Detached Garage Feeder Grounding and Bonding Red Tag
A 200 A, 120/240 V single-phase service supplies a detached garage through a 100 A feeder. The feeder is 3 AWG copper THWN in PVC with an 8 AWG copper equipment grounding conductor, protected by a 100 A breaker at the house panel. At the garage, a 100 A subpanel has a separate neutral bar and ground bar. The installer drove two 8 ft ground rods 8 ft apart and ran 6 AWG copper from the ground bar to both rods. He also installed a green bonding screw in the subpanel neutral bar and ran a bare 8 AWG conductor from the neutral bar to a metal water pipe supplying a hose bib. The house service has 2/0 copper service-entrance conductors, a 4 AWG grounding electrode conductor to the water pipe, and the main bonding jumper installed. The inspector red-tags the job.
1. Identify the grounding and bonding violations at the garage and explain the hazard created by each.
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The green bonding screw in the garage neutral bar is a violation. It bonds the neutral to the panel enclosure and to the equipment grounding conductor, creating a parallel path for neutral current on the enclosure and the 8 AWG EGC. That can energize metal parts and cause shock, and it defeats the separation required for a subpanel fed by a feeder.
The bare 8 AWG from the neutral bar to the metal water pipe is also wrong. The water pipe is not a neutral load; it must be bonded to the grounding electrode system or to the equipment grounding conductor, not to the neutral bar. Connecting it to neutral puts neutral current on the plumbing and creates a shock hazard.
The 8 AWG copper EGC is correctly sized for a 100 A feeder, and the 6 AWG copper to the ground rods is acceptable because a grounding electrode conductor to a rod need not be larger than 6 AWG copper. The two rods are spaced at least 6 ft apart, so that part is fine. The fix is to remove the green screw, keep the neutral bar isolated, terminate the water-pipe bond on the ground bar, and verify that the garage grounding electrode system is bonded to the equipment grounding conductor and enclosure.
2. What size grounding electrode conductor is required for the 200 A house service, and does the 4 AWG copper to the water pipe comply? What about the 6 AWG to the rods?
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The service grounding electrode conductor is sized from the largest service-entrance conductor. The largest ungrounded service conductor is 2/0 copper. Table 250.66 requires a 4 AWG copper grounding electrode conductor for 2/0 copper service conductors, so the 4 AWG copper to the metal water pipe complies, assuming the connection is made within the required 5 ft of the pipe entry and the pipe is a qualifying grounding electrode.
For a ground rod, the grounding electrode conductor is not required to be larger than 6 AWG copper. The installed 6 AWG copper therefore complies. If a single rod were used, it would still need to meet the 25 ohm requirement or be supplemented by a second electrode; two rods are a common compliant approach. The important distinction is that the service GEC is sized by Table 250.66, while the rod connection is capped at 6 AWG copper. The garage water pipe still must be bonded, but to the grounding system, not to the neutral bar.
3. Explain why a separate building requires a grounding electrode even though an equipment grounding conductor is run with the feeder.
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The equipment grounding conductor is the fault-current path back to the source; it is what allows the overcurrent device to open during a ground fault. The earth is not a reliable fault-return path and cannot be used in place of the EGC. A separate building supplied by a feeder still must have a grounding electrode system because the structure needs a local earth reference for lightning, surge currents, and accidental contact with higher-voltage lines, and to stabilize voltage to earth.
At the garage, the grounding electrode system connects to the equipment grounding conductor and the panel enclosure, not to the neutral bar. The neutral must remain isolated at the subpanel so that normal neutral current does not travel on the EGC, the enclosure, or the earth. This is why the green screw is wrong: it would create a parallel neutral path and a shock hazard. The correct arrangement is a 4-wire feeder with separate neutral and EGC, a local grounding electrode system bonded to the EGC, and a single neutral-to-ground bond at the service only.