Sizing requirements for boxes and conduit bodies used as pull or junction boxes are stipulated in Section 370-28. While the boxes within the scope of 370-16 are calculated from the sizes and numbers of conductors, boxes in 370-28 are calculated from the sizes and numbers of conduits (raceways).
For raceways containing conductors of No. 4 or larger, and for cables containing conductors of No. 4 of larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run must comply with Section 370-28(a)(1) through (3). This section provides requirements for pull and junction boxes used on systems 600 volts, nominal, or less to ground. Pull and junction boxes used on systems over 600 volts must comply with the installation requirements of Section 370-71.
This month concludes the five-part series covering sizing requirements for pull and junction boxes. Part I covered straight pulls, including the box’s minimum width and depth. Part II covered angle pulls and boxes with more than one row of raceway entries. Part III covered “U” pulls and the minimum distance between raceway entries enclosing the same conductors. Last month’s In Focus covered boxes containing both straight and angle pulls.
370-28(a)(2) Boxes Containing Splices
Section 370-28(a)(2) contains a small change from the last edition of the National Electrical Code (NEC). Previously, this section only applied to boxes containing angle or “U” pulls. Now, boxes containing splices must also comply with this calculation method... even if the box only contains straight pulls. A straight-pull junction box with spliced conductors (No. 4 or larger) must comply with the minimum size requirements of Section 370-28(a)(1) and (2). Simply calculate by both methods, compare the dimensions, and select the largest size.
For example, a box is needed for eight 2-inch conduits...four on the left side and four on the right. Each conduit will contain 2/0 conductors that will be spliced within the box. What is the minimum length required for the left/right (“X”) dimension?
The minimum distance required because of the straight pull calculation method, in Section 370-28(a)(1), is 16 inches (2 H 8 = 16). However, because of the spliced conductors, compliance with the angle or “U” pull calculation method is also required. Since each side contains the same size and number of raceways, either can be used for the calculation.
The largest conduit on the left side is 2 inches, therefore multiply two by six (12). Add to that number the other trade-size conduits on the same side of the box (12 + 2 + 2 + 2 = 18). The minimum length required for the “X” dimension is 18 inches. Prior to the 1999 NEC, the minimum size for the “X” dimension would have been 16 inches.
Where a raceway or cable enters the wall of a box (or conduit body) on the opposite side of a removable cover, Table 373- 6(a) may determine the distance from that wall to the cover. The distance from the entry wall to the cover is permitted to comply with the distance required for one width per terminal in Table 373-6(a). The distances specified are based on the wire size (AWG or kcmil) within the raceway and not the raceway’s trade diameter. Without this exception, the depth of the box would be determined by the angle pull method.
For example, a pull box contains two conduit entries. While one 2-inch conduit enters the bottom, the other enters the back of the box. The raceways will enclose No. 3/0 conductors. What depth is required for this box? Without the exception, the required depth would be 12 inches. Look at the first column of Table 373-6(a) and find the conductor size (3/0-4/0). Follow that line across, and select the distance specified in the “1” column under “Wires per Terminal.”
The minimum depth required for this box is 4 inches. The distance from the raceway entering the bottom of the box to the top is calculated by the angle pull method. Multiply the 2-inch raceway by six, and since no other conduits are entering the bottom, nothing is added to that number. The minimum length required for the top/bottom (“Y”) dimension is 12 inches.
Compliance with one other dimension must be observed. Since the two 2-inch raceways enclose the same conductors, the distance between raceway entries must not be less than six times the trade diameter of the largest raceway. Therefore, the conduit entries must be at least 12 inches apart.
370-28(a)(2) Transposing Cable Sizes
Pull or junction boxes enclosing cables containing conductors of No. 4 or larger must also comply with the requirements of Section 370-28(a)(1) and (2). The first step for sizing boxes containing cables is to convert each cable size into a raceway size. To transpose cable size into raceway size, simply find the minimum trade size raceway required for the size and number of individual conductors within the cable. Use Appendix C, or Tables 4 and 5 of Chapter 9, to find the raceway size.
For example, a junction box is needed for two cables...one entering the left side and one entering the right. Each cable contains three No. 2 XHHW conductors. First, convert the cable size into a raceway size. Appendix C, Table C1 shows that the minimum size raceway required for three No. 2 XHHW conductors is 1 inch. Next, size the junction box. The straight-pull calculation method must be employed because the cables are on opposite walls. Since the conductors within the cable would require a one-inch raceway, use 1 inch as the cable’s trade diameter. Multiply the trade diameter by eight
(1 x 8 = 8). The minimum length required for the left/right dimension is eight inches.
370-28(a)(3) Conduit Bodies
Conduit bodies (and boxes) with dimensions less than those required in Section 370-28(a)(1) and (2) are permitted under the conditions found in Section 370-28(a)(3). Where the conduit body (or box) has been approved for and is permanently marked with the maximum size and number of conductors allowed, compliance with the straight or angle pull sizing method is not required.
For example, two 3-inch rigid metal conduits are entering an LB conduit body. The conduit body is a threaded type with 3-inch conduit entries. Although the length required for the angle pull is 18 inches (3 x 6 = 18), the conduit body’s length is only nine inches. Because the conduit body has been approved for and is permanently marked, this installation is permitted.
Caution is advised where using conduit bodies enclosing larger-size conductors. Conduit bodies do not have to be large enough to contain the same size and number of conductors permitted in the raceway that enters the conduit body. A conduit body may have markings indicating the maximum size and number of conductors... and this marking may be less than what is permitted within the entering raceway.
Therefore, look on (or in) the conduit body for specific conductor limitations.
MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classed and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached via e-mail at charles@charlesRmiller.com.