Choose the best answer:
1. 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. If your grounding electrode conductor goes down to the water entrance in the basement, what grounding/bonding connections must you make on the fifth floor? The owner pays for janitorial service, but provides no other ongoing oversight.
a. Nothing additional
b. A Table 250-66 local bonding connection
c. A Table 250-122 local bonding connection
d. Depends on whether there are multiple occupancies
2. Suppose you have a hot-air gas furnace with a 15A circuit to run the blower, and no other gas appliances. How many grounding/bonding connections must you deliberately make, as a minimum, to portions of this system? The building has a 200A service.
a. One: a No. 14 equipment grounding conductor (EGC) at the furnace
b. One: a No. 4 bonding connection from the service
c. Both of the above
d. None of the above
3. What size equipment grounding conductor must you run in each of two PVC raceways containing one set each of 350kcmil conductors, protected with a 600A fused switch?
a. No. 4
b. No. 3
c. No. 2
d. No. 1
4. Suppose the run in the previous question was the main feeder for the building and the 600A fused switch was the service disconnect with no additional circuit protective arrangements. If you make the run in listed Type MC cable with four 350kcmil insulated conductors and one No. 3 bare conductor, what would the inspector likely tell you to do?
a. Get cable with a No. 1 EGC.
b. Use a raceway method with No. 1 EGC.
c. Get ground fault protection of equipment (GFPE) with a trip set not over 300A.
d. Any of the above
5. Which of the following standard duplex receptacle installations requires a bonding jumper between the receptacle and the enclosing metal box?
a. Handy box, with fiber retention washers for the mounting screws
b. Old work wall case with mounting ears, washers removed
c. With listed yoke for grounding
d. In a raised cover
e. Two of the above
f. Three of the above
Answers and Discussion
1. b, Sec. 250-104(a)(4).. You don’t want potential differences between water pipes and nearby electrical enclosures. The grounding electrode connection in the basement addresses a ground reference for the system, which is a much different question. Only a local bonding connection serves to eliminate local potential differences.
2. c, Sec. 250-104(b), 250-104(c). The appliance has an immediate connection to the grounding conductor of its electrical supply circuit. The potential could differ from the rest of the piping if it were allowed to float. It could be a dramatic factor if a service union at an appliance were opened and a spark resulted, since you don’t usually purge the gas out of the piping at that point before loosening the union.
3. d, Sec. 250-122(f)(1). You size the equipment grounding conductor in both raceways based on the full rating of the overcurrent protective device, as given in the table. Under some conditions, fault current will feed into the EGC from both ends of the parallel circuit, and each one of the EGCs needs to be able to function independently, as if it were the only one in the circuit.
4. d, Sec. 250-122(f)(2). The first two options represent alternative approaches that result in applying the standard procedure, as in the prior question. The second option in particular simply allows an easy method to meet the principal rule. The third option, new in the 1999 NEC, addresses the division-of-current issue by assuring that a small, individual leg of the EGC won’t, at least for an extended period, carry more fault current than the ampacity of the conductor supplying the fault. The GFPE device must be listed for this function.
5. e, Sec. 250-146(a-b). Responses “b” and “c” are okay. Response “d” violates Sec. 250-146(a) unless you find a listed cover/receptacle combination. They don’t commonly exist, except for some specialized devices, such as some pin-and-sleeve devices—especially for hazardous (classified) locations.