Conduit Fill Calculator
Calculate conduit fill percentage per NEC Chapter 9 to ensure your wiring installation is code compliant.
Results
Visualization
How It Works
Conduit fill refers to the percentage of a conduit's internal area occupied by conductors. NEC Chapter 9 limits fill to prevent heat buildup and allow easy wire pulling.
The Formula
Fill % = (Total Wire Area / Conduit Internal Area) × 100
NEC limits: 1 wire = 53%, 2 wires = 31%, 3+ wires = 40%
NEC limits: 1 wire = 53%, 2 wires = 31%, 3+ wires = 40%
Variables
- Wire Area — Cross-sectional area of each THHN conductor (from NEC Chapter 9 Table 5)
- Conduit Area — Internal cross-sectional area of the raceway (from NEC Chapter 9 Table 4)
- Fill % — The percentage of conduit area occupied by wires
Example
Four 12 AWG THHN wires in 3/4" EMT: Wire area = 4 x 0.0133 = 0.0532 sq in. Conduit area = 0.533 sq in. Fill = 0.0532/0.533 = 10.0%. Well under the 40% limit for 3+ wires.
Tips
- NEC fill limits apply to the total area including wire insulation, not just the copper.
- Use THHN/THWN-2 wire areas from NEC Chapter 9 Table 5 for accurate calculations.
- Mixed wire sizes require adding each individual wire area separately.
- Equipment grounding conductors count toward conduit fill.
- If your fill is close to the limit, consider upsizing the conduit for easier wire pulling.