Detached Workshop: Grounding Electrode System and Feeder EGC
A 30 ft x 40 ft detached workshop is fed from a dwelling by a 100 A feeder: two 1/0 AWG aluminum ungrounded conductors, one 1/0 AWG aluminum neutral, and a 6 AWG copper equipment grounding conductor, installed in Schedule 80 PVC in a 24 in deep trench. The subpanel has separate neutral and ground bars. A metal underground water line supplies the shop and enters through the floor slab. The electrician proposes to (a) bond the panel's ground bar to the water pipe with 6 AWG copper, landing within 12 in of where the pipe enters the slab; (b) drive one 8 ft x 5/8 in copper-clad rod and connect it to the ground bar with 6 AWG copper; and (c) leave the neutral bar isolated from the enclosure. The single rod measures 68 ohms to earth.
1. Evaluate the grounding electrode conductor sizing and the water pipe connection.
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Is the water pipe an electrode? Yes - a metal underground water pipe in direct contact with earth for 10 ft or more is a grounding electrode, and where one is present it must be used. It also must be supplemented by at least one other electrode, which the rod provides.
GEC size: the grounding electrode conductor at a separate structure is sized from the largest ungrounded feeder conductor. Largest ungrounded = 1/0 AWG aluminum. For an aluminum ungrounded conductor of 1/0 or smaller, the table requires only 8 AWG copper. The installed 6 AWG copper exceeds the minimum and is acceptable.
Connection location: the connection must be made within 5 ft of the point where the pipe enters the building. Twelve inches is inside that limit -> compliant.
Watch items:
- The GEC must be a continuous, irreversibly spliced conductor; a split-bolt in a crawl space is not acceptable.
- The water pipe may be used as an electrode but must never serve as the equipment grounding conductor for the feeder or the panel.
- The pipe also needs to be bonded to the grounding system regardless of electrode status, so this connection serves double duty.
2. The single rod reads 68 ohms. Does this comply, and what is the fix?
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No. A single rod, pipe, or plate electrode must have a resistance to earth of 25 ohms or less. At 68 ohms the installation fails.
Fix: install a second made electrode (rod, pipe, or plate) and bond it to the first. The second rod must be spaced at least 6 ft from the first; typical practice is a spacing equal to at least the driven length (8 ft) so the two resistances add in parallel rather than interfering. Connect it with a conductor no smaller than 6 AWG copper.
Key point that trips people up: once the second electrode is installed, no further resistance measurement is required. You are not chasing 25 ohms with additional rods - the code's requirement is satisfied by the second electrode plus the supplemental-electrode rule for the water pipe. Trying to drive rods until the meter reads 25 ohms is good engineering but is not what compliance requires.
Practical note: two rods 8 ft apart in average soil typically land around 30-40 ohms; the value matters for lightning and surge performance, so if the shop has sensitive equipment, more electrode is better even though it is not required.
3. Is the 6 AWG copper equipment grounding conductor adequate for the 100 A feeder, and is the isolated neutral correct?
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EGC sizing: for a 100 A overcurrent device, the minimum copper equipment grounding conductor is 8 AWG. The 6 AWG installed exceeds the minimum -> acceptable.
(Do not confuse this with the electrode conductor sizing in the previous task. The equipment grounding conductor that runs with the feeder is sized from the overcurrent device; the grounding electrode conductor is sized from the largest ungrounded conductor.)
Isolated neutral: correct. Because a 6 AWG equipment grounding conductor is run with the feeder, the neutral is a grounded circuit conductor only. The neutral bar must be isolated from the enclosure and from the ground bar at the subpanel. Bonding it here would:
- put normal neutral return current on the enclosure, the EGC, and the grounding electrode system, and
- create a parallel neutral path through the earth, which is exactly the 'objectionable current on grounding conductors' condition the code prohibits.
Also verify: the panel enclosure, the water pipe, and the rod are all bonded to the equipment grounding conductor and to each other, and the feeder EGC terminates on the ground bar, not the neutral bar.