Detached Garage Feeder: Neutral, Ground, and the Electrode System
A 100 A, 24-space panel is installed in a detached garage 60 ft from the house. The feeder runs in Schedule 80 PVC underground with three 3 AWG Cu THWN conductors (two ungrounded and one neutral) plus one 8 AWG Cu conductor, protected at 100 A in the main panel. A copper water line runs underground from the house to a hose bib on the garage. The garage footing contains 20 ft of ½ in. rebar, and the electrician planned to use it as the grounding electrode. In the garage panel, the neutral bar and the equipment-grounding bar are separate pieces but are joined by a factory-installed bonding screw the installer never removed. The inspector red-tags the job.
1. The inspector requires the neutral and ground to be separated at the garage panel. Explain why, what the underground metal water line has to do with it, and what must be changed.
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Rule: 250.24(A)(5) prohibits grounding the grounded conductor on the load side of the service disconnect, and 250.32(B) requires that a feeder to a separate structure include an equipment grounding conductor with the neutral isolated at the panel.
Why the water line matters: The older allowance for a 3-wire feeder with the neutral re-grounded at the second building applied only where there was no continuous metallic path between the structures. The underground copper water line is exactly such a path, so a 3-wire arrangement was never permitted here. (The exception also no longer exists for new work in current editions.)
What to change: Remove the bonding screw/strap so the neutral bar is isolated from the enclosure; land the 8 AWG Cu equipment grounding conductor on a separate grounding terminal bar; terminate the neutral only on the insulated neutral bar. Then bond the metal water piping to the grounding terminal bar per 250.104 with a jumper sized per Table 250.66 - for 3 AWG feeder conductors that is 8 AWG Cu.
Feeder itself: Four conductors were already pulled, so the raceway and conductors are acceptable; the defect is entirely at the panel termination.
2. Does the garage need a grounding electrode system? Size the grounding electrode conductor. Can the 20 ft of ½ in. rebar serve as the electrode, and what changes if only 15 ft of rebar were present?
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Required: Yes. 250.32(A) requires a grounding electrode system at each separate structure supplied by a feeder, connected to the equipment grounding terminal bar.
Sizing: 250.66 sizes the grounding electrode conductor from the largest ungrounded feeder conductor. 3 AWG Cu falls in the "2 AWG or smaller" row → 8 AWG Cu minimum.
Concrete-encased electrode: 250.52(A)(3) recognizes at least 20 ft of ½ in. or larger bare copper or zinc-galvanized reinforcing bar (or 20 ft of 4 AWG bare copper) in a concrete foundation or footing in direct contact with earth. Twenty feet of ½ in. rebar qualifies, and where a qualifying electrode is present it must be used (250.50). The connection must be made with a listed clamp suitable for concrete encasement and, per 250.52(A)(3), made to the rebar within the concrete.
If only 15 ft existed: It is not a concrete-encased electrode. You would need another electrode - typically a ground rod. With a single rod, 250.53(A)(2) requires a supplemental electrode (a second rod) unless a single rod measures 25 Ω or less, and the conductor to the rod need not exceed 6 AWG Cu per 250.66(A).
3. Is the 8 AWG Cu conductor adequate as the feeder equipment grounding conductor? What if the feeder breaker were 60 A? What if the load required 200 A?
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Basis: Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device protecting the circuit - not from the conductor size, except as required by 250.122(B).
- 100 A device → 8 AWG Cu. The installed conductor is correct.
- 60 A device → 10 AWG Cu would be sufficient.
- 200 A device → 6 AWG Cu.
Watch the upsizing rule: If the ungrounded feeder conductors are increased in size for voltage drop or other reasons, 250.122(B) requires the equipment grounding conductor to be increased proportionally. A 3 AWG Cu feeder that is bumped up to 1/0 for a 60 ft underground run no longer pairs with an 8 AWG ground.
Also verify: The equipment grounding conductor must be run with the circuit conductors inside the same raceway (300.3(B)), and it must be continuous and effectively bonded at every enclosure per 250.4(A)(3) and 250.96.