Junction Box Sizing Calculator (NEC 314.28)

Quickly calculate NEC-compliant junction box dimensions for straight pulls, angle pulls, and splices. This tool helps electricians and engineers ensure code compliance with NEC 314.28 requirements.

Disclaimer: For guidance only. Confirm with local code and a qualified electrician. Based on NEC 314.28. Last updated: June 2023.

How this calculator works

This calculator determines the minimum dimensions for junction boxes based on the National Electrical Code (NEC) Article 314.28. The code specifies different calculation methods for straight pulls, angle pulls, and splices to ensure conductors can be properly installed and maintained.

Straight Pulls vs Angle Pulls

Straight pulls require a box length of at least 8 times the trade diameter of the largest raceway. Angle/U-pulls require a box length of at least 6 times the trade diameter of the largest raceway plus the sum of the trade diameters of other raceways in the same row.

Angle/U-pull calculations require information about conduit entries on different sides of the box. This feature will be implemented in a future update.

Splice calculations require information about the number and size of conductors being spliced. This feature will be implemented in a future update.

Results

Enter conduit details and click Calculate to see results.

Example Calculations

Here are some practical examples of how to use this calculator for common scenarios:

Example 1: Straight Pull with 1″ Conduit

For a straight pull with a single 1″ conduit, the minimum box length is calculated as 8 × 1″ = 8 inches.

Example 2: Straight Pull with Multiple Conduits

For a straight pull with two 1-1/4″ conduits and one 2″ conduit, the calculation is based on the largest conduit: 8 × 2″ = 16 inches.

Example 3: Angle Pull

For an angle pull with a 1-1/2″ conduit and a 1″ conduit in the same row: (6 × 1.5″) + 1″ = 9″ + 1″ = 10 inches.

Frequently Asked Questions

What is the NEC 314.28 code for junction boxes? +

NEC 314.28 specifies the minimum dimensions for pull and junction boxes based on the size and number of conduits entering the box, and the type of pull (straight, angle, or U-pull).

When do I need to use the angle pull calculation? +

Use the angle pull calculation when conductors enter and leave the box through different walls at an angle (not in a straight line). This includes U-pulls where conductors enter and exit on the same wall.

How do I account for multiple conduits in the same row? +

For straight pulls, only the largest conduit determines the box size. For angle pulls, you must add the diameters of all other conduits in the same row to the calculation (6 × largest conduit + sum of other conduits).

What safety margin should I use? +

A 10% safety margin is commonly used to account for variations in installation and potential future expansion. For critical applications or when future modifications are likely, a 20% margin may be appropriate.