Detached Garage Feeder: Neutral, Ground, and Electrode Decisions
A 200 A service supplies a dwelling. A detached garage 60 ft away is fed from the house by a 100 A feeder consisting of three #3 Cu THWN conductors plus a #8 Cu equipment grounding conductor, installed in Schedule 80 PVC. At the garage, a main-lug load center has separate neutral and equipment-grounding bars and no main bonding jumper installed. Two 8 ft ground rods are driven 8 ft apart outside the garage, and a metal water line runs underground from the house into the garage and terminates at a hose bib. The installer landed the grounding electrode conductor and every equipment grounding conductor on the neutral bar "because that's where the open holes were." The inspector red-tags the job.
1. Is landing the equipment grounding conductors and the grounding electrode conductor on the neutral bar at the garage a violation? Explain the hazard and describe the correct termination.
Read the worked answer
Yes - violation.
- The garage panel is on the load side of the service disconnecting means. 250.24(A)(5) and 250.142(B) prohibit connecting equipment grounding conductors or the grounding electrode conductor to the grounded (neutral) conductor there.
- Because the feeder includes a #8 Cu equipment grounding conductor, 250.32(B) requires the grounded conductor to be isolated from the enclosure and from all grounding at the separate structure.
- The hazard is objectionable current: normal neutral return current would split onto the EGC, the panel enclosure, the ground rods, and the metal water line. That puts continuous current on metal parts you can touch, causes electrolytic corrosion of the water line, and can raise the touch voltage on the panel enclosure.
Correct installation:
- All equipment grounding conductors (feeder EGC, branch-circuit EGCs) land on the equipment grounding bar.
- The grounding electrode conductor to the two rods lands on the equipment grounding bar (or the enclosure grounding terminal).
- The neutral bar stays isolated; remove the bonding screw/strap if one were present.
- Verify with a continuity test that neutral bar to enclosure reads open and EGC bar to enclosure reads closed.
2. Size the grounding electrode conductor at the garage and evaluate the two-rod installation. Would a single rod have been permitted?
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Requirement for an electrode at the garage. 250.32(A) requires a grounding electrode system at each separate building or structure supplied by a feeder or branch circuit. This is true even though an equipment grounding conductor is run with the feeder - the EGC handles faults, the electrode system handles lightning and earth potential.
GEC sizing. 250.66 sizes the GEC by the largest ungrounded feeder conductor. The largest ungrounded conductor is #3 Cu, which falls in the "2 AWG or smaller" row → #8 Cu minimum. The #8 Cu installed is therefore correct in size (the problem was where it landed, not what was used).
Two rods vs. one. 250.53(A)(2) permits a single rod only if it demonstrates 25 ohms or less to earth; otherwise a supplemental electrode is required. 250.53(A)(3) requires rod electrodes to be at least 6 ft apart. Driving two rods 8 ft apart is the standard, code-compliant way to satisfy this without taking a resistance measurement. A single rod would only be acceptable with a documented 25-ohm-or-less test.
Both rods must be bonded together and connected to the GEC by an irreversible connection (or a listed clamp accessible per 250.68).
3. What must be done with the metal water line entering the garage, and how is that bonding conductor sized?
Read the worked answer
It must be bonded. 250.104 requires metal water piping systems in or attached to a building or structure to be bonded to the grounding system. A metal line running underground from the house into the garage is exactly the kind of metallic path that can carry fault current or lightning energy between structures and create a shock hazard if it is not bonded.
Sizing. The bonding jumper is sized from Table 250.102(C)(1), using the largest ungrounded conductor of the feeder supplying the structure. With a #3 Cu feeder, the table row for "2 AWG or smaller" gives #8 Cu. A larger conductor is always permitted; smaller is not.
Where it lands. On the equipment grounding terminal bar at the garage panel - never on the neutral bar.
Additional point. Because this water line is metallic and in direct contact with the earth for a substantial distance, evaluate whether it also qualifies as a grounding electrode under 250.52(A)(1) (10 ft or more in contact with earth). If it does, it becomes part of the grounding electrode system and must be connected to the GEC as well; the two-rod installation would be supplemented by, not replaced by, that connection. The safe design is to bond it to the EGC bar and also treat it as an electrode - the cost is a few feet of #8 Cu, and the failure mode of leaving it unbonded is a difference of potential between two buildings that a person can bridge.