Authoritative Engineering Content
Data Source and Methodology
Authoritative source: National Electrical Code (NEC) — NFPA 70 (2020), Article 314.16 and Table 314.16(B). Official NFPA code page (free view with account).
All calculations strictly follow the formulas and allowances provided by this source.
- Insulated conductors that enter and terminate/splice in the box: count 1 each.
- Pass-through (unspliced) insulated conductors: count 1 each.
- Equipment grounding conductors (all): count as 1 volume using the largest EGC present.
- Each device yoke: counts as 2 volumes using the largest conductor connected to that yoke.
- Integral internal clamps: count as 1 volume of the largest insulated conductor entering the box.
- Internal fittings (e.g., hickeys): count as 1 volume each of the largest insulated conductor in the box.
For additional background see: ECM: Box Fill Calculations, JADE Learning: NEC 314.16.
The Formula Explained
18→1.50, 16→1.75, 14→2.00, 12→2.25, 10→2.50, 8→3.00, 6→5.00 (cubic inches)
Total required volume:
$$V_{req} \;=\; \sum_{i} n_i \cdot V(\mathrm{AWG}_i)\;+\; \mathbf{1}_{\mathrm{EGC}>0}\cdot V(\mathrm{AWG}_{\mathrm{EGC,max}})\;+\; 2\!\!\sum_{j} y_j \cdot V(\mathrm{AWG}_{y_j})\;+\; \mathbf{1}_{\mathrm{clamp}}\cdot V(\mathrm{AWG}_{\max})\;+\; f \cdot V(\mathrm{AWG}_{\max})$$ where:
- $n_i$ = insulated conductors of size $\mathrm{AWG}_i$ that enter and terminate/splice (pass-through counts as 1).
- $\mathrm{EGC,max}$ = largest equipment grounding conductor AWG (counted once if any EGCs are present).
- $y_j$ = number of device yokes connected to conductors of $\mathrm{AWG}_{y_j}$ (each counts as 2 conductors).
- $\mathrm{AWG}_{\max}$ = largest insulated conductor AWG present in the box (used for clamp and fitting allowances).
- $f$ = number of internal support fittings.
Glossary of Variables
- Box marked volume (in³): Manufacturer-stamped cubic-inch capacity of the box.
- Insulated conductors: Current-carrying conductors that enter and terminate or splice in the box; pass-throughs count once; pigtails within same box add no extra count.
- Equipment grounding conductors (EGC): Green/bare conductors; all EGCs together count as one conductor volume using the largest EGC present.
- Device yoke: A strap holding one or more devices (e.g., duplex receptacle). Each yoke counts as two conductors of the largest conductor on that yoke.
- Internal clamps: Integral cable clamps inside the box; count as one conductor of the largest insulated conductor present.
- Internal fittings: Accessories like hickeys; each counts as one conductor of the largest insulated conductor present.
- V(AWG): NEC Table 314.16(B) volume per conductor size (in³): 18→1.50, 16→1.75, 14→2.00, 12→2.25, 10→2.50, 8→3.00, 6→5.00.
How It Works: A Step-by-Step Example
Scenario. Box contains: six 12 AWG insulated conductors; one duplex receptacle on 12 AWG (one yoke); two EGCs with largest EGC 12 AWG; integral clamps present; no fittings. Box marked volume = 20.3 in³.
- Conductor allowance: 6 × V(12) = 6 × 2.25 = 13.50 in³.
- EGC allowance: present → 1 × V(12) = 2.25 in³.
- Device yoke: 1 yoke → 2 × V(12) = 4.50 in³.
- Clamp: present → 1 × V(12) = 2.25 in³.
- Fittings: none → 0.
- Total required: 13.50 + 2.25 + 4.50 + 2.25 = 22.50 in³.
- Comparison: 20.3 in³ box does not comply (short by 2.2 in³).
FAQ
Do pigtails count toward box fill?
No additional count for pigtails that originate and terminate in the same box. Their conductors are already included per NEC 314.16(B).
How are equipment grounding conductors counted?
All EGCs taken together count as a single conductor volume based on the largest EGC present.
Do pass-through conductors count?
Yes. Each insulated pass-through (unspliced) conductor counts as one conductor volume.
How do devices like duplex receptacles count?
Each yoke counts as two conductor volumes using the largest conductor attached to that yoke.
Do internal clamps and fittings count?
Internal clamps count as one conductor volume; each internal fitting counts as one, both based on the largest insulated conductor in the box.
Which AWG sizes are supported?
18, 16, 14, 12, 10, 8, and 6 AWG, using the cubic-inch values in NEC Table 314.16(B).
Can I use this for multi-gang boxes?
Yes. Enter the total manufacturer-marked volume (sum of gangs) and include all conductors/devices in the calculation.