Bonding to ground consists of connecting to the ground bus bar (terminals strip) in the main panel or (if the ground wire reaches there first) to the fat ground wire (grounding electrode conductor) coming out of the panel and going to a ground rod or to the main cold water pipe (if metal).
Size for this bonding ground wire for the pipe(s) should be that of the largest circuit wires passing near the pipe section in question.
If you did run a ground wire from an "isolated" section of metal plumbing down to the panel, other sections of plumbing can be connected with ground wires tapped (clamped) onto this first ground wire.
Also done quite often is running a ground wire (bonding jumper) from an isolated section of metal pipe along (or vaguely following) a section of plastic pipe as far as where metal pipe resumes. Six gauge bonding jumpers should be installed across the inlet and outlet of water heaters and water meters.
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