renosteinke Wrote:
George...it IS a fine point, and one I've never seen contested- or even discussed.
I absolutely agree that the separate building needs a ground rod; lightning just might hit it, after all!
But, as I read the "25 ohm" requirement, that applies to the ground rod at the service drop, and every other ground rod- even on a separate building- is a 'supplemental' rod for that requirement. I do not see a requirement for the "25 ohms" to be met for each and every rod, or structure.
How would you test it, anyway? I can't imagine you separating the two for testing, when they are required to be connected to each other.
Reno
I have copied the sections that would appear to apply. What are we missing? Were is the language that would allow us to treat the required grounding electrode system as a 250.54 electrode. Supplementary electrodes are "connected to the equipment grounding conductors specified in 250.118" and no I don't think connection via the Grounding Electrode Conductor is what the code making panel intended.
The last sentence of 250.32 reads in part "the grounding electrode(s) required in Part III of this article shall be installed." 250.54 reads in part
"250.54 Supplementary Grounding Electrodes.
Supplementary grounding electrodes shall be permitted"
Supplementary electrodes are not "the grounding electrode(s) required in Part III of this article."
In short I think you are off your soundings on this one. The language is clear in requiring the installation of "the grounding electrode(s) required in Part III of this article." Since that means that single rod and plate electrodes must have a resistance to ground of twenty five ohms or less I have to believe that two electrodes are required.
250.32 Two or More Buildings or Structures Supplied from a Common Service.
(A) Grounding Electrode. Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s), the grounding electrode(s) required in Part III of this article at each building or structure shall be connected in the manner specified in 250.32(B) or (C). Where there are no existing grounding electrodes, the grounding electrode(s) required in Part III of this article shall be installed.
III. Grounding Electrode System and Grounding Electrode Conductor
250.50 Grounding Electrode System.
If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system. Where none of these electrodes are available, one or more of the electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.
250.54 Supplementary Grounding Electrodes.
Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as the sole equipment grounding conductor.
250.56 Resistance of Rod, Pipe, and Plate Electrodes.
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.