Ground rods for rocky ground mount array

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bbartmasse r

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Are two ground rods always needed for ground mounted arrays? Our site is basically a big rock, we've drilled and epoxied for footers. Definitely not a chance of getting a ground rod in there, maybe a ground plate.

Are they always required? I always thought driving a ground rod for a ground mounted array was funny because usually one of the footers is so far in the ground that if you connected to it, it would be a better ground than any rod you tried to drive.

Anybody have any creative solutions for rocky sites?

thanks
 
bbartmasse r said:
Are two ground rods always needed for ground mounted arrays? Our site is basically a big rock, we've drilled and epoxied for footers. Definitely not a chance of getting a ground rod in there, maybe a ground plate.

Are they always required? I always thought driving a ground rod for a ground mounted array was funny because usually one of the footers is so far in the ground that if you connected to it, it would be a better ground than any rod you tried to drive.

Anybody have any creative solutions for rocky sites?

thanks
Click to expand...

250.53 and 250.54 show some alternatives, but we are dealing with just such a situation and the AHJ has given us the OK to consider the concrete and steel footers of the array to be the electrode.
 
ggunn said:
250.53 and 250.54 show some alternatives, but we are dealing with just such a situation and the AHJ has given us the OK to consider the concrete and steel footers of the array to be the electrode.
Click to expand...

what gives the AHJ the authority to decide such a thing? If the code [or local amendments] does not say something is a legal GE it is not a legal GE short of going through an official waiver process if such a thing exists in that jurisdiction.

Although you might decide that it is a metal underground structure that is allowed to serve as a GE.
 
Are they always required?

I'd say nope.

690.47
(D) Additional Auxiliary Electrodes for Array Grounding.
A grounding electrode shall be installed in accordance
with 250.52 and 250.54 at the location of all ground- and
pole-mounted PV arrays and as close as practicable to the
location of roof-mounted PV arrays. The electrodes shall be
connected directly to the array frame(s) or structure. The dc
grounding electrode conductor shall be sized according to
250.166. Additional electrodes are not permitted t() be used
as a substitute for equipment bonding or equipment grounding
conductor requirements. The structure of a ground- or pole-mounted PV array shall be permitted to be considered
a grounding electrode if it meets the requirements of 250.52.


250.52 Grounding Electrodes.
(A) Electrodes Permitted for Grounding.

(2) Metal Frame of the Building or Structure. The metal
frame of the building or structure that is connected to the
earth by one or more of the following methods:
(1) At least one structural metal member that is in direct
contact with the earth for 3.0 m (10 ft) or more, with or
without concrete encasement.

(7) Plate Electrodes. Each plate electrode shall expose not
less than 0.186 m2 (2 ft2) of surface to exterior soil. Electrodes
of bare or conductively coated iron or steel plates
shall be at least 6.4 mm (1/4 in.) in thickness. Solid, uncoated
electrodes of nonferrous metal shall be at least
1.5 mm (0.06 in.) in thickness.

(8) Other Local Metal Underground Systems or Structures.
Other local metal underground systems or structures
such as piping systems, underground tanks, and underground
metal well casings that are not bonded to a metal
water pipe.

(B) Not Permitted for Use as Grounding Electrodes.
The following systems and materials shall not be used as
grounding electrodes:
(1) Metal underground gas piping systems
(2) Aluminum
 
petersonra said:
what gives the AHJ the authority to decide such a thing? If the code [or local amendments] does not say something is a legal GE it is not a legal GE short of going through an official waiver process if such a thing exists in that jurisdiction.

Although you might decide that it is a metal underground structure that is allowed to serve as a GE.
Click to expand...
I can decide anything I want but the AHJ has the authority to pass or fail my inspection. 250.52(A)(3) is sufficiently vague that the AHJ's interpretation calls the shot.
 
cuba_pete said:
Well, "authority" is in the name...

Here's an article.

Basically, if they are the people you would normally pull a permit through and they inspect it before and after then they are the authority.
Click to expand...

It is certainly true that the AHJ has whatever authority it was given by whatever law was passed making it the AHJ.

Unfortunately a fair number of people here do not understand the difference between the AHJ and the local inspector.
 
ggunn said:
I can decide anything I want but the AHJ has the authority to pass or fail my inspection. 250.52(A)(3) is sufficiently vague that the AHJ's interpretation calls the shot.
Click to expand...

This provision is not especially vague. If it meets the criteria it is a legal GE. You might also have a legal GE under 250.52(A)(8).

The inspector has an obligation to inspect in accordance with the rules the AHJ has laid down. That requires s/he abides by what the enacted code actually says. It does not always work that way for various reasons, but that is the way it is supposed to work.
 
It may be worth pointing out that the 2011 and 2017 codes don't require an array electrode at all.

If you have a ground mount array with many metal supports that collectively are many times 10ft of metal in the earth, then a reasonable AHJ will not require more, in my opinion. The expoxy mentioned in the original post gives me some pause, though.
 
jaggedben said:
It may be worth pointing out that the 2011 and 2017 codes don't require an array electrode at all.

If you have a ground mount array with many metal supports that collectively are many times 10ft of metal in the earth, then a reasonable AHJ will not require more, in my opinion. The expoxy mentioned in the original post gives me some pause, though.
Click to expand...

If a legal GE is required, then a reasonable inspector would require compliance and not make up his own rules.
 
petersonra said:
If a legal GE is required, then a reasonable inspector would require compliance and not make up his own rules.
Click to expand...

You're entitled to your opinion about what constitutes reasonableness. The second paragraph of 90.4, in my opinion, provides cover for an AHJ to declare that such an array has adequate grounding.
 
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