Detached Garage Subpanel: Neutral Bonding and Grounding Electrode
A detached garage is fed from a 200A dwelling panel by a 60A feeder. The contractor installed 6/3 NM-B with a bare equipment grounding conductor and a 60A breaker at the house. At the garage, the feeder terminates in a 12-space panelboard. The installer bonded the neutral bar to the panel enclosure with the green screw and connected the bare feeder ground to the neutral bar; the two ground rods are connected to the same bar with a #6 copper conductor. The homeowner reports that a 20A GFCI receptacle in the garage trips when a table saw starts, and a noncontact voltage tester shows voltage on the panel cover and on the metal casing of a refrigerator. The feeder is about 85 ft long. The inspector must determine whether the grounding and bonding arrangement is correct and what must be changed before the panel is energized.
1. What is causing the GFCI tripping and the voltage on the panel cover? Identify the code violations.
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The neutral is bonded to the enclosure and to the equipment grounding conductor at the garage panel. That creates a parallel neutral return path on the bare EGC and on the grounding electrode conductor. Neutral current from 120V loads flows on metal parts, producing a voltage drop that appears on the panel cover and the refrigerator casing. A GFCI sees current leaving the neutral and returning on the EGC and trips.
NEC 250.24(A)(5) permits the neutral-to-ground bond only at the service, and 250.6 prohibits objectionable current on grounding paths. The garage is a separate structure, so 250.32 requires an EGC with the feeder and a separate grounding electrode system. The neutral must be isolated at the garage panel. The green bonding screw must be removed, and the feeder EGC and branch-circuit grounds must land on a separate equipment grounding bar.
2. Describe the correct grounding and bonding at the detached garage, including the grounding electrode system.
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At the garage panel, install a separate equipment grounding terminal bar isolated from the neutral. Land the feeder EGC and all branch-circuit equipment grounding conductors on that bar. Land the feeder neutral on the insulated neutral bar. Do not install the bonding screw.
Install a grounding electrode system per 250.32(A) and 250.50. Two 8 ft ground rods at least 6 ft apart are typical; a single rod is permitted only if it has a resistance of 25 ohms or less. Connect the rods to the equipment grounding bar with a 6 AWG copper grounding electrode conductor. Bond metal water piping and structural steel if present. The feeder EGC must be bonded to the grounding electrode.
This arrangement eliminates the parallel neutral path and provides a low-impedance fault-return path through the EGC. The neutral remains isolated at the garage, and the grounding electrode system stabilizes voltage to earth. Verify continuity of the EGC and the grounding electrode conductor before energizing.
3. If the existing feeder had been a 3-wire feeder with no EGC and the garage panel were existing, would the same correction apply? Explain the limited exception.
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For a new feeder, no. A 4-wire feeder with an EGC is required, and the neutral must be isolated at the separate structure.
For an existing installation only, 250.32(B)(2) may permit a 3-wire feeder without an EGC if specific conditions are met: there is no continuous metallic path between the buildings, and ground-fault protection of equipment has not been installed on the supply side of the feeder. Under that exception, the grounded conductor may serve as the grounding means at the separate structure, and a neutral-to-ground bond at the garage is part of the permitted arrangement.
The inspector must verify the conditions. If a metallic water line, phone line, coaxial cable, or other metallic path exists between the buildings, the exception is not available and an EGC must be added. Because this case is a new feeder, the correct answer is the 4-wire method with a separate EGC and an isolated neutral.