NEC Article 314 - Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures
Educational Notice: This content provides interpretations and summaries of NEC Article 314. For official code text, please refer to NFPA 70 - National Electrical Code. All content is for educational purposes and should not replace professional judgment or official code consultation.
Overview
NEC Article 314 covers the installation and use of outlet boxes, device boxes, pull boxes, junction boxes, conduit bodies, fittings, and handhole enclosures. This article is crucial for ensuring proper electrical connections and maintaining system safety.
Key Sections
314.16 - Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies
One of the most frequently referenced sections in electrical work, 314.16 establishes the maximum number of conductors allowed in boxes based on box volume and conductor size.
Box Fill Calculation Requirements
Safety Alert: Overfilled boxes can lead to damaged conductors, poor connections, and fire hazards. Always calculate box fill before making connections.
Standard Box Fill Formula:
- Each conductor = 1 count (based on largest conductor in box)
- Equipment grounding conductors = 1 count total (regardless of number)
- Cable clamps = 1 count total (if any clamps present)
- Fixture studs/hickeys = 1 count each
- Device yokes = 2 counts each
Common Box Sizes and Conductor Limits
| Box Size | Volume (cu in) | #14 AWG | #12 AWG | #10 AWG |
|---|---|---|---|---|
| 4" × 1¼" | 12.5 | 6 | 5 | 4 |
| 4" × 1½" | 15.5 | 7 | 6 | 5 |
| 4" × 2⅛" | 21.0 | 10 | 8 | 7 |
| 4⅛" × 1¼" | 14.0 | 6 | 5 | 4 |
| 4⅛" × 1½" | 16.5 | 8 | 7 | 6 |
| 4⅛" × 2⅛" | 24.5 | 12 | 10 | 8 |
314.17 - Conductors Entering Boxes, Conduit Bodies, or Fittings
Key Requirements:
- Conductor Protection: All conductors entering boxes must be protected from abrasion
- Unused Openings: Must be closed with appropriate materials
- Cable Clamps: Required for most cable entries (with specific exceptions)
314.20 - Metal Boxes
Metal boxes must be grounded in accordance with Article 250, except where isolated grounding is used per 250.146(D).
314.21 - Repairing Noncombustible Surfaces
When boxes are installed in noncombustible walls or ceilings, gaps around the box must not exceed ⅛ inch (3 mm).
314.23 - Supports
Boxes must be securely fastened in place. Key support requirements:
Box Support Methods:
- Structural mounting: Direct attachment to building structure
- Box supports: Listed mounting brackets and supports
- Conduit support: Under specific conditions with rigid/IMC conduit
Pro Tip: When supporting boxes with conduit, ensure the conduit is properly secured within 3 feet of the box and the box doesn't exceed specific size limitations.
314.24 - Depth of Outlet Boxes
Outlet boxes must have sufficient depth to accommodate conductors and devices:
- Minimum depth: Based on conductor size and number
- Device accommodation: Must allow proper installation without conductor damage
314.25 - Covers and Canopies
All boxes must have covers, and the covers must be suitable for the conditions of use.
314.27 - Outlet Boxes
Installation Requirements:
- Flush mounting: In finished surfaces, boxes must be flush with surface
- Combustible surfaces: Special requirements for installation in wood and similar materials
- Accessibility: Boxes must remain accessible after installation
314.28 - Pull and Junction Boxes and Conduit Bodies
For conductors 4 AWG and larger, specific sizing requirements apply:
Straight Pulls:
- Minimum length: 8 times the trade diameter of the largest raceway
Angle Pulls:
- Distance to opposite wall: 6 times the trade diameter of the largest raceway
- Additional raceways: Plus the sum of diameters of other raceways on the same wall
Practical Applications
Residential Wiring
- Switch boxes: Proper sizing for multiple switches and conductors
- Outlet boxes: Ensuring adequate space for GFCI and AFCI devices
- Junction boxes: Sizing for splice connections in accessible locations
Commercial Applications
- Device boxes: Accommodating multiple circuits and larger conductors
- Pull boxes: Proper sizing for feeder and branch circuit installations
- Conduit bodies: When and where they can be used for conductor changes
Common Violations and How to Avoid Them
Box Fill Violations
Problem: Too many conductors in a box Solution: Use larger boxes or reduce conductor count through proper circuit design
Improper Support
Problem: Boxes not adequately secured Solution: Use appropriate mounting methods for the installation type
Accessibility Issues
Problem: Boxes covered by finish materials Solution: Ensure all boxes remain accessible with proper covers
Calculation Examples
Example 1: Standard Outlet Box Fill
Scenario: 4×4×1½" box with:
- 2 - #12 THWN conductors (hot and neutral)
- 1 - #12 equipment grounding conductor
- 1 - duplex receptacle
- Cable clamps present
Calculation:
- 2 conductors: 2 × 2.25 = 4.5 cu in
- 1 grounding conductor: 1 × 2.25 = 2.25 cu in
- 1 device: 2 × 2.25 = 4.5 cu in
- Cable clamps: 1 × 2.25 = 2.25 cu in
- Total: 13.5 cu in
- Box volume: 16.5 cu in ✓ Compliant
Example 2: Pull Box Sizing
Scenario: Straight pull with 2" rigid conduit Calculation: 8 × 2" = 16" minimum length required
Related Code Sections
- Article 250 - Grounding and Bonding
- Article 300 - General Wiring Methods
- Article 312 - Cabinets, Cutout Boxes, and Meter Socket Enclosures
- Table 314.16(A) - Metal Boxes Volume Requirements
- Table 314.16(B) - Volume Allowance Required per Conductor
Interactive Tools
Calculator tools will be available in a future update.
Study Questions
- What is the minimum box volume required for 6 #12 AWG conductors with one device and cable clamps?
- How do you calculate the minimum length for a pull box with straight pulls?
- When are cable clamps required for cable entries into boxes?
- What are the grounding requirements for metal outlet boxes?
This article summary is based on the 2023 National Electrical Code. Always consult the current adopted code in your jurisdiction and verify with local authorities having jurisdiction.