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How is the 5' measured when connecting the grounding electrode conductor to a metallic water service? Is it along the pipe or 5' horizonal from entrance?
I would interpret it as being along the pipe. From the point the pipe enters the building to a distance of 5' along the pipe can be used for the grounding electrode system.
How is the 5' measured when connecting the grounding electrode conductor to a metallic water service? Is it along the pipe or 5' horizontal from entrance?
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Answer: Along the length of the interior metal water pipe that is being used for the grounding electrode. This is implied also by NEC ART 250.30(A)(4)(2)Exception.
Purpose being the limitation and reduction in the possibility of the interruption of current return path by means of plumbing system maintenance and/or any damage.
Measuring 5' from entrance horizontally could add to the grounding electrode any number of plumbing couplings and elbows and such that can fail or be removed and replaced during repair, maintenance retrofitting, at any time.
Measuring 5' along the length of the pipe will usually limit the number of fittings of consequence to the PRV which you are required to jumper by NEC ART. 250.53 (D)(1)limiting and reducing as much as possible the probability of interruption of continuity of the grounding path.