# Pull- and Junction-Box Calculations, Part II

314.28 Pull and Junction Boxes

The National Electrical Code (NEC) has specific requirements detailing how to calculate the maximum numbers of conductors in boxes (outlet, device, junction, pull, etc.) and conduit bodies. Provisions for boxes and conduit bodies enclosing conductors 18 AWG through 6 AWG are in 314.16. These boxes are calculated from the sizes and numbers of conductors.

Boxes and conduit bodies enclosing conductors larger than 6 AWG (under 600V) must also comply with the provisions of 314.28. These boxes are calculated from the sizes and numbers of conduits or raceways. Last month’s Code in Focus started a discussion of pull- and junction-box sizing requirements. This month, the discussion continues with straight-pull calculations.

314.28(A)(1) Straight Pulls

In straight pulls, the length of the box must not be less than eight times the trade size (metric designator) of the largest raceway. The type of pulls must be known before the size of the box can be calculated. A box having two raceway entries, on walls opposite of each other, will obviously be a straight pull. A box having two raceway entries, on walls that are not opposite of each other, will be an angle pull (see Figure 1). A box having four raceway entries, two on one side and two on the opposite side, could contain either straight pulls or U pulls. A box having four raceway entries, one on each side, could contain either straight pulls or angle pulls (see Figure 2).

Where raceways enter on all four sides of the box and all the pulls are straight, two separate calculations are needed. First, find the trade size (metric designator) of the largest raceway for the left/right (horizontal) dimension and multiply by eight. Next, find the trade size (metric designator) of the largest raceway for the top/bottom (vertical) dimension and multiply by eight. For example, the left and right side of a box contains one 3-inch conduit each. The top and bottom of the same box contains one 2-inch conduit each. Both pulls will be straight pulls.

What are the minimum dimensions required for this box? The largest size raceway for the left/right (horizontal) dimension is a 3-inch conduit. Therefore, the minimum length for this dimension is 24 inches (3 x 8). Since the largest size raceway for the top/bottom (vertical) dimension is a 2-inch conduit, the minimum height of the box is 16 inches (see Figure 3).

The minimum size box required for the example in Figure 3 is 24 by 16 inches. What if, upon installation, this box is rotated 90 degrees? Of course, one dimension would be well within the required specifications, but the other dimension would be in violation of the Code. One solution, which eliminates the possibility of installing the box incorrectly, is to buy a square box equal to the largest dimension.

Where different size conductors enter the box, select the largest. For example, a pull box is needed for 10 raceways. All the pulls will be straight pulls. Two 3-inch conduits enter the left side of the box. One 4-inch and one 2-inch conduit enter the box on the right side. One 3-inch and two 2-inch conduits enter the top of the box. Three 2-inch conduits enter the bottom of the box. Since the largest size raceway for the left/right (horizontal) dimension is a 4-inch conduit, the minimum length for this dimension is 32 inches (4 x 8). The largest size raceway for the top/bottom (vertical) dimension is a 3-inch conduit. Therefore, the minimum length for this dimension is 24 inches (3 x 8) (see Figure 4).

314.28(A)(2) Angle or U Pulls

Boxes and conduit bodies containing conductors of 4 AWG or larger (under 600V), and containing angle or U pulls must be sized in accordance with the specifications in 314.28(A)(2). Where splices or where angle or U pulls are made, the distance between each raceway entry inside the box and the opposite wall of the box must be at least six times the trade size (metric designator) of the largest raceway in a row. This distance must be increased for additional entries by the amount of the sum of the diameters of all other raceway entries. This specification applies to raceways in the same row on the same wall of the box. Boxes containing angle or U pulls are not calculated the same as boxes containing straight pulls.

Calculate the dimensions of a box with angle pulls by starting with one wall where the raceways enter the box and find the distance to the opposite wall of the box. The calculations are based on the same wall where the conduits enter, to the opposite wall of the box, not to where the conductors actually go.

First, select one wall of the box and multiply the trade size (metric designator) of the largest raceway by six. If there are no other raceway entries on the same wall, the calculation for that wall is finished. If the box only contains two raceways where angle pulls are made, the box will be square. For example, the right side of a box contains one 2-inch conduit, and the bottom contains one 2-inch conduit. What are the minimum dimensions of this box? Since there are only two raceways entering this box and the pulls are angle pulls, the box will be square. The minimum size required for this box is 12 inches by 12 inches (2 x 6) (see Figure 5).

The distance between raceway entries enclosing the same conductors must not be less than six times the trade size (metric designator) of the largest raceway. [314.28(A)(2)] Caution is advised when installing a box with only two raceways where the pull will be an angle pull.

Although raceways installed in a symmetrical manner may look more professional, in this type of installation they could be in violation of the Code. For example, the right side of a box contains one 2-inch conduit, and the bottom contains one 2-inch conduit. If a 12-inch square box is installed, the raceways must not be centered. The distance between the raceway entries must be at least 12 inches (see Figure 6).

Next month’s Code in Focus 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.