Originally Posted By: roconnor
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Tim … good job explaining the basic service grounding requirements. The rule I always followed for a new house is two (2) grounding points for a service ground, unless testing is performed.
For a new house the code allows a service ground to a metal water service pipe with at least 10 of length in the earth, but there also must be a supplemental ground rod (2 ground points). This supplemental ground rod is not required to meet the 25 ohm resistance the way I read the code, unless you take a very restrictive position (a little much in my opinion).
Also, I have heard that some water companies now require an isolation coupling between the house and the street connection to help avoid shock hazards for water company workers, so you have to be aware when a house is very close to the street (or an existing ground is to an abandoned service pipe) that the pipe is embedded at least 10 feet into the ground.
For plastic water service pipes, or pipes embedded less than 10 feet, the service ground can be just a single ground rod with a resistance of less than 25 ohms. But, this requires testing (I am also a p/t AHJ, and the contractor is required to perform the testing to prove the 25 ohm resistance ... the AHJ isn't responsible for testing).
The code provides for the installation of a second ground rod where this resistance is not achieved (2 ground points). Again, where two ground points are provided testing is not required. It is usually easier for a contractor to just install the two ground rods to avoid the resistance testing.
There are other options when you have a plastic water service pipe, like a ground ring around the house or concrete encased electrode within or near the footing at least 20 feet long, although these options are not as common.
Older homes may only have one ground point to the metal water service pipe, as the supplemental ground rod is somewhat a newer requirement. In my opinion this is acceptable for an existing home as long as it is not an abandoned service and it?s a tight connection before the water meter within 5 feet of the house entrance. On some houses the ground clamp is loose or may have been re-attached to a section of pipe that wasn't? cleaned, which needs to be corrected.