314.28 Pull and Junction Boxes
Article 314 of the National Electrical Code covers the installation and function of all boxes and conduit bodies used as outlet, device, junction or pull boxes, depending on their use. This article also covers conduit bodies, handhole enclosures and installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies. [314.1] Boxes and conduit bodies containing conductors 18 AWG through 6 AWG must meet the installation provisions in 314.16.
Boxes enclosing these conductors are calculated from the sizes and numbers of conductors. Boxes and conduit bodies enclosing conductors larger than 6 AWG, under 600 volts, must meet the installation requirements in 314.28. Boxes containing conductors of 4 AWG or larger, under 600 volts, are calculated from the sizes and numbers of raceways.
As long as the there are conductors larger than 6 AWG in the raceways, the actual sizes and numbers of conductors are not relative to the calculation. Where pull and junction boxes are used on systems over 600 volts, the installation must comply with the specifications in 314.70. Last month’s column discussed U pulls. This month, the discussion continues with pull- and junction-box calculations.
314.28(A) Minimum Size Pull and Junction Boxes
Up to this point, minimum size boxes have been calculated for either straight pulls, angle pulls or U pulls. Straight- and angle-pull methods have been covered individually because none of the examples included both types of pulls in the same box. However, there are many installations where more than one type of pull is located in a single box. A pull or junction box must be of sufficient size to meet the minimum size specifications for each type of pull located within the box (see Figure 1).
Where a box contains both straight and angle pulls, calculate using both methods separately. After finishing the calculations, compare and choose the largest size for each dimension. First, compare and select the largest size for the horizontal (left/right) dimension. Next, compare and select the largest size for the vertical (top/bottom) dimension. For the purpose of these illustrations, the left/right dimension is referred to as the X dimension, and the top/bottom dimension is the Y dimension. To review, straight-pull dimensions are calculated by multiplying the largest conduit by eight. Unlike angle pulls, no additional conduit entries are added to the sum of the straight-pull calculation.
For example, a box with four conduits will contain one straight pull and one angle pull. While the left side of the box contains one 3-inch and one 2-inch conduit, the right side contains only one 3-inch conduit. The bottom of the box contains one 2-inch conduit, and no conduit is entering the top of the box. Calculating by the straight-pull method, the minimum length for the X dimension is 24 inches (3 x 8 = 24).
Since the left/right dimension also contains an angle pull, calculate by the angle-pull method. The largest conduit on the left side is 3 inches, therefore multiply three by six (3 x 6 = 18). Add to that number the other trade size conduit on the same side of the box (18 + 2 = 20). Not considering the dimension required for the straight pull, the minimum length required for the angle pull is only 20 inches. But the box must be sized to meet the applicable requirements for all the pulls. The minimum length required for the AX@ dimension is 24 inches (see Figure 2).
In this example, calculating the top/bottom (vertical) dimension is easy. Since no conduit enters the top of the box, a straight-pull calculation is not necessary. Multiply the 2-inch conduit by six. Since no other conduit enters on the same wall, no additional raceway diameters are added (2 x 6 + 0 = 12). The minimum length required for the AX@ dimension is 12 inches (see Figure 3).
The previous example demonstrated a box with a simple combination pull consisting of one angle and one straight pull. It is not uncommon to have boxes with multiple conduit entries on all sides. For example, the left side of a box contains two 3-inch conduits, and the right side contains one 2-inch and two 3-inch conduits. The top of the same box contains four 3-inch and two 2-inch conduits. The bottom contains one 4-inch and three 3-inch conduits. This box will contain both straight and angle pulls. Since this box contains both types of pulls, perform both types of calculations and select the largest for each dimension.
First, find the minimum length required for the X dimension. For the straight-pull calculation, multiply the largest raceway by eight. The minimum distance required because of the straight pull is 24 inches (3 x 8 = 24). Because the X dimension also has angle pulls, additional calculations are necessary. Since the largest conduit on the right side is 3 inches, multiply three by six (3 x 6 = 18). Add to that number the other conduits on the same side of the box (18 + 3 + 2 = 23). The angle-pull calculation for the left side of the box is only 21 inches (6 x 3 + 3 = 21). Because of the straight pull, the minimum length required for the X dimension is 24 inches (see Figure 4).
Next, find the minimum length required for the Y dimension. For the straight-pull calculation, multiply the largest raceway by eight. The minimum length required because of the straight pull is 32 inches (4 x 8 = 32). Since this dimension also contains angle pulls, additional calculations must be performed. The largest raceway entering the top of the box is a 3-inch conduit, therefore multiply three by six (3 x 6 = 18). Add to that number the other conduits on the same side of the box (18 + 3 + 3 + 3 + 2 + 2 = 31). Since the largest conduit on the bottom is 4 inches, multiply four by six (4 x 6 = 24). Add to that number the other conduits on the same side of the box (24 + 3 + 3 + 3 = 33). Although a 32-inch box is large enough for the straight pull, a larger box must be installed to provide enough room for the angle pulls. The minimum length required for the Y dimension is 33 inches (see Figure 5).
As demonstrated in the last illustration, the straight-pull calculation is not always larger than the angle-pull calculation. Therefore, it is necessary to perform both calculation methods and select the largest size for each dimension.
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.