Yes, you're likely on the right track with NEC 314.28(A) which replaced 312.6 and now governs pull and junction boxes, conduit bodies, and handhole enclosures. However, before diving directly into the calculation, there are a few important points to clarify

1. Identify the Box Type and Purpose:
Pull Box vs. Junction Box

Is this box used only for connecting wires (junction box), or is it also used to pull conductors through (pull box)? If it's for pulling, the rules are more stringent.

Straight Pulls vs. Angle or U-Pulls

This significantly affects the calculation.

2. NEC 314.28(A): The Key Section

This section provides the rules for determining the minimum size of pull and junction boxes. It's divided into two main parts: 314.28(A)(1): Straight Pulls

For straight pulls (where conductors enter one side of the box and exit directly opposite), the minimum length of the box must be at least eight times the trade diameter of the largest raceway entering the box.

314.28(A)(2): Angle or U-Pulls

This is where it gets more complex. The distance from the raceway entry to the opposite wall must not be less than six times the trade diameter of the largest raceway, PLUS the sum of the diameters of all the other raceways on the same wall or row. For U-pulls, double the six times the trade diameter requirement.

3. Steps for Determining Minimum Box Size (Using NEC 314.28(A))


Determine the Largest Raceway

Identify the raceway (conduit) with the largest trade diameter entering the box.2.

Identify Other Raceways on the Same Wall

If it's an angle or U-pull, identify all other raceways on the same wall or row as the largest raceway.3.

Calculate Minimum Dimensions



Straight Pull

Minimum length = 8 x (Trade Diameter of Largest Raceway)

Angle or U-Pull

Minimum distance = (6 x Trade Diameter of Largest Raceway) + (Sum of Trade Diameters of Other Raceways on the Same Wall). Double the 6x factor for U-pulls.4.

Consider Wire Bending Space (NEC 314.28(A)(3))

For conductors No. 6 AWG and larger, and for cables containing conductors No. 6 AWG and larger, the minimum bending space at terminals must be provided as specified in 312.6(B). This might require a larger box than the straight/angle pull calculations alone.
5.

Enclosure Volume (NEC 314.16)

Although 314.28(A) dictates minimum dimensions, you also need to ensure the box has sufficient cubic inch capacity for the number and size of conductors and devices within the box. NEC 314.16 provides the rules for calculating the required volume. This calculation will be important for boxes with many conductors or splices.

Example (Angle Pull)

Let's say you have a box with the following: One 3-inch conduit entering one wall. Two 2-inch conduits entering the same wall. It's an angle pull.1.

Largest Raceway

3-inch conduit.2.

Other Raceways

Two 2-inch conduits.3.

Minimum Distance

(6 x 3 inches) + (2 inches + 2 inches) = 18 inches + 4 inches = 22 inches.Therefore, the distance from the entry point of the 3-inch conduit to the opposite wall must be at least 22 inches.

Key Considerations



Trade Size vs. Actual Diameter

Make sure you're using the trade size of the conduit, not the outside diameter.

Conduit Bodies

If you are using conduit bodies with specific markings, follow the manufacturer's instructions and the marking information, it may have exceptions to the 6x rule.

Splices and Taps

If you're making splices or taps within the box, you must have sufficient space and volume as per NEC 314.16. The space for splices is in addition to the minimum dimensions calculated for the pull.

Derating

Ensure that the box provides enough space for heat dissipation from the conductors. Overcrowded boxes can lead to conductor overheating and insulation failure.

Manufacturer's Instructions

Always consult the manufacturer's instructions for the specific box you're using.

Steps to Take if the Box is Undersized


Verify the Calculations

Carefully double-check your calculations using the correct NEC sections and trade diameters.2.

Consult with a Qualified Electrician or Engineer

This is crucial. An experienced professional can properly assess the situation, perform the necessary calculations, and recommend the correct solution.3.

Replace the Box

If the calculations definitively show the box is too small, you'll likely need to replace it with a larger one that meets the NEC requirements.4.

Consider Conduit Bodies (with caution)

In some limited situations, using properly sized conduit bodies to make the bends might be an option, but this requires careful analysis and compliance with all applicable code sections. Don't assume this is a simple fix.

Important Safety Note: Working with electrical systems can be dangerous. If you are not a qualified electrician, do not attempt to perform any electrical work yourself. Always consult with a licensed and experienced electrician for any electrical installations or modifications. Electrical work performed incorrectly can lead to serious injury, death, and property damage. Flag for review