Detached Garage Feeder: One Ground Rod, a Green Screw, and 20 ft of Rebar
A 60 A feeder runs 70 ft from a house panel to a detached garage. It consists of two 6 AWG Cu ungrounded conductors, a 6 AWG neutral, and a 10 AWG Cu equipment grounding conductor, all THWN in PVC conduit, terminating in a 12-space panel with a 60 A main breaker. The installer drove one 5/8 in. x 8 ft ground rod, ran a bare 8 AWG Cu grounding electrode conductor to it, and left the green bonding screw in the garage panel, connecting the neutral bar to the enclosure. There is no metal water piping in the garage. The slab contains 20 ft of No. 4 rebar, accessible at the panel location, and no ground resistance test was performed.
1. Evaluate the grounding electrode system and the grounding electrode conductor size.
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GEC size: acceptable. Table 250.66 is applied to the largest ungrounded feeder conductor (6 AWG Cu), which requires 10 AWG Cu. The installed 8 AWG exceeds the minimum, so sizing is not the defect. Even the caps in 250.66(A) through (C) would allow no larger than 6 AWG for a rod and 4 AWG for a concrete-encased electrode, so an 8 AWG is comfortable either way.
Electrode system: noncompliant on two counts.
- A single rod must be supplemented - 250.53(A)(2). One rod, pipe, or plate electrode must be supplemented by an additional electrode from 250.52(A)(4) through (A)(8), unless the single rod has a resistance to earth of 25 ohms or less and that is documented. No resistance test was performed, so the exception cannot be claimed.
- The concrete-encased electrode is present and must be used - 250.50 and 250.52(A)(3). At least 20 ft of bare or zinc-galvanized reinforcing bar not smaller than 1/2 in. encased in concrete in contact with earth is a concrete-encased electrode. 250.50 requires all available electrodes to be bonded together into the grounding electrode system. The 20 ft of No. 4 rebar in the slab is available, so it must be part of the system.
The practical fix: bond the rebar with a listed concrete-encased electrode connector and a conductor sized per 250.66 (not required larger than 4 AWG Cu for this connection), run it to the panel grounding bar, and that bonded electrode satisfies the supplemental-electrode requirement of 250.53(A)(2) without a second rod or a resistance test. Alternatively, drive a second rod at least 6 ft from the first and bond both.
2. The green bonding screw is still installed in the garage panel. Explain the consequence and what the Code requires.
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What the screw does. It bonds the neutral bar to the panel enclosure. In the garage, with a 60 A main breaker and a feeder that already includes an equipment grounding conductor, the neutral must be isolated from the enclosure and the EGC must be bonded to the enclosure - 250.32(B)(1). The green screw makes the neutral and the EGC one conductor at two points: the service, and the garage.
Why that matters. The house neutral and the garage EGC now form a parallel path. Some of the neutral return current from garage loads travels back toward the house on the EGC, the conduit, and the grounding electrode system instead of entirely on the neutral.
- The EGC and every bonded metal surface in the garage sit at a potential above earth whenever the garage is loaded - the classic shock-from-the-box symptom.
- That current is objectionable current on a grounding path, prohibited by 250.6.
- The grounding electrode and the earth itself become part of the return circuit, which is exactly the condition the separate EGC exists to prevent.
- The neutral loses its dedicated overcurrent path, and neutral current can circulate in ways the overcurrent device was never intended to see.
- Any future interruption of the neutral turns the whole grounding system into a shock hazard.
Correction: remove the green screw, land the feeder neutral on the insulated neutral bar, land the 10 AWG EGC on the equipment grounding bar, and bond the grounding bar to the enclosure. Verify with a continuity test that the neutral bar reads open to the enclosure. The bonding screw belongs only at the service, or at a separately derived system where a system bonding jumper is required.
3. The owner wants to add a 40 A, 240 V EV charger in the garage. Does the existing grounding and bonding arrangement support it, and what else must you check?
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The grounding arrangement supports it, once corrected. The 10 AWG EGC in the existing 60 A feeder would be undersized for a 40 A branch circuit, but that is not how it works: the feeder EGC is sized by Table 250.122 for the feeder overcurrent device (60 A -> 10 AWG Cu), not for whatever the largest downstream branch circuit happens to be. Separate, properly sized branch-circuit conductors and EGC run from the garage panel to the charger.
What actually has to be checked:
- Feeder capacity. A 60 A feeder cannot carry the existing garage load plus a 40 A continuous EV load. EV charging is a continuous load, so the branch circuit is sized at 125% of the charger's rated input current (625.42), and the calculated load must be redone for the feeder and the house service. This is where the project is most likely to fail - the grounding question gets all the attention and the load calculation is what kills it.
- Neutral isolation. Confirm the green screw is gone; an EV charger is a 240 V load with no neutral, which makes it tempting to skip verifying the neutral bar. Do not.
- GFCI/protection and disconnect. Follow the charger's listing and instructions (110.3(b)) and the applicable sections for the equipment grounding, disconnecting means, and any required GFCI protection.
- Bonding of any new metal piping or gas line installed for the garage equipment - each must be bonded per 250.104 where it is likely to become energized.
If the feeder calculation fails, the options are a larger feeder with a proportionally larger EGC per Table 250.122, or a load management device that limits the charger so the service calculation holds.