8.4 Boxes and Box Fill

Why Box Fill Matters on the TX Journeyman Exam

Box-fill calculations are a core Calculations-part topic. The exam presents a box with a list of conductors, devices, and fittings, then asks for the minimum box volume required — or whether a given box is compliant. You must know the volume allowances in Table 314.16(B) and how each item in a box is counted. There are at least six distinct categories, and miscounting one is the most common reason candidates miss the question.

Article 314 — Outlet, Device, Pull, and Junction Boxes

314.16 — Number of Conductors in Boxes

Boxes must be sized to provide adequate free space for all contained conductors and devices. There are two methods:

• 314.16(A) — Boxes 6 in.³ or smaller: Use Table 314.16(A), which lists the maximum number of conductors by box size (based on #14 and #12 conductor equivalents).
• 314.16(B) — Box Volume Calculations: Compute the required volume from the allowances in Table 314.16(B). This method is used whenever the box contains conductors of mixed sizes, devices, clamps, or EGCs — which describes most real-world boxes and virtually all exam questions.

Table 314.16(B) — Volume Allowances

Each item in a box gets a volume allowance based on the largest conductor in its category:

Conductor volume allowances by size (Table 314.16(B)):

How Each Item Is Counted

• Conductors entering the box: Each conductor that enters from outside counts once, based on its size. A conductor running through the box without a splice counts once. A conductor that originates entirely inside the box (a pigtail, for example) does not count.
• Device yoke (switch or receptacle): Each yoke counts as 2 × the largest conductor connected to that device. A single-gang box with one receptacle and #12 conductors on it = 2 × 2.0 = 4.0 in.³ for the device alone.
• Internal cable clamps: If the box has internal cable clamps (a metal box with built-in clamps), count one allowance for all the clamps, based on the largest conductor in the box. External cable connectors (NM connectors screwed to the outside) are not counted.
• Equipment grounding conductors: Count the largest EGC once as a full allowance (for example, #12 EGC = 2.0 in.³). Each additional EGC (14–6 AWG) adds 0.25 in.³ based on the largest of the additional EGCs.
• Fixture studs and hickeys: Each counts as 1 × the largest conductor on the stud.

Worked Example — Box-Fill Calculation

Problem: A box contains the following:

• 3 #12 THHN conductors entering from the top
• 2 #14 THHN conductors entering from the side
• 1 duplex receptacle (device yoke) with #12 conductors connected
• 2 internal cable clamps
• 2 #12 equipment grounding conductors

Calculate the minimum box volume required.

Step 1 — Conductors entering the box:
3 #12 = 3 × 2.0 = 6.0 in.³
2 #14 = 2 × 2.0 = 4.0 in.³
Subtotal = 10.0 in.³

Step 2 — Device yoke (receptacle):
Largest conductor on device = #12 → 2 × 2.0 = 4.0 in.³

Step 3 — Internal cable clamps:
All clamps counted once; largest conductor in box = #12 → 1 × 2.0 = 2.0 in.³

Step 4 — Equipment grounding conductors:
First (largest) #12 EGC = 2.0 in.³
One additional #12 EGC = 0.25 in.³
Subtotal = 2.25 in.³

Step 5 — Total required volume:
10.0 + 4.0 + 2.0 + 2.25 = 18.25 in.³

Select a box: A 4 in. square box with 1-1/4 in. depth has 18.0 in.³ — not enough (18.0 < 18.25). A 4 in. square box with 1-1/2 in. depth has 21.0 in.³ — sufficient. The minimum compliant box is 4 in. square, 1-1/2 in. deep.

Related Article 314 and 300 Rules

314.24 — Depth of Boxes

Boxes must be deep enough to prevent damage to conductors when devices or fittings are installed. A box that is too shallow can pinch conductors against the device strap.

300.14 — Minimum Free Conductor Length at Boxes

At least 6 in. of free conductor must be provided at each outlet box, junction box, or switch point, measured from the point where the conductor emerges from the raceway or cable sheath. At least 3 in. of that conductor must extend outside the box opening. This ensures conductors can be terminated without strain and devices can be replaced safely.

314.17 — Conductors Entering Boxes

Conductors entering a box must be secured. Where raceways or cables enter the box, they must be clamped or secured to the box. Nonmetallic cable (NM) must be secured within 12 in. of the box. Metal raceways are secured at the box by their listed fittings or connectors. Knockout closures or listed fittings are required to protect conductors from sharp edges.

314.29 — Access to Boxes

Boxes must be accessible without removing any part of the building finish or structure. A junction box buried above a finished ceiling with no access panel is a code violation. Exception: boxes covered by removable panels (suspended ceiling tiles, access doors) are considered accessible.

Key Takeaways for the Exam

1. Table 314.16(B) allowances: #14 = 2.0 in.³, #12 = 2.0 in.³, #10 = 2.5 in.³, #8 = 3.0 in.³, #6 = 5.0 in.³.
2. Device yoke = 2 × largest conductor on that device — the most commonly missed allowance.
3. Internal cable clamps = 1 × largest conductor (all clamps counted as one unit).
4. EGCs: first and largest gets a full allowance; each additional (14–6 AWG) adds 0.25 in.³.
5. 300.14 requires 6 in. of free conductor at each box, with 3 in. outside the box opening.