Choose the best answer:
1. Suppose you have a 480V-208Y/120V transformer mounted on building steel 20 ft from the column-width panel (with main) it supplies. The panel sits in the webbing of a vertical I beam, and the supply feeder runs in rigid nonmetallic conduit. The Xo terminal and the transformer enclosure are bonded. Where could you bring the grounding electrode conductor?
a. The transformer
b. The panel
c. The panel if it has an insulated neutral bar
d. More than one of the above
2. Suppose you install a 13.8kV-480Y/277V transformer and secondary panel with a main disconnect on the top floor of a 5-story office building. Assume no effectively grounded building steel, but there’s an adjacent 2-in. copper water line running exposed through utility closets and firestopped floors to its basement entrance. Where could you connect the grounding electrode conductor? The building owner pays for janitorial service, but provides no other ongoing oversight.
a. The basement water entrance
b. The adjacent water pipe
c. A made electrode
d. More than one of the above
3. Your neighbor decides to build a detached wood-frame workshop on reinforced concrete footings. There isn’t much load, so you go out with a 60A 120/240V feeder from his basement panel. How many wires (minimum) do you need to pull into the feeder raceway?
a. RNC=4/GRC=3
b. RNC=4/GRC=4
c. RNC=3/GRC=3
d. RNC=3/GRC=4
4. In the prior question, if you plan to use the reinforcing as the electrode, what do you need to do before the concrete truck first arrives? Your neighbor’s using 8-ft re-bars.
a. Arrange for 20-ft re-bars instead
b. Bond the bars together using listed fittings.
c. Make sure the usual tie wires are tight.
d. Add 20 ft of No. 4 copper to the footing.
5. Suppose you wire a new house on a slab, with the service disconnect and panel let into a stud cavity on the inside of a bedroom. If you feed the panel with an exposed 3-ft run of flexible metal conduit from the meter socket, what arrangements do you have to make for the telephone installer coming the next day? You’re using rigid nonmetallic conduit for the service riser.
a. Add an approved lug to the meter socket
b. Route the grounding electrode conductor on the outside of the building.
c. Either of the above
d. Nothing
Answers and Discussion
1. a, Sec. 250-30(a)(2). You must terminate the grounding electrode conductor in the same location as the main bonding jumper. This prevents the building framework from becoming a grounded circuit conductor in parallel with the one in the conduit. Under prior codes, you could terminate at the system disconnecting means, even with the bonding jumper upstream at the transformer. If you did that in this case, current would routinely flow through the steel framing and into the Xo terminal. The 1999 NEC tries to minimize such currents.
2. a, Sec. 250-30(a)(3). Since there isn’t qualified supervision of this installation, the exception doesn’t apply, only the main rule. Although prior codes allowed separately derived system grounding connections to be made locally, that didn’t address frequent instances where other trades would interrupt water piping system continuity. Here the 1999 NEC requires you to make the connection at the water entrance.
3. c, Sec. 250-32(b)(2). Here again, the 1999 NEC tries to get rid of parallel grounded conductor pathways. If you go out to the new building in rigid nonmetallic conduit, you can reground at the second building since there isn’t any parallel return path other than through the earth, and you can ignore that. On the other hand, if you go out in metal conduit, the conduit, after regrounding, would become a parallel path for grounded conductor current.
4. c, Sec. 250-50(c). The 20 ft rule for metallic elements in a concrete encased electrode doesn’t mean each element has to be 20 ft long. Just as in the case of bonding reinforcing steel in swimming pools, you can rely on the usual steel tie wires, but they have to be made up tight.
5. c, Sec. 250-92(b)(1). You have to provide for intersystem bonding, and the greenfield not being “nonflexible” no longer qualifies as an eligible service raceway for this purpose. Either “a” or “b” responses meet the requirements.
