Conduit Fill

Enclosed Cable Pathway Fill — Explanation

An enclosed cable pathway is a cable pathway whose contents are accessed at the ends or by removing a cover/lid, fully surrounding the cables.

Types of enclosed pathways

1. Without longitudinal access (“end-access”; e.g., conduit).
2. With longitudinal access (“length-access”; with hinged or removable lid/cover).

Fill ratio definition

The pathway fill ratio \(F\) expresses how much of the pathway’s cross-section is occupied by cables (ignores interstitial gaps and bends):

\[ F \;=\; \frac{\text{Integrated cross-sectional area of all cables}}{\text{Cross-sectional area of pathway internal space}} \]

Symbols:

• \(F\) = pathway fill ratio
• \(D_c\) = cable diameter; \(N\) = number of identical cables
• \(A_c = \tfrac{\pi D_c^{2}}{4}\) = area of one round cable
• \(A_p\) = cross-sectional area of pathway internal space

Pathway cross-section

• Circular: \(\displaystyle A_p = \frac{\pi D_p^{2}}{4}\) where \(D_p\) is the internal diameter.
• Rectangular: \(\displaystyle A_p = H \times W\) where \(H\) and \(W\) are internal height and width.

Simplified (identical round cables in circular pathway)

\[ F \;=\; N \left( \frac{D_c}{D_p} \right)^{2} \]

Industry standard fill factors

• 1 cable: \(53\%\)
• 2 cables: \(31\%\)
• 3+ cables: \(40\%\)

Reverse calculation — maximum number of cables

• Circular pathway: \[ N \;=\; F \,\frac{D_p^{2}}{D_c^{2}} \]
• Rectangular pathway: \[ N \;=\; F \,\frac{H\,W}{\frac{\pi D_c^{2}}{4}} \;=\; \frac{4FHW}{\pi D_c^{2}} \]

Rounding guidance

• For small counts (≤3), rounding up can be unrealistic (e.g., summed cable diameters exceeding \(D_p\)).
• Always verify rounded values against the target fill factor.
• Higher counts better correlate with the target \(F\); rounding down is the conservative choice.

These formulas assume straight runs, identical round cables, and circular or rectangular pathways.