These NEC box fill practice questions are worked out step by step using Table 314.16. Box fill is worth learning properly because the method never changes — only the numbers do. Count what is in the box, give each item the volume the code assigns it, add it up, and pick a box at least that size. The trick is knowing what counts as one conductor, what counts as two, and what does not count at all.
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Written by Kevin, a licensed
Michigan master electrician — 26 years in the trade and 17 of them as an electrical
inspector. Every answer below names the NEC section it comes from, so you learn where to
look it up rather than memorising it.
Exam tipAll the clamps together count once. All the fittings together count once. Each yoke or strap counts twice. Every equipment grounding conductor in the box, however many, counts once between them. Get those four rules straight and box fill stops being a guess.
Tap an answer to see whether you got it right, the code section it comes
from, and the reasoning worked out in full. Nothing to sign up for.
Question 1Box & Pull Boxes
One 14-3 NM (on a switch) and one 12-2 NM (on a receptacle) enter a box with cable clamps. What is the total box fill?
Correct answer: B. 23.5 in³
NEC 314.16(B) — box-fill calculation
Box fill (314.16)
Count each item per Table 314.16(B)(1):
14-3 conductors: 3 × 2.00 = 6.00
12-2 conductors: 2 × 2.25 = 4.50
All grounds (×1, largest 12): 2.25
All clamps (×1, largest 12): 2.25
Switch yoke: 2 × 2.00 = 4.00
Receptacle yoke: 2 × 2.25 = 4.50
Total = 23.5 in³
314.16(B) — box-fill calculation
23.5 in³
How to do it
Add volume allowances: each conductor by its Table 314.16(B) size, all clamps = 1 (largest), all grounds = 1 (largest), and each device on a yoke = 2 (largest). The total must be ≤ the box's cubic inches.
Common mistakes
Counting a device (switch/receptacle) as 1 — it is 2.
Forgetting the clamp and grounding allowances.
Sizing an allowance from the wrong (not the largest) conductor.
Question 2Box & Pull Boxes
What is the minimum cubic-inch volume needed for nine 14 AWG conductors?
Correct answer: B. 18.0 in³
NEC 314.16(B)(1) · Table 314.16(B)(1)
Box fill (314.16)
Each 14 AWG conductor counts as 2.00 in³ (Table 314.16(B)(1)).
Step 1 — Volume per conductor: 2.00 in³
Step 2 — Multiply by the count
9 × 2.00 = 18.0 in³
314.16(B)(1) · Table 314.16(B)(1)
18.0 in³
How to do it
Add volume allowances: each conductor by its Table 314.16(B) size, all clamps = 1 (largest), all grounds = 1 (largest), and each device on a yoke = 2 (largest). The total must be ≤ the box's cubic inches.
Common mistakes
Counting a device (switch/receptacle) as 1 — it is 2.
Forgetting the clamp and grounding allowances.
Sizing an allowance from the wrong (not the largest) conductor.
Question 3Box & Pull Boxes
What is the volume of a 4 x 1-1/2 in. square box?
Correct answer: A. 21in
NEC 314.16(A)
21in — NEC 314.16(A)
Question 4Box & Pull Boxes
How many bends can conduit have before needing a pull box?
Correct answer: B. 360
NEC 358.26 / 344.26 (360° max)
360 — NEC 358.26 / 344.26 (360° max)
Question 5Box & Pull Boxes
How many inches of free conductor shall be left in each outlet junction box?
Correct answer: A. 6 in
NEC 300.14
6 in — NEC 300.14
Question 6Box & Pull Boxes
NM cable shall be secured within how many inches of every outlet box, junction box, or cabinet?
Correct answer: A. 12 in
NEC 334.30
12 in — NEC 334.30
Question 7Box & Pull Boxes
For box fill, how many cubic inches does one 14 AWG conductor count as?
Correct answer: B. 2.00 in³
NEC 314.16(B)(1) · Table 314.16(B)(1)
Each conductor size has a fixed volume allowance - read it straight from Table 314.16(B)(1) (per 314.16(B)(1)).
14 AWG = 2.00 in³ each.
Answer: 2.00 in³.
314.16(B)(1) · Table 314.16(B)(1)
Question 8Box & Pull Boxes
For box fill, how many cubic inches does one 12 AWG conductor count as?
Correct answer: B. 2.25 in³
NEC 314.16(B)(1) · Table 314.16(B)(1)
Read the per-conductor volume from Table 314.16(B)(1).
A device (receptacle/switch yoke) counts as how many conductor volumes for box fill?
Correct answer: B. 2
NEC 314.16(B)(4) — device/yoke allowance
Per 314.16(B)(4), each device on a single yoke or strap counts as TWO conductor volumes.
The size used is the largest conductor connected to that device.
Answer = 2 conductor volumes.
314.16(B)(4) — device/yoke allowance
Question 10Box & Pull Boxes
All equipment grounding conductors in a box count as how many, and at what size?
Correct answer: B. one total, the largest
NEC 314.16(B)(5) — equipment grounding conductors
All grounds together count as ONE conductor volume, based on the LARGEST EGC in the box.
(Add a second '1' if isolated-ground conductors are also present.)
314.16(B)(5) — equipment grounding conductors
These are 10 of 803 questions
The full course carries 654 code and calculation questions plus 149 photo questions taken on real job sites — my own photographs from 17 years of electrical inspections. Every one of them is worked out like the ones above.
NEC 2023 and NEC 2026 both included, switch between them any time
Every question names the code section, so you learn to find it in the book
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