Box fill calculation
Box fill is the check that an outlet or junction box has enough space for everything inside it. Under NEC 314.16, you count each item in the box as a number of conductor volumes, multiply by the volume allowance for the conductor size from Table 314.16(B)(1), and compare the total to the box volume. If the required volume is larger than the box, you need a bigger box or a marked extension ring.
What box fill is and why it matters on the exam
Box fill limits what goes in a box so conductors and splices are not crushed or overheated. The rules are in NEC 314.16 and apply to boxes and conduit bodies holding conductors 6 AWG and smaller. Larger conductors, 4 AWG and up, are handled by the pull and junction box sizing rules in 314.28.
Exam box fill questions test the counting rules more than the math. It is one of the core journeyman electrician exam calculations.
How to find the box volume
The box volume is the starting point, and it comes from one of two places under 314.16(A):
- Standard metal boxes: use the volume listed for that box size in Table 314.16(A).
- Nonmetallic boxes and other boxes: use the volume marked on the box by the manufacturer.
Plaster rings, extension rings, and domed covers that are marked with their volume can be added to the box volume. A ring marked 4.5 cubic inches on a 21.0 cubic inch box gives you 25.5 cubic inches to work with.
The counting rules in 314.16(B)
Every item in the box converts to conductor volumes:
| Item in the box | How it counts | Size used | NEC section |
|---|---|---|---|
| Conductor entering and spliced or terminated in the box | 1 each | That conductor | 314.16(B)(1) |
| Unbroken conductor passing through | 1 | That conductor | 314.16(B)(1) |
| Unbroken loop at least twice the 300.14 free length (12 in) | 2 | That conductor | 314.16(B)(1) |
| Pigtails and jumpers that never leave the box | 0 | Not counted | 314.16(B)(1) |
| Internal cable clamps, one or more | 1 total | Largest conductor in box | 314.16(B)(2) |
| Luminaire studs or hickeys | 1 for each type | Largest conductor in box | 314.16(B)(3) |
| Each device yoke or strap | 2 | Largest conductor connected to that device | 314.16(B)(4) |
| Up to four EGCs or bonding jumpers entering | 1 total | Largest EGC | 314.16(B)(5) |
| Each EGC beyond four | 1/4 each | Largest EGC | 314.16(B)(5) |
A few details catch people:
- External connectors are not counted. Only clamps inside the box take up space.
- A device wider than a standard single gang counts two volumes for each gang it needs.
- Fixture wires from a domed luminaire canopy: up to four fixture wires smaller than 14 AWG, with or without an EGC, can be left out of the count.
- Isolated ground conductors are included in the EGC count under the 2023 rule. They do not get their own separate full allowance.
Volume per conductor
Each conductor size has a volume allowance in Table 314.16(B)(1). The two you will use most are 2.00 cubic inches for 14 AWG and 2.25 cubic inches for 12 AWG. When a box contains mixed sizes, each conductor is multiplied by its own allowance, and items like clamps and yokes use the size the rule names (largest in the box, or largest connected to the device).
Required volume = (number of conductor volumes) × (volume per conductor) Maximum conductors = box volume ÷ volume per conductor, rounded downWorked example
A 4 in square box, 1-1/2 in deep, will hold a duplex receptacle and a single-pole switch on separate yokes. Two 12/2 NM cables with ground and one 12/3 NM cable with ground enter the box, and the box has internal cable clamps. All conductors are 12 AWG. Is the box big enough?
- Insulated conductors: two 12/2 cables give 4, and one 12/3 cable gives 3. That is 7.
- EGCs: three grounds enter the box. Up to four count as 1.
- Device yokes: two yokes × 2 = 4.
- Internal clamps: 1.
- Total: 7 + 1 + 4 + 1 = 13 conductor volumes.
- Required volume: 13 × 2.25 = 29.25 cubic inches.
Table 314.16(A) gives 21.0 cubic inches for a 4 in square box 1-1/2 in deep. That is 8.25 cubic inches short, so the box fails. The 2-1/8 in deep version of the same box is listed at 30.3 cubic inches, which leaves 1.05 cubic inches to spare, so it works.
Second example: more than four EGCs
A 4-11/16 in square box, 2-1/8 in deep (42.0 cubic inches in Table 314.16(A)), is used as a junction box. Six 12/2 NM cables with ground enter, all splices, with internal clamps.
- Insulated conductors: 6 × 2 = 12.
- EGCs: six enter. The first four count as 1, and the two extra count 1/4 each, so 1 + 0.5 = 1.5.
- Internal clamps: 1.
- Total: 12 + 1.5 + 1 = 14.5 conductor volumes.
- Required volume: 14.5 × 2.25 = 32.625 cubic inches.
The box has 42.0 cubic inches, so it passes with 9.375 cubic inches left over.
Conduit bodies
Conduit bodies follow 314.16(C). For conductors 6 AWG and smaller, a conduit body must have a cross-sectional area at least twice that of the largest conduit attached to it. Splices are allowed only in conduit bodies that are durably marked by the manufacturer with their volume, and then you calculate fill the same way as a box. Short-radius bodies such as capped elbows and service-entrance elbows cannot contain splices at all.
Common exam mistakes
- Counting every ground. Up to four EGCs count as one volume. Only the fifth and beyond add a quarter each.
- Counting each clamp. All internal clamps together count once.
- Giving a device one volume. Each yoke counts as two, based on the largest conductor connected to that device, not the largest in the box.
- Counting pigtails. A jumper that starts and ends inside the box is not counted.
- Rounding up. When finding the maximum number of conductors, always round down. 30.3 ÷ 2.25 = 13.47 means 13 conductors, not 14.
- Forgetting the ring. A marked plaster ring or extension ring adds its volume to the box.
Practice questions
Frequently asked questions
How many 12 AWG wires can go in a 4 in square box?
It depends on the depth. The 1-1/2 in deep box at 21.0 cubic inches holds 9 (21.0 ÷ 2.25 = 9.33), and the 2-1/8 in deep box at 30.3 cubic inches holds 13, with nothing else in the box. Devices, clamps, and grounds use up some of that.
Do ground wires count in box fill?
Yes. Up to four equipment grounding conductors entering the box count together as one volume, based on the largest EGC. Each EGC beyond four adds one quarter of a volume, per 314.16(B)(5).
Do wire nuts count toward box fill?
No. Twist-on wire connectors are not listed as an item to count in 314.16(B). Only conductors, clamps, support fittings, device yokes, and EGCs are counted.
Does a receptacle count as two conductors?
Yes. Each yoke or strap holding one or more devices counts as two conductor volumes, sized by the largest conductor connected to that device. A two-gang device counts four.
Where can I practice box fill problems?
Try a full journeyman practice test, then study conduit fill and wiring methods. The guide to passing the exam shows where box fill fits.