How to use the box fill tables
Box fill is a volume check. Count every item in the box as a number of conductor "allowances" using the counting-rules table, multiply each allowance by the volume for its conductor size from Table 314.16(B)(1), add them up, and compare the total with the box volume. If the total is larger, use a bigger or deeper box, add an extension ring, or move a splice elsewhere.
The metal-box table gives each box's minimum volume and the most conductors of one size it can hold when nothing else is in it. Those maximums are simply the volume divided by the per-conductor allowance and rounded down, which is how this page computes them. Real boxes almost always contain clamps, a device or grounds, so the true number is lower.
Worked example
A single receptacle in a 3 × 2 × 3-1/2 in metal device box (18 in³), fed by two 12/2 NM cables with internal clamps:
- Insulated conductors entering the box: 4
- Internal cable clamps: 1
- Receptacle on one yoke: 2
- Equipment grounding conductors (all together): 1
Total 8 allowances × 2.25 in³ = 18.00 in³. That is within the 18 in³ box. Add one more 12/2 cable and the count rises to 10, or 22.50 in³, which no longer fits; a 4 × 2-1/8 in square box with a device ring (30.3 in³ before the ring) would.
Common box fill mistakes
- Forgetting the two allowances for each device yoke.
- Counting each ground separately (they count once, up to four) or ignoring them altogether.
- Using the conductor count from the metal-box table for a box that also holds clamps and a device.
- Not adding the marked volume of a plaster ring, extension ring or raised cover, which 314.16(A) allows you to include.
Box fill applies to 6 AWG and smaller conductors. Larger conductors fall under the pull and junction box sizing rules in 314.28. Values are from NFPA 70, 2023 edition; local amendments may apply.