Complete Guide to Conduit Fill
The Conduit Fill Calculator ensures ≤40% fill per NEC Chapter 9. Prevents jamming, heat, and code violations.
What is Conduit Fill?
% fill = (total wire area / conduit area) × 100. Max: 53% (1 wire), 31% (2), 40% (>2). Our Conduit Fill Calculator uses Annex C.
Why Limit Fill?
Avoids:
- Jamming: Wire damage during pull.
- Overheating: Reduced dissipation.
- Code Violation: Inspector rejection.
- Pull Difficulty: High friction.
Use with Wire Size and Derating.
NEC Fill Rules (Chapter 9)
| Wires | Max % Fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| ≥3 | 40% |
Conduit Types
NEC Table 4 dimensions:
- EMT: Most common.
- RMC: Rigid metal.
- PVC: Sch 40/80.
- FMC: Flexible.
Wire Area (Annex C)
THHN/THWN (in²):
| AWG | Area |
|---|---|
| 14 | 0.0133 |
| 12 | 0.0157 |
| 10 | 0.0211 |
| 8 | 0.0366 |
| 6 | 0.0507 |
| 4 | 0.0824 |
Calculation Steps
- Select: Conduit type & size.
- Add: Wire AWG + count.
- Compute: Total wire area.
- Check: ≤40% (or 31%/53%).
Example: 3/4" EMT
6× #12 THHN + 4× #10 THHN.
- Area: 6×0.0157 + 4×0.0211 = 0.1784 in²
- EMT area: 0.533 in²
- Fill: 33.5% → OK
Max Wires (Annex C)
THHN in 1" EMT:
| AWG | Max Count |
|---|---|
| 14 | 34 |
| 12 | 26 |
| 10 | 19 |
| 8 | 11 |
Mixed Sizes
Allowed. Calculator sums areas.
Practical Tips
- Derate: >3 current-carrying → 80%.
- Pull Box: Every 360° bend.
- Lube: Reduce friction.
- Oval: Not circular → measure.
Common Mistakes
- Ignore Insulation: Use THHN area.
- Wrong Table: EMT ≠ PVC.
- Count Grounds: Yes, all wires.
- Forget Nipples: 60% fill <24".
Advanced
- Jam Ratio: 2.8–3.2 → risk.
- Pull Tension: Software.
- Compact Stranding: Smaller area.
- Fiber Optic: Different rules.
Conclusion
Proper fill prevents damage and ensures compliance. Our Conduit Fill Calculator uses NEC Annex C. Pair with Wire Size and Construction tools. Build safe with 1000 Calculators.