Junction Box Sizing Calculator
Calculate proper junction box dimensions per NEC 314.28 requirements
Junction Box Configuration
Straight Pulls (NEC 314.28(A)(1))
Wires pulled horizontally from one side to opposite side
Wires pulled vertically from top to bottom or vice versa
Angle Pulls (NEC 314.28(A)(2))
Angle pulls: wires enter from one side and exit at 90° or 180° angle
Recommended Junction Box Size
⚠️ Electrical Safety Warning
• This calculator is for 4 AWG and larger conductor installations only
• Always follow local electrical codes and NEC 2020 Article 314.28
• Consult a licensed electrician for final design verification
• Obtain proper electrical permits before installation
• Improper junction box sizing can cause fire hazards and code violations
• Consider conduit bending radius and wire management requirements
Example: Standard Junction Box Calculation
Project Specifications
Application: Commercial electrical distribution
Horizontal Straight Pull: 2½" conduit
Left Side Angle Pull: One 4" conduit + Two 1" conduits
Bottom Angle Pull: Three 2" conduits
NEC 314.28 Calculations
• Horizontal Straight: 2.5" × 8 = 20"
• Left Angle Pull: (4" × 6) + 1" + 1" = 24" + 2" = 26"
• Bottom Angle Pull: (2" × 6) + 2" + 2" = 12" + 4" = 16"
• Minimum Length: max(20", 26") = 26"
• Minimum Height: max(0", 16") = 16"
Result: Use 30" × 20" junction box (next standard size)
Standard Conduit Sizes
Trade Sizes (inches)
NEC 314.28 Requirements
Straight Pulls
Length ≥ 8 × largest conduit size
Angle Pulls
Length ≥ 6 × largest + sum of others in row
Application
Required for conductors 4 AWG and larger
Safety
Prevents conductor damage during installation
Junction Box Types
Understanding Junction Box Sizing per NEC 314.28
What is NEC 314.28?
NEC Article 314.28 specifies minimum size requirements for pull and junction boxes to prevent conductor damage during installation and ensure safe wire bending radii. These requirements apply to conductors 4 AWG and larger.
Straight Pull Formula
Minimum Length = 8 × Largest Conduit Size
- •Applies to straight-through wire pulls
- •Same formula for horizontal and vertical pulls
- •Uses trade size of conduit, not actual diameter
Angle Pull Formula
Minimum Dimension = 6 × Largest + Σ(Other Conduits in Row)
- 6 × Largest: Six times the largest conduit in the row
- Σ(Others): Sum of all other conduits in same row
- Row: Conduits aligned perpendicular to pull direction
- Multiple Rows: Calculate each row, use largest result
Critical Safety Factors
Conductor Protection
Prevents insulation damage from sharp bends during installation.
Installation Access
Provides adequate space for proper wire management and connections.
Code Compliance
Ensures electrical installation meets national and local requirements.