Enter any 2 values (total cable cross-sectional area, number of cables, or cable diameter) into the calculator to determine the missing variable. Note: this is an idealized area-based calculation and does not apply any code-required duct/conduit fill limits.

Cable Duct Fill (Total Cable Area) Calculator

Enter any 2 values to calculate the missing variable (area-based; no fill-factor applied)

Related Calculators

Total Cable Cross-Sectional Area (Duct Fill) Formula

The following formula is used to calculate the total cross-sectional area occupied by a bundle of round cables (sum of individual cable areas). This value is commonly used when checking duct/conduit fill, but it is not the same thing as the duct’s internal capacity unless you explicitly compare it to the duct’s internal area and an allowed fill percentage.

C = N \cdot \left( \pi \cdot \left( \frac{d}{2} \right)^2 \right)

Variables:

  • C is the total cable cross-sectional area (in area units, e.g., sq mm)
  • N is the number of cables
  • d is the outside diameter of each cable (in length units, e.g., mm)

What is Cable Duct Capacity?

Duct capacity (in this context) is the duct’s internal cross-sectional area available for cables. Total cable area is the sum of the cross-sectional areas of the cables you want to install. To evaluate whether a set of cables can fit, you compare the total cable area to the duct’s internal area using an allowable fill ratio required by applicable standards/codes (for example, some conduit rules of thumb limit fill to about 40% for 3+ conductors, but requirements vary—always follow the governing code/manufacturer guidance). Overfilling can make cable pulling difficult and can contribute to excess heating in some installations.

How to Calculate Cable Duct Capacity?

The following steps outline how to calculate the total cable area used for a given number of cables and cable diameter (often used for a duct/conduit fill check).

  1. First, determine the number of cables (N).
  2. Next, measure the outside diameter of each cable (d).
  3. Calculate the area of one cable using: Area = π × (d/2)2.
  4. Multiply the area of one cable by the number of cables to get the total cable cross-sectional area (C).
  5. If you are doing a duct fill check, compare C to the duct’s internal area using your allowed fill percentage (i.e., Allowed Cable Area = Fill% × Duct Internal Area).
  6. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Number of cables (N) = 10

Diameter of each cable (d) = 5 mm

Total cable area (C) = 10 × π × (5/2)2 ≈ 196.35 sq mm.