Conduit fill calculator. NEC Chapter 9.
Configure the raceway and conductors, then press Run Calculation.
How the calculation works
Conductor cross-sectional areas come from NEC Chapter 9, Table 5, for THHN/THWN-2 insulation — the dominant building wire for raceway installations. Raceway internal areas come from NEC Chapter 9, Table 4, for the selected conduit type and trade size.
The permitted fill percentage comes from NEC Chapter 9, Table 1: 53% for a single conductor, 31% for two conductors, and 40% for three or more. The calculator applies the limit that matches the conductor count you enter, so adding a third conductor correctly drops the limit from 31% to 40% of a smaller effective allowance.
Total conductor area is compared against the permitted area. When the fill exceeds the limit, the calculator reports the smallest trade size of the same conduit type that would comply.
What this tool does not do
- Assumes THHN/THWN-2 conductor dimensions. Other insulation types have different areas in Table 5 and will give different results.
- Does not apply ampacity adjustment factors. When more than three current-carrying conductors share a raceway, NEC 310.15(C)(1) derating also applies and is a separate calculation.
- Does not check conduit body, pull box, or bend requirements under NEC 314.28 or 344.26.
- Does not account for existing conductors in a raceway being reused.
- Nipples 24 inches or shorter are permitted 60% fill under Chapter 9 Note 4; this calculator does not apply that allowance.
Results are for planning purposes and must be reviewed by a qualified professional before use on a real installation. Read the full disclaimer.
Common questions
- What is the maximum conduit fill allowed by the NEC?
- NEC Chapter 9 Table 1 permits 53% fill for one conductor, 31% for two conductors, and 40% for three or more conductors. This calculator applies the 40% limit once three or more conductors are present and adjusts automatically for one or two.
- Which conductor dimensions does the calculator use?
- Cross-sectional areas come from NEC Chapter 9 Table 5 for THHN/THWN-2 insulation. Raceway internal areas come from NEC Chapter 9 Table 4 for the selected conduit type and trade size.
- Does conduit fill account for derating?
- No. Conduit fill and ampacity adjustment are separate checks. When more than three current-carrying conductors share a raceway, NEC 310.15(C)(1) adjustment factors also apply to ampacity, which this tool does not calculate.
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