Originally Posted by
James Duffin
I think it is the first five visible feet...
From the NEC. (bold is mine)
- 250.52
Grounding Electrodes.
- - (A) Electrodes Permitted for Grounding.
- - - (1) Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 3.0 m (10 ft) or more (including any metal well casing effectively bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductors.
Interior metal water piping located more than 1.52 m (5 ft) from the point of entrance to the building shall not be used as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system.
- - - - Exception: In industrial and commercial buildings or structures where conditions of maintenance and supervision ensure that only qualified persons service the installation, interior metal water piping located more than 1.52 m (5 ft) from the point of entrance to the building shall be permitted as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system, provided that the entire length, other than short sections passing perpendicular through walls, floors, or ceilings, of the interior metal water pipe that is being used for the conductor is exposed.
Note: The 5 feet is referring to when the water piping is used as part of the grounding electrode system, not just for "bonding" of the interior metal water piping.
The reason is that, if used for the above purpose, beyond 5 feet inside the building, one could conceivably have a metal water pipe cut and repaired with a non-metallic fitting, thereby cutting the electrical path to ground out. The reasoning behind 'the first 5 feet' is that would be coming in through a wall and would be visible, allowing one to see if there was a problem with it.
Of course, though, the metal water piping *outside* is where it was likely cut off, with the metal water service pipe having been replaced with a PVC line - which means it is useless as a ground now anyway.
The above is referring to "grounding", not "bonding".