Service Grounding Electrode Conductor and Water-Pipe Electrode
A 200 A, 120/240 V single-phase service is being replaced in a rural New Mexico home. The service conductors are 2/0 AWG copper. The metal water main enters the house and has 12 ft of buried copper pipe before the meter; the meter is 15 ft inside the house, and the interior piping is mostly PEX with short copper stubs at fixtures. There is an existing 8 ft ground rod at the service, connected with a 6 AWG copper conductor. The HVAC contractor bonded the gas line to the cold-water pipe with a 10 AWG green conductor. The electrician proposes to run a 6 AWG copper grounding electrode conductor from the panel to the water pipe, jump around the meter with the same conductor, and rely on the water pipe as the only electrode because 'the rod is probably rotten.' Table 250.66 requires 4 AWG copper for 2/0 AWG copper service conductors. The inspector questions the water-pipe connection point and the gas bonding.
1. Evaluate the proposed 6 AWG grounding electrode conductor and meter jumper. What size is required, and where must the water-pipe connection be made?
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The proposed 6 AWG grounding electrode conductor is undersized. For 2/0 AWG copper service conductors, Table 250.66 requires 4 AWG copper, so the run from the service to the water pipe must be at least 4 AWG copper. The bonding jumper around the water meter must also be sized in accordance with Table 250.66, so the same 6 AWG jumper is undersized and should be 4 AWG copper. The connection to a metal underground water pipe electrode must be made within 5 ft of the point where the pipe enters the building, before the meter and before the interior PEX transition; connecting 15 ft inside at the meter is not acceptable. The water pipe may qualify as an electrode because it has at least 10 ft of buried metal in contact with earth, but the connection point and conductor size must be corrected.
2. May the water pipe be the only grounding electrode? Explain the supplemental electrode requirement.
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No. A metal underground water pipe that qualifies as a grounding electrode must be supplemented by another electrode, such as the existing ground rod, because the water pipe may be replaced or isolated later. The 12 ft buried section qualifies the pipe as an electrode, but it cannot be used alone. The 8 ft ground rod can serve as the supplemental electrode, and a conductor to a ground rod is not required to be larger than 6 AWG copper under the electrode conductor rules, so the existing 6 AWG rod conductor is not automatically a violation. The electrician should not remove or abandon the rod merely because it may be old; it should be inspected, tested if necessary, and reconnected as the supplemental electrode. The service must have a complete grounding electrode system, not a single water-pipe dependence.
3. Is the 10 AWG gas-line bond clearly a violation? Explain how to judge it.
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It is not automatically a violation just because it is 10 AWG. Bonding of gas piping that is likely to become energized is sized by the equipment grounding conductor table based on the rating of the circuit that could energize the piping, not by the grounding electrode conductor table. If the circuit likely to energize the gas line is a 20 A circuit, 12 AWG copper is the minimum, so 10 AWG is larger than required; if it is a 60 A circuit, 10 AWG is still the table value. The bond must connect to the grounding electrode system, the equipment grounding conductor, or the service equipment enclosure, and it must be installed so it is not dependent on a removable connection. The electrician should identify the likely energizing circuit and verify the connection point and any local gas-code bonding requirement. If a 100 A or larger circuit could energize the piping, 10 AWG would be undersized and would need to be increased.