200 A Service, a Metal Water Pipe, and an Eager Plumber
A contractor is upgrading a single-family dwelling to a 200 A, 120/240 V overhead service. The service-entrance conductors are 2/0 AWG copper, and the meter-main on the exterior wall has the main bonding jumper installed. The metal water service pipe enters the basement with 12 ft of burial ahead of the curb stop; between the entry point and the water heater there is a removable water meter and a dielectric union. A 1/2 in. by 8 ft copper-clad ground rod is driven at the foundation. The helper has already pulled 6 AWG copper to the water pipe and 6 AWG copper to the rod. The plumber, who is rewiring the gas piping himself, suggests landing the grounding electrode conductor on the buried steel gas service 'since it is already in the ground and it's steel.'
1. Size the grounding electrode conductor to the water pipe and to the rod. Is the helper's 6 AWG adequate?
Read the worked answer
Water pipe electrode (Table 250.66): the table is keyed to the largest ungrounded service-entrance conductor. For 2/0 AWG copper, the required copper grounding electrode conductor is 4 AWG.
- The helper's 6 AWG to the water pipe is undersized and must be replaced.
Ground rod (250.66(A)): the grounding electrode conductor to a rod, pipe, or plate electrode is not required to be larger than 6 AWG copper. The helper's 6 AWG to the rod is acceptable.
Also verify:
- The water pipe connection must be made within 5 ft of the point where the pipe enters the building, on the street side of any meter or nonmetallic fitting, and the connection must remain accessible (250.68(A) and its exception).
- The metal water piping system is also bonded per 250.104(A) sized by Table 250.66 - here 4 AWG copper - which in this layout is satisfied by the electrode conductor itself.
- The 1/2 in. x 8 ft rod must be listed and driven the full 8 ft of contact length.
2. Is the ground rod required at all, and what must be done about the water meter and the dielectric union?
Read the worked answer
Yes, the rod is required.
- A metal underground water pipe with 10 ft or more of earth contact is a permitted grounding electrode (250.52(A)(5)-type electrode), but 250.53(D)(2) requires a supplemental electrode to be installed and bonded to the water pipe electrode. The 8 ft rod satisfies that supplemental requirement; it must be bonded to the water pipe electrode with a conductor sized per 250.66, capped at 6 AWG copper for a rod.
Water meter: a removable meter interrupts the metallic path. A bonding jumper must be installed around the meter (and any other removable/nonmetallic section) so the electrode path is continuous; size it per Table 250.66, here 4 AWG copper.
Dielectric union: if the union isolates downstream interior metal piping from the electrode, the interior piping is still required to be bonded per 250.104(A) with a 4 AWG copper bonding jumper, with the connection accessible.
Sequence note: the grounding electrode conductor and the bonding jumpers serve different functions; do not use a single undersized conductor to do both jobs.
3. Evaluate the plumber's suggestion to use the buried steel gas service as the grounding electrode.
Read the worked answer
Not permitted - reject it.
- Section 250.52(B) explicitly prohibits using underground metal gas piping as a grounding electrode. The prohibition exists because a fault current or lightning surge on that piping creates a fire and explosion hazard at joints and at the service regulator.
- The gas piping is also not a permitted conductor for any grounding or bonding path.
- Interior metal gas piping may still require bonding to the grounding electrode system where likely to be energized (250.104(B)), using the equipment grounding conductor of the circuit likely to energize it, or another permitted method. Bonding is not the same as using it as an electrode, and the AHJ should be consulted on the accepted method.
Document it: note the refusal on the inspection record, because the plumber's work is separately subject to permit and inspection.