NEC Box Fill Calculator
Calculate box fill per NEC 314.16, all 5 counting rules from Table 314.16(B). Free, no account required.
1. Select Your Box
2. Conductors
Each conductor entering or passing through the box counts as 1 volume allowance per its size.
3. Clamps, Supports & Devices
Clamps and supports each add 1 volume allowance (largest conductor). Each device on a yoke adds 2.
Each device = 2 volume allowances based on largest conductor
4. Equipment Grounding Conductors
NEC 2023: Up to 4 EGCs = 1 volume allowance. Each EGC beyond 4 = additional ¼ volume.
FAIL
Box is too small. Select a larger box or reduce conductors/devices.
Volume Breakdown, NEC 314.16(B)
132.5% of box volume used
NEC 314.16(B), The 5 Counting Rules
1. Conductors: Each conductor entering or passing through = 1 volume allowance based on its own size.
2. Cable clamps: All internal clamps combined = 1 volume allowance based on the largest conductor.
3. Support fittings: All studs/hickeys combined = 1 volume allowance based on the largest conductor.
4. Devices: Each device on a yoke or strap = 2 volume allowances based on the largest conductor connected.
5. EGCs: Up to 4 EGCs = 1 volume allowance (largest EGC). Each beyond 4 = additional ¼ volume (NEC 2023).
NEC Table 314.16(B), Volume Allowances
| Wire Size | Volume (in³) |
|---|---|
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
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This calculator uses NEC 2023 (NFPA 70) Table 314.16(A) and 314.16(B) values. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction.
How the box fill calculation works
Box fill is a volume comparison. Table 314.16(A) gives the volume of standard metal boxes in cubic inches, and nonmetallic boxes are marked with their volume. Table 314.16(B) assigns a volume allowance per conductor based on the largest conductor size involved in each rule. Add up the allowances, compare against the box volume, and the fill either passes or it does not.
There are five counting rules in 314.16(B) and the reason candidates fail this topic is that each rule uses a different basis for which conductor size to charge. Conductors get one volume each based on their own size. Clamps get one volume total, based on the largest conductor in the box. Support fittings get one each based on the largest conductor. Devices get two volumes each based on the largest conductor connected to that device. Equipment grounding conductors get one volume total for all of them, based on the largest EGC present, plus one more total if there is a separate isolated ground.
Two things do not count at all: wire connectors and pigtails made up entirely inside the box, and a conductor that passes through without splice or termination counts as one only if it is at least twice the minimum length required by 314.17(B), which is 6 inches of free conductor.
Step by step
- 1
Establish the box volume
Table 314.16(A) for standard boxes, or the marked volume for nonmetallic. Add listed volumes of any plaster rings or extension rings.
- 2
Count conductors
One volume each per 314.16(B)(1), at each conductor's own size. Loop-through conductors count as one.
- 3
Count clamps
One volume total per 314.16(B)(2), based on the largest conductor in the box. Cable connectors outside the box do not count.
- 4
Count support fittings
One volume each per 314.16(B)(3) for fixture studs and hickeys, at the largest conductor size.
- 5
Count devices
Two volumes per yoke or strap per 314.16(B)(4), at the largest conductor connected to that device.
- 6
Count grounding conductors
One volume total per 314.16(B)(5) for all EGCs, at the largest EGC size, plus one more for an isolated grounding conductor.
- 7
Compare
Total allowance must not exceed the box volume.
Worked example
A 4 x 1 1/2 inch square box contains two 12/2 NM cables and one 12/3 NM cable feeding a single receptacle. Internal clamps are present. Does it pass?
- Box volume, Table 314.16(A), 4 x 1 1/2 square = 21.0 cubic inches.
- Insulated conductors: two 12/2 give 4 hot and neutral conductors, 12/3 gives 3. Total 7 at 2.25 cubic inches each per Table 314.16(B) = 15.75.
- Clamps: one allowance at the largest conductor, 12 AWG = 2.25.
- Device: one yoke, two allowances at 12 AWG = 4.50.
- Grounding: three EGCs but one allowance total at 12 AWG = 2.25.
- Total = 15.75 + 2.25 + 4.50 + 2.25 = 24.75 cubic inches.
Answer: No. 24.75 cubic inches required against 21.0 available is an overfill of 3.75 cubic inches. A 4 x 2 1/8 inch square box at 30.3 cubic inches, or adding a listed plaster ring with a marked volume, resolves it.
How this is tested on the exam
Box fill is one of the most reliably tested calculations on journeyman exams and one of the most reliably missed. The two errors that account for most failures are charging every grounding conductor separately instead of one total, and charging a device one volume instead of two. Both are single-line rules in 314.16(B) and both are worth memorizing verbatim.
Frequently asked questions
Do wire nuts count toward box fill?
No. 314.16(B) does not assign a volume to wire connectors. Pigtails that begin and end inside the box are also not counted, since they never leave the box.
How many volumes does a receptacle or switch take?
Two, per 314.16(B)(4), and they are based on the largest conductor connected to that yoke, not the largest conductor in the box. A device wider than a single 2-inch gang takes two allowances for each gang it occupies.
How do I count grounding conductors?
All equipment grounding conductors together get a single volume allowance, sized to the largest one present, per 314.16(B)(5). If an isolated equipment grounding conductor is also present it gets one additional allowance, for two total.
Does a plaster ring add usable volume?
Yes, if it is marked with its volume. Plaster rings, extension rings, and domed covers that are listed and marked with cubic-inch capacity are added to the box volume per 314.16(A).
What about a conductor that just passes through the box?
A conductor running through the box without splice or termination counts as one volume allowance, provided it is at least twice the 6-inch minimum free length required by 314.17(B). Shorter than that and it is treated as terminating.