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Study guides · NEC 314.16
Box Fill, Step by Step (NEC 314.16)
Box fill is free points on the journeyman exam — if you know the method. The calculation is always the same: add up the volume allowances for everything in the box, and check it against the box's volume in Table 314.16(A). Here's the whole thing.
Step 1 — Volume Allowances Per Conductor
Table 314.16(B) assigns every conductor a volume based on its size. Memorize this table — it's small:
| Wire size | Volume allowance |
|---|---|
| 18 AWG | 1.50 in³ |
| 16 AWG | 1.75 in³ |
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
Step 2 — What Counts (and What Doesn't)
- Each conductor that terminates or is spliced in the box counts as one allowance [314.16(B)(1)].
- A conductor that passes straight through with no splice counts as one allowance.
- A conductor looped for future use (at least twice the free-conductor length of 300.14) counts as two.
- Conductors that originate inside the box (pigtails, jumpers) don't count at all.
- Internal cable clamps: all of them together count as one allowance, based on the largest conductor [314.16(B)(2)].
- Devices (receptacle, switch on a yoke): each yoke counts as two allowances, based on the largest conductor connected to it [314.16(B)(4)]. A duplex receptacle is one yoke.
- Equipment grounding conductors: all of them together count as one allowance, based on the largest ground wire [314.16(B)(5)]. (Beyond four grounds, each extra adds a quarter allowance — rare on the exam.)
- Small fittings — locknuts, bushings — no allowance.
The exam's favorite traps: counting pigtails, counting each ground wire separately, and counting a duplex receptacle as two devices.
Step 3 — Box Volumes from Table 314.16(A)
You don't memorize these — you look them up. But know the four classics cold:
| Box | Volume |
|---|---|
| 4″ square × 1½″ deep | 18.0 in³ |
| 4″ square × 2⅛″ deep | 30.3 in³ |
| 4-11/16″ square × 1½″ deep | 25.5 in³ |
| 4-11/16″ square × 2⅛″ deep | 42.0 in³ |
Step 4 — Worked Example
The question: A 4″ square box, 1½″ deep, contains four 12 AWG circuit conductors, two 12 AWG equipment grounding conductors, one duplex receptacle, and two internal cable clamps. Is the box big enough?
Count it up (12 AWG = 2.25 in³ each):
- 4 circuit conductors × 2.25 = 9.00 in³
- 2 grounding conductors → single allowance = 2.25 in³
- 1 duplex receptacle (one yoke) → 2 × 2.25 = 4.50 in³
- 2 internal clamps → single allowance = 2.25 in³
Total: 18.00 in³. The box holds 18.0 in³ — it fits, exactly. One more conductor of anything and you'd step up to the 2⅛″ deep box (30.3 in³).
That's the whole method: allowances in, box volume out. Drill it with your book open until the counting is automatic — then it's free points on test day.
Keep Drilling
Box fill 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.