Which Size Box?

An electrician asked me a very interesting question concerning the sizing of a junction or pull box and the difference between the two. They are basically the same. Electricians use junction and pull boxes as access points for pulling and feeding conductors through raceway systems. In many installations, use of boxes is necessary, or there would be a greater number of offsets, saddles and bends between outlets or fittings than are permitted per the National Electrical Code (NEC).

Pull and junction boxes

As a rule of thumb, pull boxes are used when conductors are pulled straight through a box and terminated down-stream. On the other hand, junction boxes can be used for splicing or tapping conductors. Field experience has taught electricians that it is not wise to skimp on the size or number of boxes needed in a run. Larger boxes provide more capacity, and they make pulling, splicing, tapping or positioning the conductors easier. However, boxes should not be installed where they are not really necessary. An excessive number of boxes can introduce excessive handling and splicing of conductors.

The NEC ensures sufficient working space in pull and junction boxes by specifying minimum dimensions for determining the size used for straight or angle pulls that enclose 4 AWG or larger conductors per 370.28. Section 370.16 outlines requirements for sizing boxes containing conductors 6 AWG or smaller. Remember, specific dimensions are used to calculate the box size.

Straight pulls

For a straight pull where the raceway enters and leaves on opposite sides of the box, 370.28(A)(1) requires the length be eight times the nominal diameter of the largest raceway (trade size). For example, if the raceway installed is 3 inches, then 24 inches (3 × 8 = 24) is the minimum length of the pull or junction box. However, the size of the raceway and the number of conductors spliced or pulled through govern the width and depth. Where two or more raceways are connected on the same wall of a box, consideration must be given to locknuts and bushings when selecting the size of the box. If the dimensions of locknuts and bushings are not evaluated and the box sized accordingly, the space used may extend beyond the wall of the box. Therefore, the NEC requires a larger box. Electricians sizing boxes for field installations of fittings and raceway(s) normally use 12 times the diameter that allows some excess capacity to accommodate such wall spacing.

Sizing angle and U pulls

Angle pulls consist of conductors entering one side and leaving from any side other than the opposite. Angle pulls are determined in a different manner than straight pulls.

Section 370.28(A)(2) requires the width of a pull or junction box to be at least six times the trade diameter of the largest raceway plus the sum of the remaining raceways on the same wall. Be careful, the dimensions of the locknuts or locknuts of fittings plus the bushing of the connected raceways involved should determine the depth of the box.

Considering an angle pull, if the enter and exit walls contain raceways of 3, 2 and 1 in. respectively, multiply 3 inches by 6, and add 2 inches and 1 inch. That equals 21 inches. Therefore, the size box for an angle pull is 21 inches by 21 inches. As another rule of thumb to size a box for an angle pull, multiply the largest of the raceways by 10. Doing this will provide the extra space needed for the fittings connecting raceways to the wall of the box.

When calculating the size pull or junction box for different size conductors rated 6 AWG or smaller, the cubic inch rating of each conductor from Table 370.16(B) must be selected and multiplied by the number of conductors based on each cubic inch rating. This total cubic inch rating is then used to size the box. For example, an 8-by-8-by-4-inch box is capable of housing conductors (combination of conductors 6 AWG or smaller) with a calculated cubic inch rating of 256.

Box design

Two basic designs are recognized when installing an angle or straight pull junction box, and they are either National Electrical Manufacturers Association (NEMA) 1 for an indoor installation or NEMA 3 for outdoor installation. For a laundry list, see Table 430.91 of the 2005 NEC or Table 110.20 of the 2008 NEC.

Final comment: Always reference the NEC for selecting the minimum dimensions to calculate the (angle or straight raceway run) size box to coincide with a particular installation.

STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.