Pull Box Sizing : A Comprehensive Guide for Engineers

Proper sizing of pull boxes is essential to ensure safe, code-compliant, and maintainable electrical installations. This guide provides a practical breakdown of pull box sizing rules as per NEC Article 314, focusing on different pull configurations and calculations engineers should consider.

Box Size Calculation with Angle Pull

In angle pulls, conduits enter and exit from adjacent sides of the pull box. NEC requires the distance from the entry point to the far side wall to be:

• 6 times the diameter of the largest conduit.

Formula:

Box Width/Height = 6 × D

Where D = Diameter of the largest conduit

Box Size Calculation with U-Pull

U-pulls involve conduits entering and exiting from the same wall, forming a U-shape.

Formula:

Box Width/Height = (6 × D largest) + ∑ D other

Where:

• D largest = Diameter of the largest conduit
• ∑ D other = Sum of diameters of the other conduits on that same wall

Box Size Calculation with Straight Pull

Straight pulls occur when conduits enter and exit on opposite sides of the pull box. This setup requires:

• 8 times the diameter of the largest conduit between opposite entries.

Formula:

Box Width/Height = 8 × D

Where D = Diameter of the largest conduit

Depth Calculation of Pull Box

The depth of a pull box is critical for maintaining bend radius and cable integrity during pulls:

Conduit Size	Minimum Box Depth Recommendation
Small (≤1″)	6 inches minimum
Medium (1.25″ – 2″)	3× diameter
Large (≥2.5″)	3 – 4× diameter or greater

General Rule:

Box Depth = 3 × D largest

Deeper boxes allow for better cable management, especially in complex or high-capacity installations.

Important Considerations

• Round Up : Always round up to the nearest standard box size to maintain compliance and ease of installation.

• Plan for Future Growth : Consider additional space for future conduit additions or cabling upgrades.

• Verify with Local Codes : While NEC Article 314 sets national standards, local jurisdictions may impose additional requirements.

Northern Link provides expert guidance and tools to help you design structured cabling systems that comply with code while optimizing space and performance.