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Quizzes on the Code

Dec 1, 1998 12:00 PM, By Fred Hartwell, Senior Editor

Give the best answer:

1. Suppose you need 50VA of power at 24VAC to operate a control relay using a circuit you don’t want burdened with Chapter 3 wiring methods. To qualify for consideration as a Class 2 control circuit, what are your options?

a. Provide overcurrent protection in the field, per Table 11(a) in Ch. 9.

b. Use a listed Class 2 power supply.

c. Use a listed Class 3 power supply.

d. Two of the above

e. Any of the above

2. Assume you were successful in avoiding Chapter 3 wiring for the Class 2 conductors in the first question. If you need to route them through a panel wiring duct that includes 120V nonpower-limited control circuit conductors, what should you do?

a. Use Class 2 cabling in supplemental sleeving.

b. Use Class 3 cabling in supplemental sleeving.

c. Use 600V THHN for all conductors in the duct.

d. None of the above

e. Two of the above

f. Any of the above

3. If the panel wiring duct in Question 2 includes 277V circuit conductors, how does that change your answer (if at all)?

4. Suppose Class 2 control circuit conductors (Question 2) enter a 277V fluorescent ballast channel instead of panel wiring duct, and terminate directly at auxiliary terminals on the ballast. Which of the options applies now?

5. Now suppose you have a run of EMT complete from the panel to a remote Class 2 thermostat location, so Chapter 3 wiring methods won’t present an obstacle. Which of the options in Question 2 would work? Assume the nonpower-limited circuit is operating at 120V.

6. Suppose you run a 2A 120V control circuit for a motor contactor using the new Type ITC cable, with a 5A time-delay fuse for protection. Other than the fuse, there won’t be any power limitation on this circuit. What insulation level do you need on this cable?

a. 150V

b. 300V

c. 600V

d. Ask the AHJ.

7. Which of the following conductors could be used for fire alarm circuits?

a. 19-strand No. 16

b. 19-strand No. 14

c. None of the above

d. All of the above

Answers and Discussion

1. b, Sec. 725-41. Power supplies for Class 2 and Class 3 circuits must originate from listed sources, which precludes field construction of such a supply. This has been the case for some time. However, correctly reaching that conclusion used to involve truly convoluted examination of the notes to the Tables. Now, that has been clarified.

2. b, Sec. 725-54(a)(1) Ex. 2(b)(1). You simply cannot, as a general proposition, rely on conductor insulation ratings to establish the required separation between Class 2 or 3 circuit conductors and nonpower-limited conductors. This means simply using 600V insulation on all the conductors won’t do the job. However, since the nonpower-limited conductors aren’t operating over 150V to ground, you can run them in Class 3 cables and then supplement the isolation with either sleeving or leaving the outer jacket intact.

3. d, Sec. 725-54(a)(1) Ex. 2(b). The higher voltage-to-ground means the sleeving option won’t work. You’d have to divorce these conductors from the 277V conductors, probably by using ties to maintain a ¼-in. maintained air separation to comply with Sec. 725-54(a)(1) Ex. 2(a).

4. e, Sec. 725-54(a)(1) Ex. 3. In this case, the allowance for sleeving works at any voltage. This exception trades off the normal barrier requirements for sleeving. You can use either Class 2 or Class 3 cabling. These are very short runs, and virtually all of the runs involved can be inspected after the original installation is complete.

5. d, Sec. 725-54(a)(1) Ex. 2. Although this exception allows for some control circuits to be run as a Class 1 circuit, typically using Chapter 3 wiring methods, all the elements of this circuit need to be Class 1 eligible. The Class 2 thermostat ruins this option.

6. d, Sec. 725-27(b); Sec. 727-4. This circuit squarely conforms to the scope of Art. 725, and it cannot be a Class 2 or Class 3 control circuit because it doesn’t have a listed source. It’s well beyond the allowable energy parameters of such circuits anyway. That makes it a Class 1 control circuit, which has to have 600V insulation. On the other hand, the fact that it runs in Type ITC cable means it can have 300V insulation. These two provisions are in direct conflict, leaving the AHJ with a decision. The panel’s intent is to allow the 300V insulation level.

7. d, Sec. 760-71(a). The former stranding restrictions have been removed, due to the changed electronics of modern systems that detect and annunciate a severe strand failure during alarm testing.

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