Verification note: Results are design aids, not stamped engineering or code approval. Always check the current project specification, adopted code, manufacturer data and latest edition of the cited standard before construction or procurement.
POWER & INFRASTRUCTURE
Conduit Fill Calculator
Conduit fill percentage and jam-ratio check.
Use this as a design check only. Final conduit sizing must follow adopted code, cable bend-radius limits, pulling tension limits and actual cable mix.
Conduit fill + jam-ratio check
Enter actual raceway ID and cable OD. For NEC compliance, a raceway-type/trade-size table is preferable to nominal diameter.
Fill % = Σ cable cross-sectional area / raceway internal area × 100
NEC Chapter 9 Table 1 general fill limits: 1 cable 53%; 2 cables 31%; 3+ cables 40%.
AVIXA CTS-D simple jam ratio reference = conduit ID / average cable OD.
IEEE field-bend variant for equal cables = 1.05 × ID / OD; avoid the 2.8–3.2 region.
WORKED EXAMPLE
Example — three similar cables in a bend
Conduit fill alone does not tell the full story. For three similar cables the jam-ratio check helps identify a risk zone where cables may wedge in bends.
Source details
- NFPA 70 (NEC) — Chapter 9 conduit/raceway fill basis; adopted edition varies by jurisdiction.
- AVIXA CTS-D Formula Sheet (2024) — conduit capacity and jam-ratio reference formulas.
- IEEE 525-2007 — field-bend 1.05 jam-ratio example and cable-pulling context.