NEC Article 314 Practice Questions: Boxes & Box Fill

Box fill is one of the most predictable calculation questions on the exam — every exam has one, and the counting rules never change. Learn them once, collect the points.

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21 Article 314 questions in the bankWritten in the PSI computer-based exam formatEvery answer cites the NEC section

What Article 314 Covers on the Exam

Article 314 covers outlet, device, and junction boxes, and its exam centerpiece is the box fill calculation of 314.16: each conductor counts once, all equipment grounding conductors together count once (the 2023 NEC adds a second allowance where more than four EGCs enter), each strap with devices counts twice based on the largest conductor on the device, and internal clamps count once based on the largest conductor in the box. Multiply counts by the volume per conductor from Table 314.16(B) and compare to the box volume in Table 314.16(A).

Tested subtopics from Article 314

Box volume calculations and conductor counts — 314.16(A) and (B)
Volume allowances: devices, clamps, and equipment grounding conductors — 314.16(B)(1)–(5)
Minimum size of pull and junction boxes for larger conductors — 314.28
Boxes must remain accessible — 314.29
Support of boxes and conduit bodies — 314.23

Drill a different article

21 questions in our bank cite Article 314. Practice the other high-yield articles too:

Art. 110 — General Requirements Art. 210 — Branch Circuits Art. 220 — Load Calculations Art. 230 — Services Art. 240 — Overcurrent Protection Art. 250 — Grounding and Bonding Art. 300 — Wiring Methods Art. 310 — Conductors & Ampacity Art. 430 — Motors Art. 450 — Transformers Art. 680 — Swimming Pools & Spas

How to Study Article 314

Learn the counting rules as a checklist you run in order: (1) each insulated conductor entering and terminating or spliced = 1, (2) conductors passing through unbroken = 1, (3) all EGCs = 1 (double allowance rules for additional EGC sets per 2023), (4) each device strap = 2 at the largest connected conductor size, (5) one or more internal clamps = 1 at the largest conductor in the box. Then it is arithmetic with Table 314.16(B): 2.00 in.³ for 14 AWG, 2.25 for 12 AWG. For pull boxes, memorize 314.28: straight pulls need 8× the largest raceway trade size; angle pulls need 6× plus the sum of the other raceways in the same row.

Then put it under exam conditions: the free diagnostic scores you across every category and shows whether boxes & box fill is actually a weak area — or whether your study time belongs somewhere else.

Sample Questions (From Our Real Question Bank)

Here are 4 real questions from our question bank. The free diagnostic quiz has 10 more.

NEC NEC Table 314.16(B)Electrical Wiring Methods & Materials

1. Per NEC 314.16, what is the volume allowance in cubic inches for each 14 AWG conductor counted in a box fill calculation?

A1.75 cubic inches
B2.00 cubic inches
C2.25 cubic inches
D2.50 cubic inches

Show explanation

NEC Table 314.16(B) specifies 2.00 cubic inches for each 14 AWG conductor in box fill calculations.

NEC NEC 314.16(B)Calculations - Box Fill

2. You are installing a 4" × 2-1/8" deep device box. The box contains three 12 AWG conductors, two 12 AWG equipment grounding conductors, one internal cable clamp, and one single-gang device (yoke). Per NEC 314.16, how many cubic inches of volume is required?

A13.50 cubic inches
B15.75 cubic inches
C18.00 cubic inches
D20.25 cubic inches

Show explanation

Per NEC Table 314.16(B), 12 AWG = 2.25 cu in each. Box fill: 3 conductors (3 × 2.25 = 6.75) + 1 EGC count (1 × 2.25 = 2.25) + 1 clamp count (1 × 2.25 = 2.25) + 1 device/yoke (2 × 2.25 = 4.50) = 15.75 cu in. Note: Per 314.16(B)(4), each device or equipment yoke counts as 2 conductor volumes.

NEC NEC Table 314.16(B)Wiring Methods

3. Per NEC Table 314.16(B), how many cubic inches of volume are assigned to each 14 AWG conductor in a box fill calculation?

A1.50 cubic inches
B1.75 cubic inches
C2.00 cubic inches
D2.25 cubic inches

Show explanation

Per NEC Table 314.16(B), each 14 AWG conductor is assigned a volume of 2.00 cubic inches for box fill calculations. The 1.50 value applies to 18 AWG conductors, and 2.25 applies to 12 AWG conductors.

NEC NEC 314.16(A)Wiring Methods

4. A 4×4×1-1/2 inch square metal box has a listed volume of how many cubic inches per NEC Table 314.16(A)?

A18 cubic inches
B21 cubic inches
C28 cubic inches
D30.3 cubic inches

Show explanation

Per NEC Table 314.16(A), a 100×38 mm (4×1-1/2 inch) square metal box has a listed volume of 21.0 cubic inches. The 4×2-1/8 inch (100×54 mm) square box has a larger volume of 30.3 cubic inches. Knowing the volume for each standard box size is essential for box fill calculations under 314.16.

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Frequently Asked Questions

How do I calculate box fill under NEC 314.16?

Count volume allowances: one per insulated conductor terminating or spliced in the box, one for all equipment grounding conductors together, two per device strap (at the largest conductor connected to that device), and one for internal cable clamps (at the largest conductor in the box). Multiply each allowance by the volume for that conductor size in Table 314.16(B) — e.g., 2.00 in.³ for 14 AWG, 2.25 in.³ for 12 AWG — and the total must not exceed the box volume.

How big does a junction box need to be for a straight pull?

Per 314.28(A)(1), for raceways containing conductors 4 AWG or larger, the length of a box used for a straight pull must be at least 8 times the trade size of the largest raceway. For angle or U pulls, the distance is 6 times the largest raceway plus the sum of the trade sizes of the other raceways in the same row (314.28(A)(2)).

Do pigtails count in box fill calculations?

No — conductors that originate and terminate inside the same box, such as pigtails and bonding jumpers, are not counted under 314.16(B)(1). This is a favorite trick in exam questions.