NEC box fill: 314.16 counting and volume

Where to turn

• 314.16(A) gives the volume a box must have, and says where to read it. Standard boxes carry a table of marked volumes in cubic inches. Boxes with a marked volume use that marking.
• 314.16(B) tells you how many conductor-sized allowances each item uses, and a second table gives the volume per conductor by AWG size.
• 314.16(C) covers conduit bodies, which have their own rules.

In 2023 the counting subsections ran (B)(1) through (B)(5) for conductors, clamps, support fittings, devices and equipment grounding conductors. Confirm that numbering in the 2026 book and tab the page.

The method

1. List what is in the box: conductors, clamps, fittings, devices, grounding conductors.
2. Count one allowance for each conductor that comes in from outside and is spliced or terminated in the box, and one for each conductor that runs through unbroken. A conductor that starts and ends inside the box without leaving is not counted.
3. Add one allowance, sized to the largest conductor, if the box has one or more internal cable clamps. Fixture studs and hickeys are handled in the support fitting rule.
4. Add two allowances, sized to the largest conductor connected to it, for each device yoke or strap.
5. Add one allowance, sized to the largest equipment grounding conductor, for all equipment grounding conductors together.
6. Multiply each allowance by the volume per conductor for its size, then total.
7. Compare with the box volume. The box must be at least the total.

Worked example

Three 12 AWG copper NM cables with ground enter one box. Each cable carries two insulated conductors. All conductors are spliced or terminated inside. There is one single-pole switch and internal cable clamps.

• Conductors: 6 allowances
• Clamps: 1 allowance
• Switch yoke: 2 allowances
• Equipment grounding conductors: 1 allowance
• Total: 10 allowances at 2.25 cubic inches each for 12 AWG, so 22.5 cubic inches

Now compare. A 4-inch square box 1-1/4 inches deep is marked 18.0 cubic inches and fails. A 4-inch square box 2-1/8 inches deep is marked 30.3 cubic inches and passes. A box marked 21.0 cubic inches also fails, just by 1.5.

Common traps

• Counting grounds one by one. All equipment grounding conductors together count once.
• Missing the device. A switch or receptacle on a yoke is two allowances, not one.
• Counting jumpers that never leave the box. They are exempt from the count.
• Using the wrong size for clamps and devices. Use the size of the largest conductor in the box, or the one connected to the device.
• Mixed sizes. Value each allowance at its own size. Do not price every item at the smallest wire.
• Ignoring the marked volume. If a box carries its own volume marking, use it instead of the table.
• Depth and extensions. A plaster ring or extension changes the volume. Read the marking on it.

Working it with the book open

The exam is open book, but the clock is not. Tab 314.16, the volume table and the per-conductor allowance table so that each lookup takes seconds. Write the count as a short column of numbers on scratch paper, one line per item, and total it once. Most misses come from a missing line, not from arithmetic. If an answer lands within a cubic inch or two of a standard box volume, recount before you commit.

How to practice

Build five boxes on paper with different conductor sizes, one with two devices, one with a mixed 12 and 14 AWG set. Work the count, then pick the smallest standard box from the volume table. Check your counts by listing every item, not by estimating.

Related guides: conductor ampacity and conduit fill.

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• Electrician exam information, Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.