Box fill: what it means for the Texas exam

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://bondingjumper.com/charts/bondingjumper/box-fill.svg" alt="An open device box from the front holding a receptacle on its yoke, two cables entering through internal clamps, a bundle of bare grounds and a wire nut splice, with count labels beside each item: two for the device, one for the clamps, one for all the grounds, one per conductor" width="100%"><p><a href="https://bondingjumper.com/guides/box-fill?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=box-fill.svg">Each item gets a count, each count gets a volume, and the total must fit the box. by Bonding Jumper</a></p>

The idea in two steps

1. Count the items in the box, as conductor equivalents, using the counting rules.
2. Multiply each count by the volume allowance for its conductor size from the NEC's table, add them, and compare with the box volume.

Article 314 holds the method. We do not reproduce its tables or volume figures here; read them in your own 2026 NEC.

How the counting works

Each conductor, clamp, device and the grounds are counted under 314.16(B); read the counting rules in your own 2026 NEC. Studying them from the book itself is the best way to learn them, because the details matter and a summary from memory is easy to get wrong. The pattern to remember is simple: count what is in the box, give each count a volume, add the volumes together and compare the total with the size of the box.

Every count is then valued at the allowance for the right conductor size, so a mix of 12 AWG and 14 AWG conductors in one box needs care.

Box fill vs conduit fill

Conduit fill depends on each conductor's area including its insulation, so the insulation type matters. Box fill uses a volume allowance by conductor size only, whatever the insulation. The two calculations look alike but use different tables. A raceway usually ends in a box, so you often meet both on the same run.

Common mix-ups

• Counting each ground separately. All of them together count once.
• Forgetting the device counts double.
• Using the smallest conductor size for a device count. It is based on the largest conductor connected to that device.
• Using the box's trade size instead of its marked volume.

On the Texas journeyman exam

Box fill sits in wiring methods and materials, 12 scored items across both portions under the PSI bulletin, and is a Calculations portion topic. Study to count a described box and find the minimum volume or the largest number of conductors allowed.

Practice it

The NEC box fill guide works the counting and volume steps with the book open. Every term in this set is in the electrical terms glossary.

Know the terms? Take the free 10-question check

Take the free 10-question check. It needs no signup and explains every answer, so you see whether theory, grounding or wiring methods costs you the most points. When you want the full method worked topic by topic, the Bonding Jumper course comes second.

Not affiliated with or endorsed by TDLR, PSI or NFPA. NEC and National Electrical Code are registered trademarks of NFPA.

Sources

Show the list

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