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The sizing requirements for pull boxes, junction boxes, handhole enclosures, and conduit bodies exist to prevent conductor insulation damage. Those requirements are in 314.28, and they apply to all conductors 4 AWG and larger ( Fig. 1 ). To illustrate how these requirements prevent conductor insulation damage, let's consider two extremes in a straight pull situation.
Suppose you have a 2-inch raceway coming into a 10-inch square box, and a 2-inch raceway leaves it on the opposite side. If you're pulling several 1/0 AWG conductors through the box, how are you going to pull them through one side and into the other without bending them so much you damage the insulation? Answer: You're not.
Now, suppose you have those same two raceways mounted on a 4-foot box. Obviously, you have plenty of space, and you won't need to make severe bends in the conductors. Because you don't have the luxury of installing 4-foot boxes for every wire pull you do, you need to calculate the box size you really need.
The scenario we just discussed is a straight pull, one in which the conductor enters the box on one side and exits on the other - but not at an angle and not with any splices. It's a "straight-through" installation. The NEC addresses these separately from other types of pulls.
Straight pulls
Why would you install a pull box for a straight pull? Why break the raceway with a box at all? One reason is a very long conductor run; another is to restart the 360 degrees bend limitation of the applicable raceway article. The pull box provides another lubrication point, plus it allows you to reduce the force needed to pull that run in the first place.
For straight pulls, 314.28(A)(1) requires that the length of the box be at least eight times the trade size of the largest raceway. To correctly apply 314.28(A)(1), just multiply the largest raceway size by eight. The box must be...