Yes, the pull box size is typically determined by the contractor, but with input and approval from the engineer or designer of record and subject to inspection. Here's a breakdown of why and what considerations they take into account

Why the Contractor Typically Sizes Pull Boxes:
Field Conditions Expertise

The contractor has the best understanding of the actual field conditions, conduit routing, installation techniques, and available space. They know how the conduits will be physically arranged.

Pulling Equipment

Contractors know what kind of pulling equipment they'll be using (e.g., rope, cable grip, winch). The pull box needs to be large enough to accommodate this equipment and the workers performing the pull.

Labor and Cost

The contractor is responsible for the labor and cost of installation. They'll try to optimize the pull box size to minimize material costs and installation time, while still meeting code requirements.

Coordination

The contractor is responsible for coordinating with other trades (e.g., civil, structural) to ensure that the pull box location and size are compatible with other building elements.

Considerations in Sizing Pull Boxes (Based on NEC and Common Practices)



NEC Article 314.28 (PULL AND JUNCTION BOXES AND CONDUIT BODIES)

This is the primary code section that governs pull box sizing. The following summarizes the key sizing rules.
Straight Pulls

For straight pulls, the length of the pull box must not be less than eight times the trade diameter of the largest raceway.

Angle or U Pulls

For angle or U pulls, the distance from the raceway entry point to the opposite wall of the pull box must not be less than six times the trade diameter of the largest raceway, plus the sum of the trade diameters of all other raceways on the same wall. The distance between raceway entries enclosing the same conductor must not be less than six times the trade diameter of the largest raceway.

"Hit and Turn" (Angle Pull) Scenario

Your situation (6-8 4" conduits with a hit and turn) falls under the angle pull rules. This is where the calculations get a bit more involved. The most critical considerations are the largest raceway and the sum of all the raceways entering the wall.

Practical Considerations for Underground Pull Boxes



Working Space

Ensure adequate space for workers to stand and operate pulling equipment within the pull box.

Sump/Drainage

Underground pull boxes should have a sump or drainage system to prevent water accumulation.

Cover/Access

The cover must be rated for the appropriate load (pedestrian, vehicular) and allow for easy access for maintenance.

Practical Considerations for Overhead Pull Boxes



Support

Pull boxes can be heavy, especially when filled with conductors. Ensure adequate structural support for the box and its contents.

Accessibility

The pull box must be accessible for maintenance and future wiring changes. This may require platforms or ladders.

Material Selection



Underground

Typically concrete, fiberglass, or polymer concrete.

Overhead

Typically steel or aluminum. The material should be suitable for the environment (e.g., corrosion resistance).

Manufacturer Data

Consult the manufacturer's data sheets for the pull boxes you are considering. They will provide dimensions, load ratings, and other important information.

Permitting and Inspection

Local codes and inspectors may have specific requirements for pull box sizing and installation. Be sure to check with the local authority having jurisdiction (AHJ).

Example Calculation (Illustrative)

Let's assume you have 8 4" conduits entering one side of an underground pull box, and you need to make a 90-degree turn:1.

Largest Raceway

4"2.

Sum of Other Raceways

7 x 4" = 28"3.

Required Length

(6 x 4") + 28" = 24" + 28" = 52"Therefore, the minimum length of the pull box (from the entry point to the opposite wall) would need to be 52 inches. You would then need to select a commercially available pull box that meets or exceeds this dimension. Furthermore, this calculation ONLY addresses minimum length. Width and depth need to be adequate for workers and pulling equipment.

In Summary: The contractor usually performs the pull box sizing. Calculations are based on NEC Article 314.28, specifically considering straight pulls versus angle/U pulls. Contractors consider field conditions, pulling equipment, labor costs, and coordination with other trades. Engineers or designers typically review and approve the contractor's pull box sizing to ensure it meets the design requirements.* The AHJ (electrical inspector) has the final say and can require changes if the installation doesn't comply with the code.It's strongly recommended that you consult with a qualified electrical engineer or designer to ensure that your pull box is sized correctly and meets all applicable codes and standards. They can perform the necessary calculations and help you select the appropriate pull box for your specific application. Flag for review