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Practice question · NEC 314.16(B)(4)
How Many Box Fill Allowances for a Duplex Receptacle?
How many box fill allowances for a duplex receptacle trips people up because the device looks like one thing in the box. For box fill, a device yoke does not count as one. Under 314.16(B)(4), it counts as two conductor volumes based on the largest conductor in the box.
Exam-style question (Sparky QOTD)
A duplex receptacle (device yoke) is installed in a box wired with 12 AWG conductors. How many 12 AWG volume allowances does the device count as?
A. 2
B. 3
C. 4
D. 1
Answer: A) 2. Each yoke or strap containing a device counts as two conductor volumes, so for 12 AWG that is 2 × 2.25 in³ = 4.5 in³.
Where to Find It
Open your Code book to Article 314, section 314.16(B)(4), and keep Table 314.16(B) open beside it. Section 314.16(B)(4) gives the counting rule for a device yoke. Table 314.16(B) gives the volume per allowance. For 12 AWG, that volume is 2.25 in³, so two allowances take up 4.5 in³ in your box fill calculation.
Why Two Allowances
Rule (B)(4) is clear: each yoke or strap containing a device counts as two conductor volumes. A duplex receptacle is a device on a yoke, so it counts as two, not one. The size of those volumes follows the largest conductor in the box, which in this stem is 12 AWG.
From Table 314.16(B), one 12 AWG allowance is 2.25 in³. Two allowances are 2 × 2.25 in³ = 4.5 in³. That is the amount of box volume the receptacle uses in the calculation, before you add the allowances for the conductors themselves and anything else the counting rules require.
Tab Article 314 at 314.16(B) and learn it as the five counting rules. Device equals 2 is rule (B)(4). If you want more reps on this exact skill, work through our box fill calculations guide with your book open.
The Trap
Option D, 1, is the visual trap: one receptacle, one count. The Code does not count it that way. Options B and C, 3 and 4, overcount the yoke. The stem gives you a single duplex receptacle on a single yoke, and 314.16(B)(4) assigns that yoke exactly two volume allowances. Do not add extra allowances because the receptacle has two places to plug in a cord.
Exam Tip
For every box fill question, count in a fixed order and say the rule as you go. When you reach a device, pause and say "device equals two" before you write the number down. That pause stops the most common box fill error on the exam: counting a receptacle or switch yoke as a single conductor.
Keep Drilling
This is one of 33 NEC topics. Sparky's 1,087 codebook-first questions drill all of them — every question a lookup rep with your book open, 15 minutes a day.