Box Fill Calculation Example: 4-Inch Square Box
A National Electrical Code (NEC) 314.16 box fill calculation is one sum: add the volume allowance for every counted conductor, one allowance for the internal clamps, a double allowance for each device yoke, and one for the grounds as a group, then check the total against the box's rated volume. On a 4-inch square by 1½-inch box (21.0 cubic inches, Table 314.16(A)) carrying two 12/2 cables to a receptacle with clamps and grounds, the required volume is 18.00 cubic inches, so the box is adequate with 3.00 to spare. Below, that example step by step, the point where a third cable forces a bigger box, and how mixed conductor sizes change the fittings math. Default edition NEC 2020; the Table 314.16(B) allowances are unchanged since 2017.
Want the answer without the longhand? The Box Fill Calculator takes the box volume, conductor counts, device yokes, clamps, and grounds and returns the required volume, the pass/fail, and the full breakdown.
The five things you count
Every allowance in a box fill calculation comes from Table 314.16(B), keyed to conductor size: 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG, 3.00 for 8 AWG. The method in 314.16(B) then adds five categories:
- Conductors (B)(1): each conductor that enters the box and terminates or passes through counts once, at its own size. Pigtails that never leave the box do not count.
- Clamps (B)(2): one allowance if any internal cable clamp is present, at the largest conductor in the box. One clamp or six, still one allowance.
- Support fittings (B)(3): one allowance per luminaire stud or hickey, at the largest conductor. None in these examples.
- Device yokes (B)(4): a double allowance for each strap holding a device, at the largest conductor on that yoke.
- Grounds (B)(5): one allowance for all equipment grounding conductors together, at the largest ground present.
Example 1: two 12/2 cables, one receptacle
A 4-inch square by 1½-inch box (21.0 cubic inches) is used as a receptacle box. Two 12/2 NM cables enter through internal clamps, and a duplex receptacle sits on a single-gang mud ring. Every conductor is 12 AWG, so a = 2.25 cubic inches throughout.
Conductors: two blacks + two whites = 4 × 2.25 = 9.00 in³ (B)(1).
Clamps: one allowance at 12 AWG = 2.25 in³ (B)(2).
Receptacle yoke: 2 × 2.25 = 4.50 in³ (B)(4).
Grounds: two bare 12 AWG grounds, counted once = 2.25 in³ (B)(5).
Required: 9.00 + 2.25 + 4.50 + 2.25 = 18.00 in³.
The box holds 21.0 cubic inches against 18.00 required, so it is adequate with 3.00 to spare. The mud ring is marked with its own cubic-inch capacity that adds to the box under 314.16(A); we ignored it here, so the real margin is larger. Working from the bare box volume is the conservative move; it never overstates the room you have.
Example 2: the third cable that forces a bigger box
Bring a third 12/2 cable into the same 4×1½ box, say the receptacle is now mid-run with a switch leg pulled off it. The conductor fill climbs to six conductors:
Conductors: 6 × 2.25 = 13.50 in³ (B)(1).
Clamps: 2.25 in³ (B)(2).
Receptacle yoke: 4.50 in³ (B)(4).
Grounds: 2.25 in³ (B)(5).
Required: 13.50 + 2.25 + 4.50 + 2.25 = 22.50 in³.
Now 22.50 required exceeds the 21.0 box by 1.50 cubic inches, so the box is too small. Two clean fixes: step up to a 4×2⅛ square box (30.3 cubic inches, adequate with 7.80 to spare), or use a mud ring marked at 1.50 cubic inches or more to make up the difference on the existing box. Both are legitimate; the deeper box is usually the easier call on a rough-in.
Example 3: mixed sizes and the largest-conductor rule
Mixed conductor sizes are where box fill catches people. The clamp, device, and ground allowances are all based on the largest conductor in the box, not on each conductor's own size. Take a 4×1¼ square box (18.0 cubic inches) with one 12/2 cable and one 14/2 cable, a receptacle wired on the 12 AWG, internal clamps, and grounds. The largest conductor is 12 AWG, so every fitting allowance uses 2.25 cubic inches.
Conductors: two 12 AWG (2 × 2.25 = 4.50) + two 14 AWG (2 × 2.00 = 4.00) = 8.50 in³ (B)(1), each at its own size.
Clamps: one allowance at the largest conductor, 12 AWG = 2.25 in³ (B)(2).
Receptacle yoke: 2 × 2.25 = 4.50 in³ (B)(4), largest conductor.
Grounds: one allowance at the largest ground, 12 AWG = 2.25 in³ (B)(5).
Required: 8.50 + 2.25 + 4.50 + 2.25 = 17.50 in³.
The 18.0 box clears it with 0.50 to spare. Notice that the conductors themselves are counted at their own sizes (the 14 AWG conductors add 2.00 each, not 2.25), but the clamp, yoke, and ground allowances all jump to the 12 AWG figure because they follow the largest conductor. Miss that and you undersize the box.
Standard 4-inch square volumes
| 4-inch square box (Table 314.16(A)) | Minimum volume |
|---|---|
| 4 × 1¼ | 18.0 in³ |
| 4 × 1½ | 21.0 in³ |
| 4 × 2⅛ | 30.3 in³ |
These are the Table 314.16(A) minimums for a plain 4-inch square metal box. A raised cover, plaster ring, or extension ring adds its own marked cubic inches on top (314.16(A)). The larger 4-11/16 square boxes run from 25.5 up to 42.0 cubic inches when a 4-square runs short and depth alone will not close the gap.
NEC reference
NEC 2020. Volume allowances from Table 314.16(B); conductor, clamp, support, device-yoke, and grounding counting rules from 314.16(B)(1) through (B)(5); standard box volumes from Table 314.16(A). Table 314.16(B) covers 18 through 6 AWG only; a box with 4 AWG or larger conductors is sized by 314.28 instead, a dimension-based calculation. The ground allowance shown here is the NEC 2020 rule, all equipment grounding conductors as one allowance at the largest; confirm the wording if your jurisdiction has adopted a later edition, which has refined how additional grounds are counted.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
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NEC Code Quickstart
This guide covers one calculation. NEC Code Quickstart walks all twelve exam calculations start to finish, each with a worked example and the exact code reference: ampacity and derating, breaker sizing, voltage drop, box and conduit fill, grounding, dwelling load, range and dryer demand, motor circuits, and transformer current. Written for the 2023 NEC with notes for the 2026 edition.
Related
- Box Fill Calculator: run any box through the 314.16(B) allowances and check it against its volume.
- How Many Wires Fit in a Single-Gang Box: the same method applied to device-box capacity, with a table by depth.
- Conduit Fill Calculator: conductor fill for EMT, RMC, IMC, and PVC per NEC Chapter 9, Table 1.