Boxes containing conductors smaller than 4 AWG are calculated from the sizes and numbers of conductors. Boxes containing 4 AWG or larger, under 600 volts, are calculated from the sizes and numbers of raceways. Where pull and junction boxes are used on systems over 600 volts, the installation must comply with the specifications in 314.70. Three types of calculation procedures are provided: straight pull, angle pull and U pull. Last month's column discussed U pulls. This month, the discussion continues with U-pull calculations.
314.28(A)(2) Angle or U Pulls
Where splices or angle or U pulls are made, the distance between each raceway entry inside the box and the opposite wall of the box must not be less than six times the trade size (metric designator) of the largest raceway in a row. A U pull is where two raceways enclosing the same conductors are located on the same wall of a box. Calculate a box containing both angle and U pulls by the same method as angle pulls. For example, a box will contain angle and U pulls. The bottom of the box contains two 2-inch conduits that will enclose the same conductors. On the bottom and right side of the box is one 3-inch conduit. The two 3-inch conduits make up an angle pull that will enclose the same conductors. The top and left side of the box contains no conduits.
All conductors are larger than 6 AWG. Since the largest conduit on the bottom is 3 inches, multiply three by six (3 x 6 = 18). Add to that number the trade size of the other raceways on the bottom of the box (18 + 2 + 2 = 22). The minimum length required for the bottom to top dimension is 22 inches. There is only one conduit on the right side, therefore multiply three by six (3 x 6 = 18). Because of the single conduit on the right side, nothing is added to 18 inches. The minimum length required for the right to left dimension is 18 inches (see Figure 1). The distance between raceway entries enclosing the same conductors must be at least six times the trade size (metric designator) of the larger raceway. Calculating the dimension for a box containing one U pull will require an addition step for the walls adjacent to the wall where the raceways enter. For example, the bottom of a box contains two 2-inch conduits that will enclose the same 3/0 copper conductors. There are no other raceway entries in the box. The minimum distance from the wall with the raceways to the opposite wall (top/bottom dimension) is 14 inches (2 x 6 + 2 = 14).
Since the two conduits enclose the same conductors, there is a minimum distance between them. Multiply one of the 2-inch conduits by six (2 x 6 = 12). Unlike the requirement for sizing an angle pull, do not add anything else to this number. The minimum distance between the two raceways on the bottom is at least 12 inches (see Figure 2).
The previous calculation provided the minimum distance between the two raceways but did not provide the distance for the horizontal (left/right) dimension. The minimum horizontal dimension is 12 inches plus the space needed for the two raceways and their fittings. When calculating the distance, include enough room for the proper installation of the locknuts and bushings. In the previous example, the trade size of each conduit is 2 inches. Approximately 3 inches is needed for 2-inch locknuts and bushings. The approximate distance needed for the horizontal (left/right) dimension is 18 inches (12 + 3 + 3 = 18, see Figure 3). Regardless of where they are located, raceway entries that enclose the same conductors cannot be less than six times the trade size (metric designator) of the largest raceway. Even if the raceways are on different walls, the distance between them must be at least six times the trade size of the larger raceway. For example, the left side of a box contains one 2-inch conduit. The bottom of the box also contains one 2-inch conduit. The two conduits entering this box will enclose the same conductors.
Calculate the minimum left/right (horizontal) dimension by multiplying the 2-inch conduit by six (2 x 6 = 12). Since no other conduit enters on the same wall of the box, nothing is added to the 12 inches. Calculate the minimum top/bottom (vertical) dimension by multiplying the 2-inch conduit by six (2 x 6 = 12).
Again, nothing is added to the 12 inches. In accordance with this provision, the minimum size box required is 12-by-12 inches. If a 12-inch square junction or pull box is installed, placement of the conduits is critical.
When a single conduit is installed in one wall of a box, it is often positioned in the center of that wall. This gives the box and conduits a symmetrical appearance. If the left conduit is installed in the center of the left side and the bottom conduit is installed in the center, the installation will be in violation of 314.28(A)(2). Because the raceways enclose the same conductors, the distance between them cannot be less than six times the trade size (metric designator) of the largest raceway (see Figure 4).
This does not mean that sizing the box in accordance with the calculation procedures will not work. A 12-by-12 box is permitted if the distance between the raceway entries is at least 12 inches. This can be accomplished by moving the left conduit up and the bottom conduit to the right (see Figure 5).
Since there is no stipulation limiting the size junction or pull box, a larger box could be installed to compensate for having to locate the conduits off-center. The raceways could be installed in the center of each wall if the distance between the raceway entries is at least six times the trade diameter of the larger raceway.
For example, a box is needed for two 2-inch conduits. One conduit will enter the left side and the other will enter the bottom. Although the minimum size box required is only a 12-by-12, by using a larger box, the conduits can be centered (see Figure 6).
Next month's column continues the discussion of pull- and junction-box calculations. EC
MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com or www.charlesRmiller.com.