Detached Shop Grounding Electrode Conductor
A new detached shop is 90 ft from a dwelling and is supplied by a 200 A feeder from the house panel. The feeder is 4/0 aluminum XHHW-2 in 2-inch PVC, with a 2/0 aluminum neutral and a 6 AWG copper equipment grounding conductor. At the shop, the electrician installs a 200 A main-breaker panel, two 5/8-inch by 8 ft ground rods 8 ft apart, and a 6 AWG copper grounding electrode conductor from the panel ground bar to the rods. The shop has no metal water piping, but the building steel and metal siding are bonded to the panel. The neutral bar in the shop panel is isolated from the enclosure. The AHJ red-tags the grounding electrode conductor. The contractor argues that 6 AWG is adequate because the rods are the only electrodes and the feeder already has an equipment grounding conductor.
1. What is the minimum size copper grounding electrode conductor required for this feeder, and why is the installed 6 AWG incorrect?
Read the worked answer
Size the grounding electrode conductor from Table 250.66 using the largest ungrounded feeder conductor. The largest ungrounded conductor is 4/0 aluminum. The Table 250.66 row for 3/0 or 4/0 copper and 4/0 or 250 kcmil aluminum requires 4 AWG copper or 2 AWG aluminum. Therefore the minimum copper grounding electrode conductor is 4 AWG.
6 AWG copper corresponds to a smaller feeder conductor row and is undersized here. The equipment grounding conductor in the feeder is sized from Table 250.122 for fault current, not from Table 250.66 for the grounding electrode system. The grounding electrode conductor must be continuous and connected to the electrodes with listed clamps; it may not be spliced unless the splice is irreversible or the connection is accessible.
2. Does the shop require a grounding electrode system even though the feeder includes an equipment grounding conductor? What electrodes must be used?
Read the worked answer
Yes. NEC 250.32(A) requires a grounding electrode system at a separate building or structure supplied by a feeder. The equipment grounding conductor in the feeder provides the fault-current path and bonds the shop panel enclosure, but it does not replace the grounding electrode system.
The installed ground rods are acceptable electrodes, and two rods are required unless a single rod has a resistance to earth of 25 ohms or less. If the building steel qualifies as a structural metal electrode, it must be bonded and included in the grounding electrode system. If a concrete-encased electrode is present in the footing, it must be used. Metal water piping is absent, so no water-pipe electrode is required; the 4 AWG copper grounding electrode conductor must connect the panel ground bar to the rods and to any other qualifying electrode.
3. Does the isolated neutral bar in the shop panel comply, and is the bonding of metal siding required?
Read the worked answer
The isolated neutral bar is correct for a 4-wire feeder that includes an equipment grounding conductor. The grounded conductor must be separated from the equipment grounding conductors and the panel enclosure at the separate structure. The neutral is bonded at the service, not again at the shop panel. If the shop panel were the service disconnecting means, the neutral would be bonded there instead.
Bonding metal siding is not generally required unless the siding is likely to become energized or is part of an electrical grounding path. The structural building steel, if it is metal and qualifies under 250.104(C), must be bonded where required. The red tag is properly based on the undersized grounding electrode conductor, not on the isolated neutral bar.