What is the 8x rule for junction boxes?

Electrical & MechanicalNEC 314.28 Code Compliance

Junction Box Sizing Calculator (NEC 314.28)

Calculate minimum required length, width, and depth for electrical junction boxes and conduit pull boxes containing No. 4 AWG and larger conductors.

Electrical & Mechanical Field StandardsNEC 314.28 Code Compliance

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

Under NEC Section 314.28, for straight pulls containing 4 AWG or larger conductors, the minimum pull box length must be at least 8 times the trade diameter of the largest raceway (e.g. an 8x2' = 16-inch box for 2-inch conduit). For angle pulls or U-pulls, the distance must be at least 6 times the largest raceway plus the sum of all other conduits on that wall.

NEC 314.28 applies strictly to systems with conductors sized 4 AWG or larger.
Angle pulls require at least 6x diameter between entry and exit raceways.
Always allow sufficient box depth for conduit ground bushings and locknuts.

Interactive Calculator

Adjust inputs for real-time results

Largest Trade Size Conduit

Conduit Pull Type

Sum of Other Conduits on Same Wallinches

inches

Sum of trade diameters of all other raceways entering the same wall.

Calculated OutputNEC 314.28 Compliant

Minimum Required Box Dimension

14.0"inches

Recommended Standard Box Size: 16" x 16" x 6"
Code Rule Applied: 6 × Largest Conduit + Sum of Others (NEC 314.28(A)(2))
Straight Distance Between Conduits: 12.0" in

Note: Applies to raceways containing conductors 4 AWG or larger. Depth must be sufficient for locknuts and bushings.

Straight vs. Angle Pull Rules Explained

Pulling heavy 4/0 or 250 kcmil copper conductors around tight corners causes severe wire insulation damage if junction boxes lack adequate radius clearance.

Straight pulls only require longitudinal clearance to pull slack, while angle pulls require diagonal bend clearance.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

The 8X rule (NEC 314.28(A)(1)) dictates that in straight conduit pulls where cables enter one wall and exit the opposite wall, the distance between opposite walls cannot be less than 8 times the trade size of the largest conduit.