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Grounding rod spacing

Grounding rod spacing

(OP)
Hi,

If designing grounding for a new fabrication facility that is 500’W x 1000’L and that it is intended to have a perimeter ground grid, plus we plan on installing 10’L ground rods periodically, what's the appropriate spacing for those rods?

NEC 250.53 (B) requires a minimum of 6-ft between electrodes (for multiple rods situation), but no maximum spacing requirement. The article below suggests 2X of rod length (e.g., ~20 feet spacing for 10 feet L rod). However, it still results in way too many rods for an industrial facility in that size. Is it required to install them in this manner or is there any variance? Suggestions?

http://www.esgroundingsolutions.com/i-am-going-to-...

Thanks,
JS

RE: Grounding rod spacing

In my opinion, you have to follow the calculation method recommended in IEEE 80/2000-2013. You have to know the grounding fault maximum current, protection device fault clearing time, the ground soil resistivity and other.
You will start by calculating the touch and step permissible potentials and so will get the minimum number of vertical electrodes combined with the horizontal grounding wire length.
By the way, I think cuky2000 has an Excel program following the IEEE 80.

RE: Grounding rod spacing

Excellent, Scotty. Thank you.

RE: Grounding rod spacing

(OP)
Thanks a lot for sharing your experience and spreadsheet! It makes sense to go through the IEEE 80 process. Are there any empirical guidelines without doing those iterative calculation? In industrial facility ground design, quick and somewhat over-killed ground designs were seen in those applications. Soil tests don't seems to be always performed due to the owner's budget limit, neither is the IEEE 80 method (involved engineering efforts).

RE: Grounding rod spacing

Assuming that the facility is a building, IEEE-80 step- and touch-voltage calcs don't really apply. The building floor is a large semi-equipotential surface. Ground rods are not going to significantly change the step-voltage around the perimeter or touch-voltage to the building from what you would have with a perimeter ground wire alone. I don't know of any particular requirements for ground rod spacing for safety purposes. NFPA-780 requires lightning protection down conductors at least every 100' for structures exceeding 250' perimeter. Each down conductor would normally be connected to a ground rod (although there are other options). I would use this 100' requirement unless you find some other requirement.

RE: Grounding rod spacing

Quote (The building floor is a large semi-equipotential surface)

That will be correct if the building floor is directly in contact with the eaarth.
Many building floor slab use plastic vapor barrier that isolate the floor from ground as shown in the picture below. In this case the grounding requirement need to be addressed properly.

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