Detached Garage Feeder: Electrodes, Neutrals, and a Neutral-to-Ground Connection
A dwelling with a 200 A service is fed by a 60 A feeder to a detached, unheated garage 75 ft away. The feeder is 6 AWG copper THWN (two ungrounded, one neutral, one equipment grounding conductor) in 1-1/4 in. PVC Schedule 40, terminating in a 100 A main-breaker panel in the garage. The garage contains a metal underground water pipe serving a yard hydrant and a 20 ft length of 4 AWG bare copper in the slab footing, not yet connected to anything. The installer landed the feeder neutral and the equipment grounding conductor on the same terminal bar in the garage panel and drove an 8 ft ground rod connected to that bar. The owner reports a mild tingle at the garage sink and occasional tripping of the GFCI receptacle there.
1. What grounding electrodes are required at the detached garage, how are they connected, and what size conductor applies?
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Requirement for an electrode at the separate structure
250.32(A) requires a grounding electrode (one or more) at each separate building or structure supplied by a feeder. 250.50 then requires that all electrodes present be bonded together into a single grounding electrode system - this is not a menu where you pick one.
Electrodes present here
- Metal underground water pipe, 250.52(A)(1): qualifies if at least 10 ft is in direct contact with earth. Because it is a water pipe electrode, a supplemental electrode is required (250.53(D)(2)).
- Concrete-encased electrode, 250.52(A)(3): the 20 ft of 4 AWG bare copper embedded in the footing qualifies (minimum 20 ft, 4 AWG or larger, within 2 in. of earth).
- Rod electrode, 250.52(A)(5): the 8 ft rod, which is the supplemental electrode for the water pipe.
Conductor sizing
- Table 250.66 bases the grounding electrode conductor on the largest ungrounded feeder conductor: for 6 AWG Cu the required GEC is 8 AWG Cu.
- 250.66(A) caps the conductor to a rod or pipe electrode at 6 AWG Cu; 250.66(B) caps the conductor to a concrete-encased electrode at 4 AWG Cu. Bonding jumpers between electrodes are sized the same way (250.53(C)).
- The grounding electrode conductor terminates on the equipment grounding conductor at the garage panel, per 250.32(B)(1) - the electrodes bond to the EGC, not to the neutral.
Practical answer: pull a 4 AWG Cu bonding jumper from the footing conductor, an 8 AWG Cu GEC to the water pipe, an 8 AWG Cu to the rod, bond all three together, and land the common point on the EGC bar in the garage panel.
2. The installer landed the feeder neutral and the EGC on the same terminal bar in the garage panel. Evaluate this arrangement.
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This is a violation with real shock potential.
A garage panel fed by a 4-wire feeder that includes an EGC is not service equipment. It must be wired as a subpanel:
- The neutral bar must be isolated from the enclosure (no main bonding jumper, bonding screw removed).
- The EGC and the grounding electrode conductor land on a separate equipment grounding bar bonded to the enclosure.
Putting neutral and EGC on one bar creates a parallel return path: normal neutral current divides between the feeder neutral and the EGC/enclosure/ground electrodes. That is objectionable current on grounding conductors - 250.6(A) - and it energizes the panel enclosure, the water piping, and any bonded metal with a voltage drop during normal operation.
Corrective steps
- De-energize the feeder; verify with a tester.
- Remove the bonding screw/jumper between the neutral bar and enclosure.
- Install an isolated neutral bar (or remove the jumper that bonds the existing bar) and a separate EGC bar.
- Land feeder neutral on the isolated neutral bar; land feeder EGC, rod GEC, water pipe GEC, and footing bonding jumper on the EGC bar.
- Ohm the neutral bar to the enclosure to confirm isolation; then clamp the EGC with a low-range ammeter under load - it should read essentially zero.
3. Explain the tingle at the sink and the recurring GFCI trips, and describe how you would diagnose the cause before choosing a repair.
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Likely mechanism
The shared neutral/EGC bar puts the garage panel enclosure and the bonded water piping at a potential above earth whenever the feeder neutral carries current. At 60 A of neutral current, even 0.3 ohm of shared-path resistance produces about 18 V of touch potential - enough for a noticeable tingle at a sink where a person is also in contact with a grounded surface.
The GFCI tripping is consistent with the same defect: leakage current through the parallel path, or a neutral-to-ground connection downstream of the GFCI receptacle, will trip a Class A device at 4–6 mA. A GFCI compares current out on the hot with current returning on the neutral; any neutral current diverted onto the EGC reads as a fault.
Diagnostic sequence (do not just add bonding)
- With the load energized, clamp the feeder EGC and each grounding electrode conductor. Any steady current confirms a parallel neutral path.
- Measure voltage neutral-to-ground and enclosure-to-earth at the garage panel, loaded and unloaded.
- De-energize and isolate the neutral bar; check continuity from neutral bar to enclosure and to each electrode conductor.
- Check the panel for the bonding screw and any neutral-to-ground connection at the garage, and confirm no downstream receptacle has its neutral grounded.
- Verify the feeder neutral is not damaged or spliced with a poor connection, which would drive current onto the EGC.
Caution: do not "fix" a tingle by driving more rods or bonding the water pipe harder. Bonding per 250.104 is required for the metal water piping, but bonding is not a substitute for removing the parallel neutral path.