NEC Article 314 — Outlet Box Fill Calculations (How Many Conductors Fit in a 4×4 Box?)

NEC 314.16 sets outlet-box fill limits — the rule that prevents jamming too many conductors into a single box. Box fill failures are the #2 most-cited rough-in violation in NJ after working space.

The fill formula:

Each item below counts as ONE conductor of the largest size in the box, multiplied by the cubic-inch volume from NEC Table 314.16(B):

• 14 AWG: 2.0 cu in
• 12 AWG: 2.25 cu in
• 10 AWG: 2.5 cu in
• 8 AWG: 3.0 cu in
• 6 AWG: 5.0 cu in

What counts (per NEC 314.16(B)):

• Each conductor passing through or terminating = 1
• All ground conductors combined = 1 (regardless of how many)
• Each cable clamp inside the box = 1
• Each device (switch, receptacle) = 2 (one for each yoke)
• Each strap or stud = 1

Common boxes and their volumes (from Table 314.16(A)):

• Single-gang plastic 18 cu in box: holds 9 × 12 AWG conductors (18 ÷ 2.25 = 8)
• Single-gang 4-square 21 cu in: holds 9 × 12 AWG
• 4×4×1.5 metal square: 21 cu in
• 4×4×2.125 deep metal square: 30.3 cu in (holds 13 × 12 AWG)

Real example — single-gang receptacle box with 12 AWG NM-B:

Two NM-B cables enter (each has 2 phase + 1 ground):

• Phase + neutral conductors = 4 (× 2.25 = 9 cu in)
• Ground conductors = 1 (× 2.25 = 2.25 cu in)
• Cable clamps if internal = 1 (× 2.25 = 2.25 cu in)
• Receptacle device = 2 (× 2.25 = 4.5 cu in)

Total = 18 cu in. Fits in a single-gang 21 cu in box, but exceeds an 18 cu in.

Hudson County rough-in trick: when in doubt, use a deep box (4×4×2.125 = 30.3 cu in). The marginal cost is minimal and it eliminates 95% of fill issues. Old work boxes in finished walls are notorious for failing on fill.

For full conduit fill (different rule) see the free Malfettone Conduit Fill Calculator at /tools/conduit-fill, which handles the NEC Chapter 9 Tables 1, 4, and 5 sizing rules.