Conduit Fill Calculator: NEC Wire Fill Percentage
Calculate the maximum number of wires allowed in a conduit (NEC rules).
NEC Conduit Fill Rules
To prevent overheating and damage during wire pulling, the National Electrical Code (NEC) limits how much of a conduit's cross-sectional area can be filled with wires.
Max Fill Percentages
| Number of Wires | Max Fill % |
|---|---|
| 1 Wire | 53% |
| 2 Wires | 31% |
| 3 or more Wires | 40% |
How Fill is Calculated
- Determine the internal area of the conduit.
- Calculate the cross-sectional area of a single wire (including insulation).
- Multiply wire area by the total number of wires.
- Divide total wire area by conduit area to get the Fill Percentage.
Electrical FAQs
Why is the limit 40%?
The 40% limit for 3+ wires ensures there is enough empty space for heat dissipation and to physically pull the wires through bends without damaging insulation.
Does the ground wire count?
Yes, all conductors, including the equipment grounding conductor, count towards the conduit fill calculation.