All interior metal pipe has to be bonded to the electrical ground if not being used as the electrical ground or grounding conductor. At any place in the piping where an appliance/equipment (eg. water meter, water heater, softener, filters) is installed that can be removed for servicing/repair or is not metalic itself then a jumper must be installed to keep a permanent path for the electtrical ground.
If a section of water pipe is isolated from electrical ground, it is possible for it to become energized from a bare wire touching it/appliance malfunction, allowing persons to be electrocuted by touching faucets connected to this section of piping. With all the pipe bonded together with jumpers the electricity will take least resistance to ground and should trip the circuit breaker.

I generally use #6 bare copper for the jumper between the hot and cold but for a correct installation it should be sized according to the size of your electrical service and depending if your water pipes are being considered as the grounding conductor or if they are just bonded to the ground, usually the later as i dont think interior pipes beyond 5' of its entrance into the building should be used as the conductor.