Short answer: Count allowances, multiply each by its volume, and compare the total with the box volume. Each insulated conductor from outside counts once. All internal clamps together count once. Each device yoke counts twice. The worked examples use the stated allowances to teach the calculation; actual box sizing must meet the applicable 2026 NEC requirements. A 14 AWG allowance is 2.00 cubic inches and 12 AWG is 2.25.
Box fill shows up on the Calculations part. The PSI outline puts Boxes and Enclosures in the Texas Journeyman Calculations part. The Texas Journeyman Practice Pack includes step-by-step solutions for every calculation question, each showing the rule, the given numbers, every step and why each wrong option is wrong. US$39 once.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the method in our own words and do not reproduce NEC text. The examples teach the calculation method; use the applicable 2026 NEC requirements for actual installations.
Where box fill lives in the 2026 NEC
NEC 314.16 covers box-fill calculations. Start with the box’s marked volume, count the applicable conductor allowances, and compare the results. The values below teach the calculation method; use the applicable 2026 NEC requirements for an installation. Texas uses the 2026 NEC for installations and exams from September 1, 2026 (TDLR rule adoption notice, TDLR exam page).
| Conductor size (AWG) | Volume per allowance (cubic inches) |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
The counting rules, in plain words
| Item in the box | How many allowances | Sized by |
|---|---|---|
| Insulated conductor that comes in from outside and ends or splices in the box | 1 each | Its own size |
| Conductor that runs through without a splice | 1 | Its own size |
| Conductor that stays entirely inside the box, such as a pigtail | 0 | Not counted |
| Internal cable clamps, one or more | 1 total | Largest conductor in the box |
| Luminaire stud or hickey | 1 for each type | Largest conductor in the box |
| Device or equipment on one yoke or strap | 2 per yoke | Largest conductor connected to that device |
| Equipment grounding conductors, up to four | 1 total | Largest grounding conductor |
| Grounding conductors beyond four (this lesson’s counting method) | 1/4 each | Largest grounding conductor |
The counting rules above follow the 2023 NEC method; actual box sizing must meet the applicable 2026 NEC requirements. Connectors mounted outside the box, such as NM connectors in the knockout, are not internal clamps.
Worked example 1: two devices in a 4-inch square box
A 4-inch square box marked 30.3 cubic inches, with a two-gang device ring, holds three 12/2 NM cables with ground, one duplex receptacle, one single-pole switch and internal cable clamps. All conductors are 12 AWG. For this example, the available volume is 30.3 cubic inches, with no additional volume credited to the device ring.
- Insulated conductors: 3 cables × 2 = 6, so 6 × 2.25 = 13.50 cubic inches.
- Internal clamps: 1 × 2.25 = 2.25.
- Devices: 2 yokes × 2 allowances = 4, so 4 × 2.25 = 9.00.
- Grounding conductors: 3, which is four or fewer, so 1 × 2.25 = 2.25.
- Total: 13.50 + 2.25 + 9.00 + 2.25 = 27.00 cubic inches.
- The calculated conductor fill is 27.00 cubic inches, below the assumed marked box volume of 30.3 cubic inches. This is the fill arithmetic under the stated assumptions.
Check by counting allowances: 6 + 1 + 4 + 1 = 12, and 12 × 2.25 = 27.00. Same answer.
Worked example 2: mixed sizes and five grounds
A box with no internal clamps (the cables use connectors outside the box) holds:
- Three 14/2 NM cables and two 12/2 NM cables, all with ground
- One single-pole switch connected only to 14 AWG conductors
- Pigtails made inside the box
- 14 AWG insulated conductors: 3 cables × 2 = 6, so 6 × 2.00 = 12.00 cubic inches.
- 12 AWG insulated conductors: 2 cables × 2 = 4, so 4 × 2.25 = 9.00.
- Pigtails: 0. These pigtails originate and terminate entirely within the box, so they add no conductor-fill allowance in this exercise.
- Switch: 2 allowances sized by the largest conductor connected to it, 14 AWG, so 2 × 2.00 = 4.00.
- Grounding conductors: 5. The first four count as 1 allowance at the largest size, 12 AWG: 2.25. The fifth adds a quarter allowance: 0.25 × 2.25 = 0.5625.
- Clamps: 0.
- Total: 12.00 + 9.00 + 4.00 + 2.25 + 0.5625 = 27.8125, about 27.81 cubic inches.
Using this lesson’s grounding-conductor counting method, the arithmetic total is 27.8125 cubic inches, which fits within a box marked 30.3 cubic inches but exceeds a box marked 27.0 cubic inches. The calculated fill is 2.4875 cubic inches below the 30.3-cubic-inch box volume.
Check: add in a different order. 2.25 + 0.5625 = 2.8125, plus 4.00 = 6.8125, plus 9.00 = 15.8125, plus 12.00 = 27.8125. Same total.
Worked example 3: how many conductors fit?
A box marked 18.0 cubic inches will hold only 14 AWG conductors, with no devices, clamps, fittings or grounds.
- Allowances that fit: 18.0 ÷ 2.00 = 9.
- Answer: 9 conductors of 14 AWG.
With 12 AWG instead: 18.0 ÷ 2.25 = 8 exactly, so 8 conductors. If the division leaves a fraction, round down; a box cannot be partly overfilled.
Mistakes that fail box fill
- Counting every ground. In this lesson’s counting method, the grounding conductors share one allowance. Counting three grounds as three adds two allowances that are not there.
- Sizing the device allowance by the wrong wire. The device counts by the largest conductor connected to it, not the largest in the box.
- Counting internal pigtails. A conductor that originates and terminates entirely inside the box generally does not add a separate conductor-fill allowance; verify that it meets the applicable rule.
- Missing the clamps. Internal clamps add one allowance, even if there are several.
- Using an old table number. Table and subsection numbers moved between editions, so confirm each one in your 2026 book.
Check yourself
Our own practice questions. Answers below each.
- A box holds four 14 AWG insulated conductors, one switch connected to them, internal clamps and two grounds. What volume is required? Answer: (4 + 2 + 1 + 1) × 2.00 = 16.00 cubic inches.
- Same box, but with six grounding conductors. What changes? Answer: Two grounds beyond four add 2 × 0.25 = 0.5 allowance, or 1.00 cubic inch. Total 17.00 cubic inches.
- Does a 10 AWG pigtail that stays inside the box change the result? Answer: Not if the pigtail originates and terminates entirely inside the box and meets the applicable conductor-fill rule. Confirm that condition before excluding it from the calculation.
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A pocket reference for tables you look up every day
Ugly’s Electrical References, 2026 Edition. A spiral-bound field reference with tables and formulas, including ampacity, conduit fill and conduit bending. Keep it in your tool bag and at your study desk; on exam day PSI names the NEC as the permitted code reference.
FAQ
How do you calculate box fill?
Count allowances: one for each insulated conductor that enters and ends or splices in the box, one for all internal clamps, two for each device yoke and one for up to four grounding conductors. Multiply by the volume per conductor for each size and compare the total with the box volume.
How much box volume does a 12 AWG conductor need?
2.25 cubic inches per allowance. A 14 AWG conductor needs 2.00 cubic inches, and a 10 AWG conductor needs 2.50.
Do ground wires count in box fill?
Yes. In the counting method used here, up to four equipment grounding conductors share one allowance based on the largest one, and each additional conductor adds a quarter allowance. This follows the 2023 method; use the applicable 2026 NEC requirement when sizing a box.
Where is the box fill table in the 2026 NEC?
The volume per conductor is in a table in 314.16 of your 2026 book. Box volumes are in Table 314.16(A), or use the volume marked on the box.
Sources checked October 2026
- TDLR rule adoption notice, 2026 NEC: https://www.tdlr.texas.gov/news/rulemaking/2026/09/01/commission-adopts-rules-12/
- TDLR exam page: https://www.tdlr.texas.gov/electricians/elecexam.htm
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70 (NEC), 2026 edition, section 314.16, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
