TradeToolsElectrical · in the TradeTools app
Box Fill Calculator
Whether the conductors, clamps, devices and grounds going into a box fit the volume the code allows for it — and the smallest listed box that does when they don't.
NEC 314.16 · Table 314.16(A) and (B)
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Worked example
Two 12-2 NM cables and a receptacle in a 3×2×2½ device box — does it fit?
| Box type | Device Box |
|---|---|
| Box | 3x2x2-1/2 · 12.5 cu.in. |
| Conductors | 4 × 12 AWG |
| Devices | 1-gang device (12 AWG) |
| Equipment grounding conductors | 2 · 12 AWG |
| Internal cable clamps | on |
| Fill | 144.0 % — over the volume allowed by 314.16 |
|---|---|
| Volume required | 18.00 cu in |
| Volume available | 12.50 cu in |
| Conductors | 9.00 cu in |
| Clamps | 2.25 cu in |
| Devices | 4.50 cu in |
| Grounding | 2.25 cu in |
| Smallest box that fits | Device Box — 3x2x3-1/2 |
Each 12 AWG counts 2.25 in³ (Table 314.16(B)): four conductors 9.00, one clamp allowance 2.25, two allowances for the receptacle yoke 4.50, one for the grounds together 2.25 — 18.00 in³. The 3×2×2½ box is listed at 12.5 in³ in Table 314.16(A); the 3×2×3½ at 18.0 in³ is the smallest device box in that table that takes it.
How it's calculated
The table behind the number
The box volume comes from Table 314.16(A) for standard metal boxes, or from the marking on a nonmetallic box or one not in the table, entered as a custom volume. Every conductor that originates outside the box and ends or is spliced inside it, or passes through, counts one allowance by its size from Table 314.16(B) — 2.00 in³ for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG. Internal cable clamps add one allowance of the largest conductor present; a fixture stud or hickey adds one each; a device yoke adds two allowances of the largest conductor connected to it; up to four equipment grounding conductors together count one allowance of the largest, and each one beyond four adds a quarter. The app adds it up, compares with the box, and names the smallest listed box that would pass.
- NEC 2023 314.16(A), Table 314.16(A)Volumes of standard metal boxes (3×2×2½ device box 12.5 in³; 3×2×3½ 18.0 in³; 4×1½ square 21.0 in³)
- NEC 2023 Table 314.16(B)Volume allowance per conductor (14 AWG 2.00 in³, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00, 6 AWG 5.00)
- NEC 2023 314.16(B)(1) to (5)Conductor, clamp, support fitting, device and equipment grounding conductor fill
Questions
Before you trust the number
Do the pigtails count?
No. Conductors no part of which leaves the box — pigtails and jumpers — are not counted. Each conductor that enters and is spliced or terminated counts once; a conductor that passes through unbroken counts once, or twice when the loop is at least twice the 6 in of free conductor 300.14 requires — 12 in or more.
Two cables, two ground wires — two allowances?
One. All equipment grounding conductors in the box together take a single allowance based on the largest of them when there are four or fewer; each one beyond four adds a quarter allowance (314.16(B)(5)). Two 12 AWG grounds are one 2.25 in³ allowance.
Does a clamp always cost volume?
Only clamps inside the box count — one allowance of the largest conductor present, however many clamps there are (314.16(B)(2)). A connector whose clamping mechanism sits outside the box, in the knockout, takes nothing.