footing Bond

Pierre,

I believe the term used is "Hydrostatic".

I think that the correct term is "hydroscopic".
"A hygroscopic or hydroscopic substance is a substance that absorbs water readily from its surroundings."
The concrete absorbs moisture from the ground and becomes more conductive.
Don

 

[infinity]

Re: Footing Bond

The way this is looked at may change under the 2005 NEC. I don't have a copy of that document, since we have not adopted it here yet, but I believe the words "If available" in section 250.50 have been changed to "If present". Yes, 20' of rebar satisfies the definition of a concrete-encased electrode, but if there are more than one section of rebar at least 20' long which are not bonded to each other, would we not have more than one concrete-encased electrode present at the building?

I still don't see this as a requirement to bond separate sections of rebar together. The wording in 250.52(3) says " An electrode which IMO means one section 20' in length. 250.52(1) says " A metal underground water pipe" which IMO opinion means one underground water pipe. If there were more than one water pipe would we be required to connect to all of them as well?