Detached Garage Feeder: Neutral, Ground, and Electrodes
A crew is running a new 100-ampere feeder from the 200-ampere service panel of a house to a detached, unheated garage 60 ft away. The garage will supply a table saw, a 240-volt welder receptacle, and general-use lighting. The trench is open and the conductors on site are 2 AWG aluminum XHHW-2 in Schedule 80 PVC. Inside the garage the crew plans a 100-ampere main-lug panel, two 8-ft ground rods driven 6 ft apart, and, to save a conductor, only the two ungrounded conductors plus the neutral. They intend to bond the neutral bar to the panel enclosure at the garage 'so the panel is grounded' and to run 8 AWG copper from the neutral bar to the ground rods. The garage footing contains 20 ft of 1/2-inch bare rebar.
1. Is the proposed three-wire feeder with the neutral bonded to the enclosure at the garage acceptable for this new installation? Explain what must change.
Read the worked answer
No. A feeder to a separate structure must include an equipment grounding conductor. Under 250.32(B)(1) the grounded conductor may not serve as the equipment grounding conductor for a new feeder; the narrow exception applies only to certain existing installations with no continuous metallic path.
What the proposed arrangement does wrong:
- Bonding the neutral bar to the garage enclosure and to the rods creates a parallel return path for neutral current through earth and through any metallic path (water pipe, CATV/phone shield, rebar) - objectionable current prohibited by 250.6.
- The panel enclosure and grounding system become energized during normal operation, a shock hazard.
- The low-impedance fault-clearing path the EGC is supposed to provide is destroyed, so a ground fault may not open the breaker.
Correct installation: pull a fourth conductor as the EGC - 8 AWG copper or 6 AWG aluminum for a 100 A device per Table 250.122 - land it on a ground bar bonded to the enclosure, keep the neutral bar isolated (no bonding screw, remove any factory jumper), and connect the ground bar to the grounding electrode system.
2. Which grounding electrodes apply here, and what sizes of grounding electrode conductor are required?
Read the worked answer
250.50 requires every electrode present to be bonded into one grounding electrode system, so the 20 ft of 1/2-inch bare rebar is a concrete-encased electrode under 250.52(A)(3) and must be used, together with the two rods. Rods spaced 6 ft apart satisfy 250.53(B), and since a single rod will almost never meet 25 ohms, 250.53(A)(2) requires the supplemental rod they are driving.
Conductor sizing: the grounding electrode conductor is sized by 250.66 from the largest ungrounded feeder conductor. For 2 AWG aluminum, Table 250.66 gives 8 AWG copper. The caps also apply: 250.66(A) - need not be larger than 6 AWG copper to a rod; 250.66(B) - need not be larger than 4 AWG copper to a concrete-encased electrode. So 8 AWG copper serves both.
Run it as a single continuous GEC to one electrode and then to the others (250.64(F)), using listed clamps or exothermic connections (250.70), and protect it where exposed to physical damage.
3. Assume instead this were an existing three-wire feeder installed legally decades ago. A plumber now runs a metal water line from the house to a bibb in the garage. What does that change?
Read the worked answer
It destroys the only basis for the exception. The existing-installation provision in 250.32(B)(1) depends on there being no continuous metallic path between the two structures; a new metal water pipe is exactly such a path. Neutral return current would now travel on the water pipe, creating shock and corrosion hazards and heating at joints.
Options:
- Best: install an equipment grounding conductor (re-pull or run a separate conductor), isolate the neutral at the garage panel, and land the new metal piping on the grounding electrode system per 250.104.
- If the piping is truly isolated, replace the metal run with a nonmetallic section - but this is fragile, since any later metallic path recreates the problem.
Either way, the water piping itself must be bonded, and the panel neutral-to-ground bond at the garage cannot remain if an EGC is added.