FIG. 01
HOW WE WORK
01
EDITORIAL STANDARD
We treat the NEC as the canonical source. When a printed conduit fill chart and a strict Chapter 9 Table 4 × Table 5 calculation disagree (this happens at boundary cases — 21.98 conductors rounds differently depending on whose table you use), we report the strict math and explain the discrepancy. Better for an installer to be one conductor under than one over.
02
HOW WE SOURCE
- NEC 2023 Chapter 9 Table 1 — fill percentages.
- NEC 2023 Chapter 9 Table 4 — conduit interior areas (every conduit type, every trade size).
- NEC 2023 Chapter 9 Table 5 — insulated conductor areas (every gauge × every common insulation).
- NFPA 70 Annex C — cross-referenced for sanity checks.
Source documents are sold by NFPA. We do not redistribute the code; we publish derived calculations and reference values that are not themselves copyrightable.
03
INDEPENDENCE
WireFillChart is not affiliated with NFPA, Southwire, Cerrowire, or any conduit manufacturer. We earn no commission from sizing recommendations. The site is supported by display advertising on content pages (the calculator and chart pages are ad-light by policy).
04
REPORTING AN ERROR
If you spot a wrong fill value, please open an issue with: the conduit type, trade size, insulation, wire gauge, and the result you expected. We treat every report as a P0 — wrong numbers are worse than no numbers.
05
DISCLAIMER
This site is a reference. Always verify with the current adopted edition of the NEC for your jurisdiction and have work inspected by a licensed authority. See the full disclaimer.