200 A Service Rebuild and a Legacy 3-Wire Feeder to a Detached Garage
A 200 A, 120/240 V single-phase overhead service to a dwelling is being rebuilt. The service-entrance conductors are 2/0 Cu. The only grounding electrode is a metallic water pipe with 12 ft of buried metal; the water utility is replacing the street-side portion with plastic, leaving roughly 8 ft of metallic pipe in contact with earth. Two 8 ft ground rods are on the truck. A detached garage 90 ft away is served by an existing feeder of three 6 AWG Cu conductors (two ungrounded plus a grounded conductor) in PVC with no equipment grounding conductor and no grounding electrode at the garage. There is no metallic path between the two buildings. A 60 A breaker protects the garage feeder. The owner wants to keep the existing feeder.
1. Size the grounding electrode conductors to the water pipe and to the ground rods, and address the plastic replacement.
Read the worked answer
Water pipe. A metal underground water pipe qualifies as a grounding electrode only with at least 10 ft in contact with earth (250.52(A)(1)). With the street-side section going plastic, the remaining 8 ft no longer qualifies as an electrode - but the interior metal piping still must be bonded per 250.104(A).
- If the pipe still qualified, Table 250.66 for 2/0 Cu service conductors gives a 4 AWG Cu GEC.
- Because it no longer qualifies, the grounding electrode system must be built on the ground rods, and the metal water piping is bonded as a metal piping system. Bonding jumper sizing follows 250.104(A) and is based on the service conductor size, so 4 AWG Cu is still a defensible answer, and installing it now avoids a rework when the pipe is cut.
- A water pipe electrode requires a supplemental electrode (250.53(D)(2)), and a single rod requires a supplemental electrode unless it measures 25 ohms or less (250.53(A)(2)). With two rods driven at least 6 ft apart, the 25-ohm measurement is not required.
Rods. 250.66(A): the GEC to a rod, pipe, or plate electrode is not required to be larger than 6 AWG Cu. Bond each rod to the others and to the service with 6 AWG Cu. Note the rod electrode itself must be at least 8 ft in length and driven below permanent moisture level, and the connection must be listed and accessible.
Judgment point: the inspector may ask for the 25-ohm reading or for the supplemental electrode. Have the two-rod installation and the 6 AWG bonding jumper visible and documented; the GEC to the water pipe stays at 4 AWG irrespective of the plastic cut.
2. Is the existing 3-wire garage feeder with no equipment grounding conductor acceptable? What changes if the feeder is replaced?
Read the worked answer
Existing feeder. 250.32(B)(1) normally requires an equipment grounding conductor run with the feeder to a separate building. The Exception for existing installations permits the grounded conductor to serve as the ground-fault return path where (a) an EGC is not provided, (b) there is no continuous metallic path between the buildings, and (c) the grounded conductor is sized adequately for the overcurrent device. All three conditions appear to be met here: PVC, no metallic path, 6 AWG Cu for a 60 A circuit.
What that exception still requires:
- A grounding electrode system at the garage (250.32(A)). A ground rod and a 6 AWG Cu GEC must be installed even though the feeder is ungrounded.
- The grounded conductor is bonded to the panel enclosure and the grounding electrode at the garage, and there must be no parallel metallic path.
- If any metallic path is later created (metal conduit, metal water line between buildings, metal gas piping), the exception is void and the installation becomes noncompliant.
If the feeder is replaced. A new feeder must be 4-wire: two ungrounded, one grounded, and one equipment grounding conductor. For a 60 A feeder the EGC is 10 AWG Cu per Table 250.122. The neutral bar at the garage must be isolated from the enclosure, the EGC terminates on the enclosure, and the grounded conductor lands on an isolated neutral.
Judgment point: do not "fix" the existing feeder by adding a rod and a separate EGC while leaving the neutral bonded at the garage - that creates a parallel return path on the new EGC and is a violation.
3. The inspector asks whether the garage feeder's grounded conductor is correctly sized. What is your response, and what if a metal water line exists at the garage?
Read the worked answer
Sizing. Under the 250.32(B)(1) Exception, the grounded conductor serving as the ground-fault return path must be sized at least to 250.122 for the feeder overcurrent device. For a 60 A device, Table 250.122 requires 10 AWG Cu, and the installed 6 AWG Cu exceeds that. It also must be adequate as a neutral for the connected unbalanced load, which for a garage panel is usually minimal.
Metal water line at the garage. This changes the analysis in two ways:
- If the metal water piping is continuous between the two buildings, there is a continuous metallic path, the Exception is unavailable, and the existing 3-wire feeder is noncompliant - a 4-wire feeder with 10 AWG Cu EGC is required.
- Even if the pipe is isolated (dielectric fitting at the street), the interior metal piping at the garage must be bonded per 250.104(B) to the grounding system at that building.
Recommendation. Document the absence of any metallic path with photos, install the ground rod and 6 AWG GEC at the garage, verify the neutral is not bonded on both ends in a way that creates a parallel path, and note on the permit that the feeder is a legacy installation under the Exception. If the owner is doing other major work at the garage, quote a 4-wire feeder replacement - the Exception is a narrowing path, not a design choice.