Conduit Fill Calculator
Calculate conduit fill percentage per NEC code from conduit size, wire gauge, and number of wires.
Check if your installation meets electrical code.
How Conduit Fill Calculations Work
The NEC (National Electrical Code) limits how much of a conduit’s cross-sectional area can be filled with wire. Overfilling causes heat buildup, insulation damage, and makes pulling additional wires impossible later.
NEC fill percentage limits:
| Number of conductors | Max fill percentage |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
Fill calculation formula:
Total Wire Area = Σ (π ÷ 4 × d²) for each conductor
Conduit Inner Area = π ÷ 4 × D²
Fill % = Total Wire Area ÷ Conduit Inner Area × 100
Worked example — 3 conductors of 12 AWG THHN in ½" EMT:
- 12 AWG THHN wire diameter: 0.133 inches
- Area per wire: π/4 × (0.133)² = 0.01385 sq in
- Total for 3 wires: 3 × 0.01385 = 0.04155 sq in
- ½" EMT inner diameter: 0.622 inches
- Conduit inner area: π/4 × (0.622)² = 0.3039 sq in
- Fill: 0.04155 ÷ 0.3039 = 13.7% — well within the 40% limit
Common conduit inner diameters (EMT):
| Trade Size | Inner Diameter (in) | Inner Area (sq in) |
|---|---|---|
| ½" | 0.622 | 0.304 |
| ¾" | 0.824 | 0.533 |
| 1" | 1.049 | 0.864 |
| 1¼" | 1.380 | 1.496 |
Practical tip: Always size conduit for future capacity. A conduit filled to 25% today allows you to pull additional circuits later without expensive rework. The incremental cost of larger conduit is minimal compared to labor costs for future upgrades.