NEC Article 314 establishes requirements for the installation and use of electrical boxes, conduit bodies, fittings, and handhole enclosures. A conduit body is a removable-cover section of a conduit system that provides access at junctions or termination points.

Scope of NEC Article 314 (314.1)

Article 314 applies to:

• Outlet boxes

• Device boxes

• Junction boxes

• Pull and splice boxes

• Conduit bodies (such as LB fittings)

• Handhole enclosures

• Fittings used to connect raceways and cables

These enclosures are used to contain splices, terminations, devices, and raceway connections.

Box Materials and Construction Rules

Round Boxes (314.2)

Round boxes cannot be used where raceways or connectors requiring locknuts or bushings attach to the side of the box.

Nonmetallic Boxes (314.3)

Nonmetallic boxes have specific limitations and generally should not be mixed with metallic raceways or fittings unless listed for such use.

Metal Boxes (314.4)

Metal boxes are grounded and bonded to maintain electrical continuity and ensure safety.

Screws and Fasteners (314.5)

Fasteners entering the wiring compartment must not damage conductors or compromise insulation. Improper screws can create safety hazards.

Box Installation Requirements

Flush Installation (314.20)

Boxes installed in walls or ceilings must be flush with the finished surface to allow proper cover installation and prevent fire spread.

Repair of Noncombustible Surfaces (314.21)

Openings around boxes in noncombustible surfaces must not exceed ¼ inch to prevent fire spread.

Support and Mounting (314.23)

Boxes must be securely fastened to the structure using approved methods such as:

• Mounting to framing members

• Brackets or bar hangers

• Listed support systems

Boxes must remain rigid and protected from physical damage.

Box Dimensions (314.24)

Boxes must have sufficient depth and internal space to accommodate devices and wiring without damaging conductors.

Boxes in Damp or Wet Locations (314.15)

Boxes installed in  damp or wet environments must:

• Be listed for the environment

• Prevent moisture entry or accumulation by being either placed correctly

• Include appropriate weatherproof covers and gaskets

Interior locations such as basements and barns are classified as damp. Other damp locations include underground, those with concrete slabs, those that directly contact the ground, and other locations that are either unprotected from weather or in contact with water.

Conductors Entering Boxes (314.17)

Conductors must enter through approved openings and be protected from abrasion.

Requirements include:

• Approved connectors or clamps

• Bushings where needed

• Protection against insulation damage

Covers and Canopies (314.25)

Boxes must be equipped with covers unless they contain a device or luminaire canopy. Covers must be secure and listed for the application.

Ceiling Fan Boxes (314.27)

Boxes supporting ceiling-suspended (paddle) fans must be listed and marked for fan support to handle vibration and weight.

Pull and Junction Box Sizing (314.28)

Boxes used with raceways must provide sufficient space for conductor bending and pulling.

Straight Pulls

Minimum length = 8× the trade size of the largest raceway.

Angle or U Pulls

Minimum distance = 6× the largest raceway size plus the sum of other raceways on the same wall.

These requirements prevent conductor damage during installation.

Accessibility Requirements (314.29)

Boxes and conduit bodies must remain accessible without removing building finishes.

They cannot be concealed behind:

• Drywall or plaster

• Paneling

• Cabinets

• Permanent structures

Access panels are permitted if they allow full access.

Handhole Enclosures (314.30)

Handholes provide underground access to conductors and must be:

• Structurally sound

• Accessible for personnel entry

• Protected from environmental damage

Box Fill and Conductor Space Limits (NEC 314.16)

Electrical boxes must provide sufficient space for conductors and devices to prevent overheating and insulation damage.

Overcrowding restricts heat dissipation and increases fire risk.

Box fill violations are among the most common inspection failures, so careful calculation is a must. Too many times it is discovered that there are too many conductors without any grace given to spacing when the box is opened. Proper box sizing is required for heat dissipation, protection of conductor insulation, and adequate spacing.

Conductor Volume Allowance

Each conductor is assigned a volume value based on its size.

What Counts Toward Box Fill

Important: Calculations are based on the largest conductor present.

Step-by-Step Example Calculation

Box contains:

• Two 12/2 cables, so 4 insulated conductors

• Equipment grounding conductors

• One switch

• Internal clamp

Step 1: Count conductors

• Insulated conductors = 4

• Grounds = 1

• Device = 2

• Clamp = 1

Total conductor count = 8

Step 2: Multiply by volume allowance

12 AWG conductors = 2.25 cu in

8 × 2.25 = 18 cubic inches minimum box volume

As a result, the box must be rated for at least 18 cubic inches.

Tips to Avoid Box Fill Violations

• Use deep boxes when installing switches or GFCIs

• Plan box size before running multiple cables

• Avoid unnecessary splices inside device boxes

• Use larger junction boxes when combining circuits

Common Cables Found in Device & Junction Boxes

The wiring method used determines which cables appear inside boxes. In residential work, NM-B cable is most common, but other wiring methods may be present if these are not standard residential conditions.

• NM-B (nonmetallic-sheathed cable) — standard residential wiring

• 14/2 & 14/3 for lighting

• 12/2 & 12/3 for receptacles and multi-wire circuits

• 10/2 & 10/3 for appliances and HVAC

• MC (metal-clad) cable for multifamily and commercial buildings, or where added protection is required

• Individual conductors in conduit (THHN/THWN-2, XHHW-2) for commercial and industrial raceway systems

• UF-B cable for feeds outdoor or underground circuits

Regardless of the wiring method, box fill calculations apply equally to all cables.

Use our conduit fill calculator to determine the calculation in your specific case.