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Posted by John Grabowski on March 2, 2009, 9:27 pm
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> You can't. If you use two ground rods far apart, and measure the
> resistance between them, you are getting the sum of the resistance to
> earth of the two rods. So for measuring resistance to earth on a
> single ground rod you typically use a special three point tester,
> which provides enough information to isolate out the resistance of
> each of the ground probes. Since the tester is fairly expensive, and
> the test fairly complicated, it is much simpler just to drive two
> ground rods and skip any measurements.
> Cheers, Wayne
I had an extra ground rod concern (had one, needed an additional one)
a while back. The inspector suggested use of a single ground plate
(approx. 15" x 20") instead of the ground rod array. The plate was
only 30" deep. This was in northern Ontario. I didn't see plates
mentioned up-thread.
*The plate is certainly a viable and approved option.
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Posted by Dioclese on March 3, 2009, 9:52 am
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> On 2009-03-02, Dioclese <NONE> wrote:
>> No, the house and detached garage were finished at beginning of
>> 2005. Detached garage, its electrical installation, its entireity,
>> were built by myself. It was within NEC code at the time with one
>> grounding rod.
> That depends on the NEC code in force at the time it was built. The
> current NEC requires using the foundation rebar as a grounding
> electrode in new construction. I believe that requirement began with
> the 2005 NEC. Of course, your garage may have no rebar in its
> foundation.
The garage slab and driveway were constructed, poured, and finished by a
company that normally does commercial slabs and county road needs. The
garage has the standard rebar beam perimeter, remainder has bags with fill
overlaid with rebar. I don't know the gauge of the rebar. The rebar was
thicker in the beams than the remainder. The rebar in the remainder was
thicker than that used in the driveway. They also went out of their way to
put a miniature beams (not rebar reinforced vertically) on the driveway
perimeter.
I went by the NEC available and approved at the time in the late 2004 era.
As did the house builder. Both the house and detached garage have their own
single grounding 8' bar (copper), #4 copper bare wire and copper crimp
clamp.
show/hide quoted text
>> Just curious. How does one connect a standard 2 cable megger, with
>> spin handle for current, for testing resistivity for a grounding bar
>> regarding the maximum 25 ohm requirement?
> You can't. If you use two ground rods far apart, and measure the
> resistance between them, you are getting the sum of the resistance to
> earth of the two rods. So for measuring resistance to earth on a
> single ground rod you typically use a special three point tester,
> which provides enough information to isolate out the resistance of
> each of the ground probes. Since the tester is fairly expensive, and
> the test fairly complicated, it is much simpler just to drive two
> ground rods and skip any measurements.
> Cheers, Wayne
Which explains why the inspector don't do such tests... They just enforce
code, not establish if something meets code with testing. The 2 grounding
bar is his out.
--
Dave
CDOs are how we got here.
A modified version, new taxes in the future, is how Congress will get us
out?
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Posted by Wayne Whitney on March 3, 2009, 12:10 pm
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On 2009-03-03, Dioclese <NONE> wrote:
> I don't know the gauge of the rebar. The rebar was thicker in the
> beams than the remainder.
Since as far as I know #3 bar is the smallest normally used in
construction, if the rebar in the beams is thicker than #3, it is
thick enough to be as part of a concrete encased electrode.
show/hide quoted text
> I went by the NEC available and approved at the time in the late
> 2004 era. As did the house builder. Both the house and detached
> garage have their own single grounding 8' bar (copper), #4 copper
> bare wire and copper crimp clamp.
To answer your other question, here at least the code version in
effect at the time of permitting applies to the project; I'm not aware
of any jurisdictions that would apply a newly adopted code to ongoing
projects, but perhaps there are some.
Under the 2002 NEC, the requirement on which electrodes to use said to
use all "available" electrodes. This led to a debate about the
concrete encased electrodes. On the one hand, for new construction,
they are available in the sense that with proper planning you could
use them. On the other hand, common construction practice was for
building sites never to see an electrician until well after the
foundation was poured, at which point the electrode was not
"available". Certainly I expect that under 2002 NEC, there were not
many inspectors requiring the use of the concrete encased electrodes.
The 2005 NEC clarified this by changing "available" to "present".
Nowadays there are plenty of jurisdictions where the builder will be
required to jack out concrete to access the rebar if they fail to
connect to it as a conrete-encased electrode.
Hope this helps.
Cheers, Wayne
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Posted by jamesgangnc on March 4, 2009, 7:40 am
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> On 2009-03-03, Dioclese <NONE> wrote:
> > I don't know the gauge of the rebar. =A0The rebar was thicker in the
> > beams than the remainder.
> Since as far as I know #3 bar is the smallest normally used in
> construction, if the rebar in the beams is thicker than #3, it is
> thick enough to be as part of a concrete encased electrode.
> > I went by the NEC available and approved at the time in the late
> > 2004 era. =A0As did the house builder. =A0Both the house and detached
> > garage have their own single grounding 8' bar (copper), #4 copper
> > bare wire and copper crimp clamp.
> To answer your other question, here at least the code version in
> effect at the time of permitting applies to the project; I'm not aware
> of any jurisdictions that would apply a newly adopted code to ongoing
> projects, but perhaps there are some.
> Under the 2002 NEC, the requirement on which electrodes to use said to
> use all "available" electrodes. =A0This led to a debate about the
> concrete encased electrodes. =A0On the one hand, for new construction,
> they are available in the sense that with proper planning you could
> use them. =A0On the other hand, common construction practice was for
> building sites never to see an electrician until well after the
> foundation was poured, at which point the electrode was not
> "available". =A0Certainly I expect that under 2002 NEC, there were not
> many inspectors requiring the use of the concrete encased electrodes.
> The 2005 NEC clarified this by changing "available" to "present".
> Nowadays there are plenty of jurisdictions where the builder will be
> required to jack out concrete to access the rebar if they fail to
> connect to it as a conrete-encased electrode.
> Hope this helps.
> Cheers, Wayne
That's just stupid. If they are not accessible then they are not
present. I'll drive a couple copper clad rods.
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Posted by Wayne Whitney on March 4, 2009, 12:00 pm
show/hide quoted text
>> The 2005 NEC clarified this by changing "available" to "present".
>> Nowadays there are plenty of jurisdictions where the builder will
>> be required to jack out concrete to access the rebar if they fail
>> to connect to it as a conrete-encased electrode.
> That's just stupid. If they are not accessible then they are not
> present. I'll drive a couple copper clad rods.
This only applies to new construction. The rebar is certainly
accessible before the concrete is poured, and it is the builder's
responsibility to see to it that it is use as a grounding electrode.
Cheers, Wayne
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> resistance between them, you are getting the sum of the resistance to
> earth of the two rods. So for measuring resistance to earth on a
> single ground rod you typically use a special three point tester,
> which provides enough information to isolate out the resistance of
> each of the ground probes. Since the tester is fairly expensive, and
> the test fairly complicated, it is much simpler just to drive two
> ground rods and skip any measurements.
> Cheers, Wayne