Published June 13, 2026

NEC 314.16 Box Fill, Broken Down

Box fill trips up apprentices and seasoned guys alike — not because the math is hard, but because it's easy to forget that devices, clamps, and grounds all "count" even though they're not conductors. Here's the plain-English version of NEC 314.16(B).

The Core Idea: Everything Becomes a "Conductor Equivalent"

Instead of measuring actual cubic inches for every odd-shaped part in a box, the NEC simplifies things: every item in the box — conductors, devices, clamps, grounds — gets converted into a number of "conductor equivalents," all based on the volume allowance for the largest conductor in the box. NEC Table 314.16(B) gives the per-conductor volume by wire size:

Wire Size	Volume Allowance
14 AWG	2.00 in³
12 AWG	2.25 in³
10 AWG	2.50 in³
8 AWG	3.00 in³
6 AWG	5.00 in³

How Each Item Counts

Conductors entering or passing through the box: 1 allowance each, based on that conductor's own size.
Devices (switches, receptacles): Each device yoke counts as 2 allowances, based on the largest conductor connected to it.
Internal cable clamps: All internal clamps together count as just 1 allowance (based on the largest conductor in the box) — no matter how many clamps there are.
Equipment grounding conductors: All the grounds together typically count as 1 allowance (at the largest ground size present), up to a point — if you have a large number of grounds, an additional fractional allowance can apply per 314.16(B)(5), so it's worth double-checking against the current code edition for boxes with many grounds.

One important note: conductors that simply originate and terminate inside the box (like a pigtail) generally don't count — it's conductors entering from outside the box that matter.

A Quick Example

Say you have a single-gang box with: two 12/2 NM cables entering (4 conductors total), the bare grounds from both cables, one internal cable clamp, and one duplex receptacle (1 device yoke).

4 conductors × 1 = 4
Grounds (all together) × 1 = 1
Internal clamp(s) × 1 = 1
Device yoke × 2 = 2

Total = 8 conductor equivalents. Since everything is 12 AWG, multiply 8 × 2.25 in³ = 18.0 in³ required. Your box needs to be rated at least 18.0 cubic inches to be compliant.

Mixed Wire Sizes

If a box has both 14 AWG and 12 AWG conductors, each conductor uses its own size's allowance — but devices, clamps, and grounds all use the allowance for the largest conductor size present in the box, even if that's not the size connected to the device itself.

Use our Box Fill Calculator — pick your box (or enter a custom volume), add your conductor sizes and quantities, devices, clamps, and grounds, and it'll total everything up and tell you if you're within NEC 314.16.

The Bottom Line

Box fill comes down to one habit: don't just count wires. Walk through conductors, then devices (×2), then clamps (1 total), then grounds (1 total, with care for larger ground counts), all converted to the volume of your largest conductor. Get in the habit of running this on every box during rough-in — it's a lot cheaper to grab a bigger box now than to redo it at inspection.

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