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1) Per Article 250.52(B)(1) Not Permitted for Use as Grounding Electrodes within the National Electrical Code states the following:
The following systems and materials shall not be used as grounding electrodes:
(1) Metal underground gas piping systems
2) The grounding electrodes used in a residential electrical distribution system are a minimum of (1) one 8 foot exterior ground rod at the meter socket area to the main panel neutral bus bar and a grounding electrode conductor installed from the main panel neutral bus bar to the street side of a city metal water supply pipe and then jumpered back across the water meter.
3) Metal Gas piping is typically bonded at a gas hot water heater area to the water heater metal supply (hot & cold) piping based on the size of the service. One contiguous bonding jumper conductor is installed from the cold to the hot water metal supply pipes and then to the metal gas pipe that supplies the hot water heater. Metal gas pipe bonding can actually take place anywhere inside the home where the water supply pipes are metal and the gas pipe is metal. However, the most common area to bond these 3 metal pipes are at a gas hot water location.
If the service amps to the main breaker = 100 amps, then a 8 AWG copper bonding conductor is installed.
If the service amps to the main breaker = 200 amps, then a 6 AWG copper bonding conductor is installed.
4) Keep in mind that grounding and bonding are 2 different applications.