From the 2000 IRC. ( the one my jurisdiction uses) :
E3508.1.1 Metal underground water pipe.Ametal underground
water pipe in direct contact with the earth for10 feet
(3048 mm) or more and electrically continuous to the points
of connection of the grounding electrode conductor and the
bonding conductors shall be considered as a grounding electrode.
Continuity of the grounding path or the bonding connection
to interior piping shall not rely on water meters or
filtering devices and similar equipment. A metal underground
water pipe shall be supplemented by an additional
electrode of a type specified in this section or in Section
E3508.2. The supplemental electrode shall be bonded to the
grounding electrode conductor, the grounded service-entrance
conductor, the grounded service raceway or any
grounded service enclosure.
Where the supplemental electrode is a made electrode in
accordance with Section E3508.2, that portion of the bond-
ing jumper that is the sole connection to the supplemental
grounding electrode shall not be required to be larger than
No. 6 copper wire or No. 4 aluminum wire.
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