General wiring rules -- Art. 300

Choose the best answer:

1. Which of the following methods provides acceptable mechanical protection against abrasion of Type NM cable when you run it through pre-cut rectangular holes in metal studs?

a. “Vee” grommets covering the bottom of the hole where the cable rests.

b. Factory knurled edges providing a smooth bearing surface

c. Grommets covering the entire perimeter of the opening

d. More than one of the above

2. Which of the following conductors (placed in rigid nonmetallic conduit in a 23-in.-deep trench) requires a warning ribbon placed above them?

a. Service entrance conductors

b. Service lateral conductors

c. Medium-voltage feeders (23kV), placed under a 4-in.-thick poured concrete sidewalk

d. Two of the above

e. All of the above

3. What special requirements apply when you wire in the cavity space of a fire-rated roof/ceiling assembly, and come down a support wire to a fluorescent fixture using a cabled wiring method?

a. Don’t attach to a support wire.

b. Don’t use a wire that’s part of the minimum ceiling design.

c. Only use a support wire recognized for this purpose in the tested ceiling design.

d. Two of the above

4.. If the cavity space in the preceding question isn’t part of a fire-rated design, how does that affect the answer? Use the same answer set as the preceding question.

5. Which of the following responses correctly shows the minimum strip length for the end of a run of Type UF cable entering the boxes as given? Assume you’ll position the trimmed sheath to just clear the clamp.

a. 7 in., interior cable clamp, back edge of 3½-in. device box recessed in drywall

b. 10 in., exterior cable clamp in KO 1 in. from the rear wall of a 1021028-in.-deep junction box

c. 6 in., rear of 4-in.-sq outlet box (1½-in. deep) with a ½-in.-rise square-to-round plaster ring

d. Two of the above

e. All of the above

6. Which of the following wiring methods could you use to run a 500 kcmil 13.8 kV feeder through an open basement joist cavity? Although only employees have access to this area, not all of them have electrical training.
a. Cable bus
b. Busway
c. Type MC cable
d. EMT
e. Two of the above
f. Three of the above
g. All of the above

Answers and Discussion

1. c, Sec. 300-4(b)(1). You need to use a grommet; knurled edges aren’t mentioned in the rule. The grommet you use needs to cover the entire perimeter of the opening, so if the cable hangs up during pulling, a cable loop doesn’t end up skinned on one side as it moves across the top of the opening.

2. d, Sec. 300-5(d), Table 300-50 Ex. 3. However unintended, service entrance conductors aren’t covered in this rule, only service lateral conductors. The medium-voltage rule warns you of situations including the following: suppose you dug up that sidewalk. You wouldn’t otherwise know it had an additional function to protect medium-voltage conductors.

3. b, Sec. 300-11(a)(1). Leave the original design alone. Developing and testing these designs is incredibly expensive, and the idea that someone might rerun the tests just to encompass branch circuit wiring on particular ceiling elements, as covered in the Exception, just won’t happen. Instead, add additional wires as required. Then, so the inspector knows you didn’t mess with the original design, be sure to color code or tag the extra wires you’re using.

4. b, Sec. 300-11(a) and -11(a)(2). Same answer, different reason. Again, add extra wires instead of messing with the ceiling design. However, you don’t have to identify the wires, since exact placement isn’t so critical. The Exception isn’t a factor, since ceiling designs aren’t likely to anticipate specific branch circuit wiring patterns.

5. d, Sec. 300-14. You need 6 in. minimum, and 3 full in. to extend outside the box opening. You’d need about 7 in. for the device box, but just 6 in. for the 4-in.-sq application. The junction box represents a special case, since you can get both hands into it to work (8 in.28 in. min.). Now, the wire need only meet the traditional 6-in. min.

6. g, Sec. 300-37. You’ll find all the methods mentioned in this section to be acceptable, which is the 1999 NEC relocation of former Sec. 710-4(a). In making the change, the Code added EMT to the list.