Select the best answer:
1. If four 12-2 Type NM cables measuring 0.48 in. by 0.20 in. in cross section are run in a raceway along a wall 70 ft between. two pull boxes, what size raceway must be installed? The cross-sectional area of the cable is 0.075 [in.sup.2].
b. 1 1/4-in.
c. 1 1/2-in.
2. Now suppose the same basic installation as in the first question is wired with similar cable that is the same thickness but only 0.46 in. wide, reducing the cross-sectional area to 0.072 [in.sup.2]. If five cables will be installed, now how large would the race-way need to be?
b. 1 1/4-in.
c. 1 1/2-in.
3. Suppose the five 12-2 Type NM cables in the second question are to be installed in Schedule 80 PVC rigid nonmetallic conduit. Now how large would the raceway need to be?
b. 1 1/4-in.
c. 1 1/2-in.
4. If PVC conduit will be exposed to direct sunlight outdoors, the manufacturer recommends using 140 [degrees] F for the maximum design temperature. Assuming you are working on a 40 [degrees] F day in an area that can reach -30 [degrees] F in the winter, how much room must be left in the piston cavities of the expansion fittings? There will be four fittings, each with a 4-in. maximum throw in a 200-ft total run. Assume the expansion fittings are evenly spaced over the run, and the conduit is free to move.
a. 1.4 in. to a maximum of 2.05 in.
b. 2.05 in. to a maximum of 2.6 in.
c. at least 3.45 in.
d. fittings not required
5. What is the combined ampacity of a parallel make-up, for a 3-phase ungrounded power load, of three sets of 600kcmil THHN single conductors in a cable tray installed under appropriate engineering supervision? The air temperature is expected to reach 120 [degrees] F.
6. What is the combined ampacity of a parallel make-up of three sets of 600kcmil THWN conductors installed under appropriate engineering supervision in three parallel ducts buried in the earth? Figure the true ampacity that could be used for actual loading, and not as would be used with Art. 220 load calculations. Assume generally applicable earth temperature and heat-transfer conditions.
ANSWERS AND DISCUSSION
1. c, Ch. 9, Table 1, Note 6. The cable must be taken as a circle with a diameter equal to the major axis of the elliptical cross section, and therefore the actual cross-sectional area doesn't matter. With a 0.24-in. radius, the area used for calculation will be [Pi] x [0.24.sup.2] = 0.18 [in.sup.2]. Four cables require 4 x 0.18 [in.sup.2] = 0.72 [in.sup.2]. Per Table 4, a 1 1/2-in. raceway is needed, with an area of 0.82 [in.sup.2].
2. c, Ch. 9, Table 1, Note 4. As in the first question, the cable must be taken as a circle. In this case the area is [Pi] x [0.23.sup.2] = 0.17 [in.sup.2]. The capacity of a 1 1/2-in. raceway to hold cables all of the same diameter is given by dividing 40% of the raceway area from Table 4 (0.82 [in.sup.2] in this case) by the area of the cable, as follows: 0.82 / 0.17 = 4.82. The decimal remainder being larger than 0.8 in. in this case, the next higher number of cables (5) would be allowed.
3. d, Ch. 9, Table 1 and Sec. 110-3(b).The conventional raceway barely worked; Schedule 80 PVC has a much thicker wall and must be calculated based on its actual cross-sectional area, which per UL requirements must be printed on the outside of each length of conduit. The actual 40% fill on 1 1/2-in PVC Schedule 80 conduit is 0.684 [in.sup.2], just large enough for four cables.
4. b. Ch. 9, Table 10. Not only do you need enough fittings to accommodate the total anticipated expansion and contraction, you must set the pistons correctly. The run will expand and contract based on the difference between its length as installed and the maximum or minimum length, which is a function of the maximum temperature differential. The summer expansion will be based on 140 [degrees] F - 40 [degrees] F = 100 [degrees] F or 4.1 in. (from the table) per 100 ft. If one fitting is installed per 50 ft, 2.05 in. must be left in each for summer expansion. The winter contraction is figured the same way, except based on [40 [degrees] F - (-30 [degrees] F)] a 70 [degrees] F temperature differential. From the table, the maximum anticipated contraction per fitting would be 0.5 x 2.8 in. = 1.4 in. Since the total throw in the fitting is 4 in., the maximum space in the piston cavity when installed would be: 4 in. - 1.4 in. = 2.6 in. Note that the Table is only for PVC. The thermal expansion coefficient for fiberglass nonmetallic conduit, is less than 1/3 that for PVC.
5. c, Appendix B, Table B-310-2 and Sec. 3188(d) with 318-11(b)(4). Paralleled single conductors in cable trays must be bound in groups. The result in this case will be three circuit groups, each in a triangular configuration. The allowance to use Sec. 310-15(b) is the gate way, under the stipulated engineering supervision, to use Appendix B. The table for cables on a messenger therefore applies. From that table, 647A x 0.89 x 3 = 1727A.
6. a, Appendix B, Table B-310-7 and Sec. 310-10. Although the Table 310-16 ampacity for these conductors is 3 x 420A = 1260A (actually somewhat higher based on an earth temperature below 30 [degrees] C), they will overheat if they carry that number of actual, not theoretically calculated amperes. The mutual heating in the adjacent ducts causes the problem. The actual number, based on the Rho 90 column, is 3 x 371A = 1113A.