2.4 Box Fill Calculations (314.16)

Box fill is a classic General Knowledge / Code hybrid item on the Arkansas Journeyman Electrician exam. You open NEC 2023 to Article 314, specifically 314.16, and apply counting rules — not Ohm's law. Prov expects you to know what counts, what does not, and how device and clamp allowances work.

The Governing Rule — 314.16

Boxes must be large enough so that the total volume of conductors, devices, clamps, and fittings does not exceed the box's marked or tabulated volume. Two tables do the heavy lifting:

Common 314.16(B) allowances (verify on exam):

What Counts as a Volume Allowance?

314.16(B)(1) — Conductor fill: Each conductor that originates outside the box and terminates or is spliced inside counts as one of its size. A conductor that passes through unbroken also counts as one. Fixture wires covered by exception and equipment grounding conductors are handled separately.

Does not count: Pigtails that stay entirely inside the box (they originate and terminate inside). Conductors that never leave the box do not add fill.

314.16(B)(2) — Clamp fill: One or more internal cable clamps count as a single volume allowance based on the largest conductor present in the box. Locknuts and bushings do not count. Clamps outside the box (external connectors) do not count.

314.16(B)(3) — Support fittings: Luminaire studs or hickeys can add allowances — read the stem; many device-box questions omit them.

314.16(B)(4) — Device or equipment fill: Each yoke or strap containing a device (duplex receptacle, switch, etc.) counts as two volume allowances based on the largest conductor connected to that device. A single switch = 2; two devices on separate yokes = 4 allowances.

314.16(B)(5) — Equipment grounding fill: All equipment grounding conductors count together as one volume allowance of the largest EGC in the box. An additional isolated/insulated EGC requirement can add one more allowance — watch for that wording.

Worked Example 1 — Device Box with NM Cable

A 4 in. square × 2⅛ in. deep metal box (Table 314.16(A) lists 30.3 in.³ for the 4 × 2⅛ square row; confirm the exact row on exam day) will contain:

• Two 12/2 NM cables with grounds (supply and load) to one duplex receptacle
• Internal cable clamps

Count for 12 AWG (2.25 in.³ each):

18.00 < 30.3 → box is adequate. The four insulated circuit conductors each count; the two grounds collapse to one allowance; clamps collapse to one; the receptacle uses two.

Worked Example 2 — Too Many Conductors

Same 30.3 in.³ box, but now three 12/2 NM cables (six circuit conductors + three EGCs), internal clamps, and one receptacle:

Still under 30.3. Add a second device yoke (another switch) on the same box:

Extra device = +4.50 → 27.00 in.³ — still OK. A third device yoke (+4.50) would reach 31.50 in.³ and fail 314.16 for this box volume.

Worked Example 3 — Mixed Sizes

When conductor sizes differ, each insulated conductor uses the volume for its own size from Table 314.16(B). Device and clamp allowances use the largest conductor connected to the device or present in the box, respectively. EGC allowance uses the largest EGC.

Example: Box contains four 14 AWG circuit conductors (2.00 in.³ each), two 12 AWG circuit conductors (2.25 each), EGCs of 14 AWG, internal clamps, one receptacle fed by the 12 AWG conductors:

Row-reading trap: Table 314.16(A) separates square, round/octagonal, and device boxes. A 4 × 2⅛ square box is 30.3 in.³, but a 4 × 2⅛ × 2⅛ device (handy) box is only 14.5 in.³ and a 4 × 2⅛ round/octagonal box is 21.5 in.³. The 18.00 in.³ fill above passes in the 30.3 in.³ square box and would fail in the 14.5 in.³ device box. Read the box shape in the stem before you read a volume.

Plastic Boxes and Marked Volume

Nonmetallic boxes are often marked with cubic inch capacity. Use the marked volume rather than guessing from dimensions. Metal boxes use Table 314.16(A) unless a larger volume is marked.

Domed Covers and Extensions

Domed covers or plaster rings with marked volume may add usable cubic inches — only if the question states a marked additional volume. Do not invent extra volume.

Arkansas Exam Lookup Habit

1. Open to 314.16 and Tables 314.16(A) and (B).
2. List every cable/conductor entering the box on scratch paper.
3. Apply the five fill categories in order: conductors, clamps, support fittings, devices, EGCs.
4. Multiply allowances by the correct in.³ from Table 314.16(B).
5. Compare total to box volume.

Box fill rewards methodical counting. On a timed Prov exam, write the tally table — it prevents double-counting grounds or forgetting that a device costs two allowances.