EC&M Pull and Junction Box Calculations can be found on Mike Holt's forum code.
Pull boxes are used to simplify wiring installation.They are made of sheet metal, cast metal or a non-metallic material and can be used to pull conductors long distances without placing too much strain on the wire or insulation.Pull boxes allow long wiring runs to be installed in shorter intervals.
Pull boxes don't have wiring connections inside, unlike junction boxes.They are only used for pulling conductors into a raceway system.In conduit runs where the number of bends between outlets exceeds the maximum allowed by the NEC, their use is mandatory.
Pull box size is determined by the number of conductors, raceways and diameter.Pull boxes and junction boxes must be sized according to NEC article 314.28.A calculation example and an overview of the sizing rules for the most common types of pulls will be provided in this post.
On the opposite sides of the box, conduit enters and leaves.The size of the largest conduit and the amount of space required by locknuts and bushings determine the depth of a box for a straight pull.The pull box must be at least eight times the diameter of the largest conduit.
The NEC article 314.28(A)(2) states that boxes and conduit bodies containing conductors 4 AWG or larger must be sized.
The distance between the raceway entry and the opposite wall of the box must be the sum of:
The pull box has three raceways and a 90 turn.The procedure for calculation would be as follows.
Straight distance between the entry and exit points of each circuit must be six times the conduit diameter.The required separations are shown in the example above.
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The distance from where the raceways enter to the opposite wall is calculated with the same procedure used for angle turns.
The entry and exit points are counted separately for each conductor run.If a circuit uses 3” conduit and a pull box for a turn, the procedure is as follows:
The straight-line distance between the entry and exit point is six times the diameter of the conduit.The required separation is 6 x 3.
When there are straight and angle pulls in the same box, the calculations must be done according to both sets of rules.The following combination pull is assumed.
Six times the largest raceway plus the sum of the rest is what this calculation is about.There is only one raceway and nothing else is added.
The largest value is chosen when both calculations are performed.The straight pull is 8 times the diameter, and the largest raceway is 6 times larger.
The pull box has to be at least 18” tall and 20” wide.The distance between the entry and exit is six times larger than the conduit.
Pull boxes are convenient, but they require materials and labor, and can increase project cost if used too much.The best way to ensure that pull boxes are only used where necessary is to hire a qualified design firm.
The electrical installation costs can be improved by energy efficiency.Smaller wiring and smaller conduit diameter are required for a lower load.In a large project such as a high-rise building, the price difference can add up quickly.