Welcome to the world of electrical engineering! Sizing pull boxes is a common task, and it's good you're paying attention to the details.You're correct about 314.28(A)(1) covering the length of the box for straight pulls. However, the NEC doesn't explicitly have a formula for determining the depth of the pull box, other than to say it must be large enough to physically accommodate the conductors without exceeding the fill requirements or damaging the insulation.Here's the breakdown of how to figure out the depth of a pull box for a straight pull, combining NEC requirements and practical considerations

Key Considerations and Relevant Code Sections:

1. Minimum Wire Bending Space at Terminals (312.6)

This is the MOST relevant code section for determining the depth. While not explicitly for pull boxes, it provides the crucial dimensions needed to safely bend conductors. You need to ensure the wires entering and exiting the box have sufficient bending space.

Table 312.6(A) (For installations at terminals) and Table 312.6(B) (For installations within cabinets and cutout boxes) provide the minimum bending space. You'll be looking at the column that corresponds to the wire size and voltage. Crucially

The bending space is measured from the end of the lug (or terminal) to the opposite wall of the enclosure. So, you need to consider the depth of any lugs/connectors you are using within the box.2.

Conductor Fill (314.16)

While not directly dictating depth, you need to ensure the box is large enough to accommodate all the conductors and any devices without exceeding the fill requirements. Excessive fill will cause heat buildup and is a code violation. This is more of a check after you've determined your depth based on bending space.3.

Conductor Size

Larger conductors require more bending space, directly impacting the required depth.4.

Number of Conductors

While not directly related to depth via a formula, the number of conductors, combined with their size and the need to maintain separation, will contribute to the overall depth required.5.

Arrangement of Conductors

How you arrange the conductors within the box significantly affects the required depth. Consider layering, staggering, and using cable ties or other means to keep the conductors organized.6.

Connectors

The physical dimensions of connectors (lugs, split bolts, etc.) add to the overall depth requirement. Choose compact connectors whenever possible.7.

Ease of Installation and Future Access

The NEC emphasizes safe and workmanlike installations. Ensure you have enough room to easily install the conductors, make connections, and perform future maintenance. A cramped box is a safety hazard.

Steps to Determine Pull Box Depth


Identify the Largest Conductor

The largest conductor will dictate the minimum bending space.2.

Determine the Required Bending Space

Use

Table 312.6(A) or Table 312.6(B), based on the voltage and conductor size, to find the minimum bending space at terminals.3. Consider Connector Depth

Add the depth of the connectors (lugs, etc.) you are using to the bending space.4.

Account for Conductor Clearance/Layering

Add additional depth to accommodate the physical space needed for the conductors to enter and exit the connectors without excessive bending or strain. Consider layering the conductors if necessary. A general rule of thumb is to allow at least a conductor diameter space between rows if layering.5.

Check Conductor Fill (314.16)

Ensure the calculated box volume (based on length, width, and your calculated depth) is sufficient to accommodate all conductors per 314.16. If it's not, you'll need to increase the depth and/or width until you meet the fill requirements.
6.

Practical Considerations

Add a bit of extra space for ease of installation and future access. It's generally better to err on the side of a slightly larger box than a too-small box.

Example

Let's say you have the following scenario: Straight pull (3) 500 kcmil conductors per phase 600V system Using compression lugs1.

Largest Conductor

500 kcmil2.

Bending Space

Look up 500 kcmil in Table 312.6(A) (Terminals). For 600V, let's assume the insulation type requires the minimum. You might see something like 10 inches.
3.

Connector Depth

Let's say the compression lugs you're using add 2 inches to the depth.4.

Clearance/Layering

You want to allow some room for the conductors to bend and to keep the rows of lugs separated. Add another 2 inches.5.

Estimated Depth

10 inches (bending space) + 2 inches (lug depth) + 2 inches (clearance) = 14 inches minimum depth.6.

Check Conductor Fill (314.16)

Calculate the volume of the box (length x width x 14 inches). Use 314.16 to calculate the allowable fill based on the number and size of conductors. If the fill requirements are exceeded, increase the depth and/or width until you comply.

Important Notes



Consult the Manufacturer's Instructions

Always follow the manufacturer's instructions for the installation of connectors and other components. They may have specific bending radius requirements.

Local Amendments

Be aware that local jurisdictions may have amendments to the NEC that could affect pull box sizing.

Use Good Judgment

Code is a minimum standard. Always use good engineering judgment to ensure a safe and practical installation.

In summary: While there's no single formula for pull box depth, using 312.6 as the foundation, combined with consideration of conductor size, connectors, and fill requirements, will lead you to a safe and compliant design. It's a good idea to document your calculations for future reference and inspection. Good luck! Flag for review