# Pull and Junction Box Calculations

314.28 Pull and Junction Boxes

Article 314 of the National Electrical Code (NEC) covers the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes. 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] Article 314 is divided into four parts: I. Scope and General, II. Installation, III. Construction Specifications, and IV. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal.

Boxes and conduit bodies containing conductors 18 AWG through 6 AWG must meet the installation requirements in 314.16. Boxes and conduit bodies enclosing conductors 4 AWG or larger (under 600V) must be installed in accordance with the requirements in 314.28.

When determining the minimum size box for conductors 18 AWG through 6 AWG, the sizes and numbers of conductors are needed to calculate the minimum size box. Calculations are different for pull and junction boxes. Boxes containing conductors of 4 AWG or larger, under 600V, are calculated from the sizes and numbers of raceways (see Figure 1). Where pull and junction boxes are used on systems over 600V, the installation must comply with the provisions in Part IV of Article 314.

Specific provisions for calculating pull or junction boxes containing 4 AWG or larger conductors are covered in 314.28. For raceways containing conductors of 4 AWG or larger, and for cables containing conductors of 4 AWG or larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run must comply with 314.28(A)(1) through (A)(3). [314.28(A)]

These provisions apply to both metallic and nonmetallic boxes. The calculation for the minimum size box is based not only on the raceways, but where the conductors enter and exit the box. Will the conductors enter and exit the box on opposite walls? Will the conductors enter and exit on perpendicular walls? Or will the conductors enter and exit on the same wall? There are three types of pulls: straight pull, angle pull and U pull (see Figure 2).

Specifications for straight pulls are in 314.28(A)(1). Specifications for angle and U pulls are in 314.28(A)(2). Where an enclosure dimension is to be calculated based on the diameter of entering raceways, the diameter shall be the metric designator (trade size) expressed in the units of measurement employed. [314.28(A)]

When using the trade size of the raceway, the result of the calculation will be in inches. When using the metric designator of the raceway, the result of the calculation will be in millimeters.

314.28(A)(1) Straight Pulls

Boxes containing straight pulls are sized according to the largest single raceway entering the box. Only one calculation is needed for boxes having raceways entering two walls. Two calculations will be necessary for boxes with raceways entering four walls when all the pulls are 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. [314.28(A)(1)] The first step is to determine the type of pull. Installing a raceway in each end of a wireway (gutter) is one example of a straight pull. The ends of this pull box provide only enough room for one raceway each (see Figure 3).

Raceways directly opposite each other are one type of installation. To be considered a straight pull, it is not necessary for the raceways to be located straight across from each other. Boxes containing raceways on opposite walls, regardless of where they are located or the amount of offset, qualify as straight pulls (see Figure 4).

Calculating the minimum dimensions of a box containing straight pull(s) is quite simple: multiply the trade size (metric designator) of the largest raceway by eight. For example, calculate the minimum dimension for a pull box with two 2-inch conduits.

The conduits are directly across from each other in the ends of a 4-inch wireway. Since the conduits are the same size, multiply either by eight. The minimum length required for this box is 16 inches (see Figure 5).

The width and depth of the pull or junction box depends on the installed raceways. Unless a raceway enters the back of the box, no requirement specifies the width or depth. The box width and depth must be large enough to provide proper installation of the raceway (or cable), including locknuts and bushings (see Figure 6).

Use the largest raceway to size pull boxes containing different-size raceways. For example, a pull box contains two conduits located on opposite walls. While one conduit has a trade size of 2 inches, the other has a trade size of 3 inches. The minimum length of this box is the trade size of the largest raceway (3 inches) multiplied by eight. The length of this pull box must be at least 24 inches (see Figure 7).

Provided the box only contains straight pulls, boxes having more than two raceway entries are calculated exactly the same. For example, three raceways will enter the left side and three raceways will enter the right side. No raceways will enter the top or bottom. All of the pulls in this box will be straight pulls.

The raceways entering the left side include one 3-inch and two 2-inch conduits. The right side contains one 4-inch, one 3-inch and one 2-inch conduit. Since all the pulls are straight, the calculation for this box only requires the largest trade size conduit. The largest raceway is 4 inches and therefore the minimum length for this box is 32 inches (see Figure 8).

No added calculation is required for additional raceways when calculating the minimum length of a box containing straight pulls only.

Because there are no raceways entering the top and bottom, the minimum width will depend on the installed conduits on each side. The width and depth of this box must be large enough to provide proper installation of the raceways, including locknuts and bushings.

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.