NEC 314.16 - Number of Conductors in Outlet, Device, and Junction Boxes

Overview

NEC 314.16 is one of the most frequently referenced code sections in electrical work. It establishes the maximum number of conductors permitted in outlet, device, and junction boxes based on the box volume and conductor size.

Box Fill Calculation Method

Step 1: Determine Box Volume

Use the marked volume on the box or refer to Table 314.16(A) for standard metal boxes.

Step 2: Count All Items

Each item in the box counts toward the fill calculation:

Conductor Count (314.16(B)(1))

• Each conductor = 1 count (based on largest conductor size in box)
• Conductors originating outside the box and terminating inside = 1 count each
• Conductors passing through without splice = 1 count each
• Looped conductors (unbroken) = 2 counts

Equipment Grounding Conductors (314.16(B)(5))

• All equipment grounding conductors = 1 count total (regardless of number)
• Based on largest equipment grounding conductor in the box

Cable Assembly Clamps (314.16(B)(2))

• One or more cable clamps = 1 count total
• Based on largest conductor in the box

Support Fittings (314.16(B)(3))

• Each fixture stud or hickey = 1 count
• Based on largest conductor in the box

Device or Equipment (314.16(B)(4))

• Each yoke or strap = 2 counts
• Based on largest conductor connected to the device

Volume Allowances per Conductor

Common Metal Box Volumes

4-inch Square Boxes

4⅛-inch Square Boxes

Single Gang Device Boxes

Calculation Examples

Example 1: Basic Outlet Box

Box: 4×4×1½" (15.5 cu in) Contents:

• 2 #12 THWN conductors (hot, neutral)
• 1 #12 equipment grounding conductor
• 1 duplex receptacle
• Cable clamps present

Calculation:

• Conductors: 2 × 2.25 = 4.5 cu in
• Grounding: 1 × 2.25 = 2.25 cu in
• Device: 2 × 2.25 = 4.5 cu in
• Clamps: 1 × 2.25 = 2.25 cu in
• Total: 13.5 cu in ≤ 15.5 cu in ✓

Example 2: Switch Box with Multiple Circuits

Box: 4×4×2⅛" (21.0 cu in) Contents:

• 4 #12 THWN conductors (2 hots, 2 neutrals)
• 2 #12 equipment grounding conductors
• 2 single-pole switches
• Cable clamps present

Calculation:

• Conductors: 4 × 2.25 = 9.0 cu in
• Grounding: 1 × 2.25 = 2.25 cu in
• Devices: 4 × 2.25 = 9.0 cu in (2 switches × 2 counts each)
• Clamps: 1 × 2.25 = 2.25 cu in
• Total: 22.5 cu in > 21.0 cu in ✗

Solution: Use larger box (24.5 cu in minimum)

Example 3: Junction Box

Box: 4⅛×4⅛×2⅛" (24.5 cu in) Contents:

• 6 #12 THWN conductors (3 circuits spliced)
• 3 #12 equipment grounding conductors
• Wire nuts (no additional count)
• No clamps (MC cable with built-in connectors)

Calculation:

• Conductors: 6 × 2.25 = 13.5 cu in
• Grounding: 1 × 2.25 = 2.25 cu in
• Total: 15.75 cu in ≤ 24.5 cu in ✓

Special Considerations

Nonmetallic Boxes

• Must be marked with their volume
• Same calculation method applies
• Cannot exceed marked volume

Conduit Bodies

• Limited conductor splicing allowed
• Must comply with 314.16 when containing splices
• Different rules for straight-through conductors

Mixed Conductor Sizes

• Use the largest conductor size for all calculations
• Example: Mix of #12 and #14 = use #12 (2.25 cu in) for all counts

Common Violations

1. Overcrowded boxes - Most common NEC violation
2. Incorrect conductor counting - Not counting all items properly
3. Using unmarked box volumes - Assuming volume without verification
4. Ignoring equipment grounding conductors - Forgetting the single count rule

Interactive Calculator

Calculator tools will be available in a future update.

Related Sections

• Table 314.16(A) - Metal Box Volume Requirements
• Table 314.16(B) - Volume Allowance per Conductor
• 314.17 - Conductors Entering Boxes
• 314.28 - Pull and Junction Box Sizing

Based on NEC 2023. Always verify with local jurisdiction requirements.