200 A Service Replacement With a Detached Garage Feeder
You are replacing the 100 A overhead service on a 1965 ranch with a 200 A meter-main and 40-space panel. The service-entrance conductors are 2/0 copper. The water service enters the basement as 1-inch copper for 12 ft, then changes to PEX at a shutoff before the meter. The existing grounding electrode conductor is 6 AWG copper clamped to the copper water pipe 3 ft from the wall. The gas range is supplied by black-iron pipe; the plumber recently bonded the gas line to the cold-water pipe with 6 AWG copper and a ground clamp. A detached garage 35 ft away has a 60 A subpanel fed by a 6/3 cable assembly with two hots, a neutral, and no equipment grounding conductor, run underground in PVC. The garage has a concrete slab with rebar, a metal door frame, and a 50 A EV receptacle is planned. The homeowner asks whether the existing grounding and garage feeder can be reused. The AHJ enforces the adopted NEC and Maine Chapter 120 without local amendments.
1. Evaluate the grounding electrode system and the gas-piping bond. What must be changed, if anything, before the service is energized?
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First classify the water pipe. A metal underground water pipe is a grounding electrode only if at least 10 ft is in direct contact with earth (250.52(A)(1)); 12 ft of copper inside the basement does not count unless the buried portion is metal. If it qualifies, 250.53(D)(2) requires it to be supplemented by another electrode, such as a ground rod. The GEC to a 200 A service with 2/0 Cu must be sized from Table 250.66; 6 AWG Cu is too small - 4 AWG Cu is the usual minimum for 2/0 Cu service conductors. The gas piping must be bonded to the grounding electrode system or to the service equipment enclosure, with a bonding jumper sized per 250.104(B). A clamp on the gas pipe is acceptable only if accessible and listed for the pipe; the cold-water bond is not a substitute if the water piping is not reliably part of the GES. Correct the GEC, add the supplemental electrode, and land the gas bond on the grounding busbar.
2. Can the existing 6/3 feeder supply the planned 50 A EV receptacle and the garage panel? Explain the code path and practical options.
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Start with the missing EGC. A feeder to a separate building must include an equipment grounding conductor and a grounding electrode system at the building (250.32). The existing 6/3 assembly has no EGC, so the garage panel is relying on the neutral or on earth; that is not acceptable for a new 50 A EV load. The 250.32(B)(2) exception for an existing 3-wire feeder is narrow and does not authorize adding a new 50 A receptacle/EVSE without an EGC. Also check ampacity: 6 AWG Cu in NM is limited to 60°C, typically 55 A; a 50 A EV branch circuit is a continuous load, and 625.42 requires sizing at 125% of the EVSE maximum current. A 40 A EVSE on a 50 A circuit needs 50 A ampacity, which is marginal; a 50 A EVSE would need 62.5 A. Reuse is therefore not a simple yes: run a new feeder with an EGC and adequate ampacity, or install a listed solution only if the AHJ accepts it for the existing installation. Do not use the neutral as an EGC.
3. What grounding and bonding is required at the detached garage if a new feeder is installed?
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At the garage, install a grounding electrode system per 250.32(A). The concrete slab may qualify as a concrete-encased electrode if it contains at least 20 ft of 1/2-inch or larger rebar or 20 ft of 4 AWG bare copper in the footing (250.52(A)(3)); otherwise use ground rods or another listed electrode. Run a GEC from the garage panel grounding bus to the electrode, sized per 250.66. Keep the neutral and equipment grounding conductors on separate terminal bars; the neutral bar must not be bonded to the enclosure at the subpanel. Bond the metal door frame only if it is likely to become energized or is part of the grounding/bonding system; the EGC to the 50 A receptacle and the metal raceway/enclosure is mandatory. Verify the feeder breaker and conductor are coordinated with the calculated load.