Detached Garage Subpanel With a Bonded Neutral
A 100 A subpanel in a detached garage is fed from the house with 1/0 aluminum XHHW-2 four-wire: two ungrounded conductors, a neutral, and a green equipment grounding conductor, protected by a 100 A breaker. In the garage panel, the installer drove the green bonding screw through the neutral bar into the enclosure and landed the grounding electrode conductor for two driven rods on the same neutral bar. The rods are spaced exactly 6 ft apart and are connected with 6 AWG copper. A metal underground water line runs from the house to a hose bib at the garage.
1. Identify what is wrong with the garage panel as wired and describe the correct configuration.
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The neutral is bonded to the enclosure at the wrong place.
- This is a separate structure fed by a feeder, so 250.32(B)(1) governs: an equipment grounding conductor must be run with the feeder and the grounded conductor must be kept isolated from the enclosure and from the EGC.
- Bonding the neutral here creates a second neutral-to-ground connection on the same system. Normal neutral return current now divides and travels back to the house on the EGC, the panel enclosure, and the grounding electrodes. That is objectionable current on a path not intended to carry it (250.6), and it puts voltage on the panel enclosure and on any metal connected to it.
Correct configuration:
- Remove the green bonding screw so the neutral bar floats.
- Land the incoming EGC on the equipment grounding bar, which is bonded to the enclosure.
- Land the grounding electrode conductor for the rods on the equipment grounding bar, not the neutral.
- Leave the system bonding jumper at the service only - one point of neutral-to-ground bonding on the entire system.
- Keep the neutral bar isolated and the EGC continuous back to the service.
2. The rods are 6 ft apart and the grounding electrode conductor is 6 AWG copper. Which of those details are acceptable and which are not?
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Rod spacing - acceptable. 250.53(B) requires rod, pipe, and plate electrodes to be at least 6 ft apart. Six feet is the minimum; more separation, ideally 25 ft, gives a better earth connection.
A second electrode - required here. Under 250.53(A)(2), a single rod must be supplemented by an additional electrode unless it is verified at 25 ohms or less. Two rods satisfy this, which is why the second rod was added. Note the requirement is a supplemental electrode, not necessarily a second rod; a concrete-encased electrode or the metal water piping could serve.
Conductor size - acceptable. Table 250.66 sized on the 1/0 aluminum feeder would require 4 AWG copper, but 250.66(A) permits the grounding electrode conductor for a rod, pipe, or plate electrode to be no larger than 6 AWG copper. So 6 AWG copper is fine.
Termination point - not acceptable. The conductor is correctly sized but lands on the neutral bar, which is the actual defect. It must terminate on the equipment grounding conductor or the grounding electrode conductor termination point bonded to the enclosure.
3. The metal water line running from the house to the garage hose bib is a key detail. Explain why it matters here.
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It creates a continuous metallic path between two structures that share a grounded system.
- If the neutral is bonded at the garage, part of the neutral return current leaves the feeder neutral and returns to the house through the metal water pipe and the earth. The pipe is now carrying current it was never intended to carry.
- That produces voltage between the hose bib and earth, a shock hazard at any person touching the bib, possible electrolytic corrosion at fittings, and a false sense that the grounding electrode system is doing the work.
- It also creates a condition where opening the feeder neutral does not de-energize the garage neutral - the pipe backfeeds it.
Conclusion: the presence of a metallic path is exactly why a four-wire feeder with an isolated neutral and a continuous EGC is required for separate structures. The water piping in the garage must also be bonded to the equipment grounding conductor per 250.104, not to the neutral.
This is also the reasoning behind the restriction that keeps the older three-wire exception from being used where a continuous metallic path exists between the buildings.