# Box fill calculations (NEC 314.16)

> Box fill is the minimum volume a box needs for the conductors, devices, clamps and fittings inside it. Count each item the way NEC 314.16(B) says, multiply each count by the volume allowance for its conductor size, and add the results.

Exams: Journeyman Electrician, Master Electrician, Residential Wireman. Boxes and raceways.

## Formula

    Minimum box volume (in³) = Total conductor count × Volume allowance for the size (in³)

## Variables

- **Count**: Conductor equivalents counted under 314.16(B) (conductors)
- **Allowance**: Volume per conductor for that size, from 314.16(B) (in³)

## Open your 2026 NEC to

- 314.16(B): Volume allowance per conductor and the counting rules
- Table 314.16(A): Volume and conductor counts of standard metal boxes

Every box has to be big enough that conductors are not crammed in, overheated or nicked when a device is pushed back. NEC 314.16 sets that minimum by turning everything in the box into conductor equivalents, then giving each equivalent a volume based on its size.

On the exam you either find the minimum volume a box needs, or check how many conductors fit in a box of known volume. Both use the same counting rules. The volume allowance per conductor is in 314.16(B): look it up in your code book for each size you use.

The traps are the items that do not count as one conductor each: a device yoke counts as two, all the equipment grounding conductors together count as one (for up to four), all internal cable clamps together count as one, and pigtails that start and end inside the box count as zero.

## Worked example

A device box will hold one receptacle and 3 insulated 12 AWG conductors that enter from outside the box. It also holds 3 equipment grounding conductors, all 12 AWG. Using NEC 314.16(B), what is the minimum box volume in cubic inches?

1. Insulated conductors from outside the box: 3, each counts once.
2. Receptacle yoke: counts as 2.
3. Equipment grounding conductors: all 3 together count as 1.
4. Internal cable clamps: 0 (all clamps together count as 1, none counts 0).
5. Total count: 3 + 2 + 1 + 0 = 6.
6. Look up the 314.16(B) allowance for 12 AWG: 2.25 in³ per conductor.
7. Minimum volume: 6 × 2.25 = 13.50 in³.

**Answer: 13.50 in³**

## How to solve it on the exam

1. List every insulated conductor that enters the box from outside: each one counts once, whether it is spliced, terminated or just passes through.
2. Add two conductor equivalents for each device yoke (receptacle or switch), sized by the largest conductor connected to that device.
3. Add one for all equipment grounding conductors together (up to four; a quarter allowance for each one beyond four), sized by the largest EGC.
4. Add one for all internal cable clamps together and one for each type of fitting such as a fixture stud or hickey, sized by the largest conductor.
5. Multiply each count by the 314.16(B) allowance for its size and add the volumes. Pigtails that start and end in the box are not counted.

## Common mistakes

- Counting a device yoke as one conductor instead of two.
- Counting each equipment grounding conductor separately instead of one for all of them (up to four).
- Counting each internal clamp separately, or forgetting clamps entirely.
- Using one allowance for a box with mixed sizes: each conductor gets the allowance for its own size, and each yoke uses the largest conductor connected to that device.
- Rounding up when finding how many conductors fit in a box. A partial conductor does not fit, so always round down.
- Counting pigtails that start and end inside the box.

## Exam tips

- Write the count as a small table on scratch paper: conductors, yokes, EGCs, clamps, fittings. It keeps you from dropping an item under time pressure.
- On the Texas exam you can use your NEC: tab 314.16 so you can open the allowance list in seconds.
- For "how many conductors fit" questions, divide the box volume by the allowance first, round down, then subtract the yokes, grounds and clamps.

## Questions people ask

### How many conductors does a device count as in box fill?

A device or equipment on a single yoke counts as two conductors, sized by the largest conductor connected to it. A device wider than a standard 2 inch gang counts two for each gang it needs.

### Do ground wires count in box fill?

Yes, but all equipment grounding conductors together count as one conductor, sized by the largest one, for up to four EGCs. Each EGC beyond four adds a quarter of that allowance.

### Do pigtails count in box fill?

No. A conductor that starts and ends inside the box, such as a pigtail to a receptacle, is not counted.

### Where are the box fill rules in the 2026 NEC?

Section 314.16. The volume allowance per conductor size and the counting rules are in 314.16(B), and the standard metal box sizes are in Table 314.16(A).

## Sources

- [TDLR: Electrician examination information](https://www.tdlr.texas.gov/electricians/elecexam.htm)
- [PSI: Texas electrician candidate information bulletin](https://test-takers.psiexams.com/api/content/bulletin/1682)

Interactive practice: https://texaselectricianexam.com/calculations/box-fill/