According to NEC^® 250.52(A)(1), metal underground water piping that is electrically continuous and in direct contact with the earth for at least 10 feet qualifies as a grounding electrode.

The grounding electrode conductor between the water pipe grounding electrode mentioned above and the service is sized from NEC^® Table 250.66 which never requires larger than 3/0 copper or 250 kcmil aluminum.

Often, water piping is installed in a building in a way that it does not qualify as an actual grounding electrode. Such water piping is still required to be joined to the serviced by a bonding jumper.

The bonding jumper from the service to “other metal water piping”, not meeting the criteria to be considered a grounding electrode in 250.52(A)(1), is sized from NEC^® Table 250.102(C)(1) and is never required to be larger than 3/0 copper or 250 kcmil aluminum.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.

2020 Code Language:

250.104(A)(1) General. Metal water piping system(s) installed in or attached to a building or structure shall be bonded to any of the following:

(1) Service equipment enclosure

(2) Grounded conductor at the service

(3) Grounding electrode conductor, if of sufficient size

(4) One or more grounding electrodes used, if the grounding electrode conductor or bonding jumper to the grounding electrode is of sufficient size

The bonding jumper(s) shall be installed in accordance with 250.64(A), 250.64(B), and 250.64(E). The points of attachment of the bonding jumper(s) shall be accessible. The bonding jumper(s) shall be sized in accordance with Table 250.102(C)(1) except that it shall not be required to be larger than 3/0 copper or 250 kcmil aluminum or copper-clad aluminum and except as permitted in 250.104(A)(2) and 250.104(A)⁠(3).