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
- 1Determine the internal area of the conduit.
- 2Calculate the cross-sectional area of a single wire (including insulation).
- 3Multiply wire area by the total number of wires.
- 4Divide total wire area by conduit area to get the Fill Percentage.
Electrical FAQs
Q
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.
Q
Does the ground wire count?
Yes, all conductors, including the equipment grounding conductor, count towards the conduit fill calculation.