Trick, I think you are confusing "bonding" with "grounding."
This is also an area that is usually modified by your local codes, so you will want to check thing out at city hall, as well.
The wire that goes to your ground rod....the "grounding electrode conductor"....is specified by the NEC to need be no larger than #6, without regard to the size of the service. Many places have raised this to a minimum of #4 solid, and further require it be in pipe for extra protection.
Metal parts of the house that are likely to become energised are required to be "bonded" to the system. Such thing include the water line...but the gas service again is an area that local codes differ about. In any event, the NEC specifies a #10 bonding jumper, but many places specify at least a #8 wire be used.
You are no longer allowed to use the water service as a "ground;" you need that ground rod. You are, howerver, required to "bond" the water line.
Water Line Service Grounding Electroide Systems (250.52) Sized per 250.66 (#4) If theres 10 ft. or Metal underground. If not its then its Bonding (See 250.104(A.1))Still sized by 250.66.
Gas piping 250.104B (Other metal Piping) #6 copper . Unless pipe is grounded by an equipment grounding conductor of a circuit thats using the Gas . The equipment grounding conductor is then an effect bond.
Where I am in what has been refered to as the soveriegn city, of the soveriegn state of SF, CA. Water is required as an electrode and/or bond depending on how you look at it, and if a UFER is involved. Water with no UFER, would be supplimented with rods. With a Ufer, which would be full sized, water would be bonded anyway, in the proccess making it an electrode....
Hot to cold at the water heater...
Gas bond, at the second fitting past the meter - required! They'll even let you use the GEC so long as it is not between the electrode and panel.
Mark Heller "Well - I oughta....." -Jackie Gleason
Perhaps I should have said you're not allowed to use the water line as your ONLY ground....250.53(D) ays the pipe shall comply with (1) and (2)...and (2) says the pipe shall be supplimented. We often forget that there are type of non-conductive pipe that pre-date plastic. Many cities still have wood water mains...and, more recently, WWII started the use of "orangeburg," a pipe made of asphalt-impregnated cardboard.
As far as the bond wire needing to be !0 or larger, I stand corrected....the NEC calls for it to be at least #8. Formerly, this was a local ammendment- it appears that the NEC has caught up! 250.104 is what I am referring to.
To be clear we are required to use a metal water line as an electrode as long as there is 10' of it in direct contact with the earth. It does not matter what is beyond that 10'. An 8' rod is an electrode so it only follows that 10' of metal water line in the ground is an electrode. It will not matter if that 10' of metal line is supplied by non-conductive mains.
In the big picture it may be better if you water line was not electrically continuous with your neighbors.
Yes we must supplement it with one or sometimes two additional electrodes. My guess is that is required as many times people disconnect the GEC from the water line and do not put it back.
I will have to look but I do not believe there is a minimum size bonding conductor for a gas line.
Bob Badger Construction & Maintenance Electrician Massachusetts