Conduit Fill

Fill percentage from the dimensions you supply, against the limit for the number of conductors.

1. What this check does

Adds the cross-sectional areas of the conductors or cables you enter and compares the total with the inside area of the conduit, against the maximum fill for that number of conductors. For three equal conductors it also reports the jam ratio.

2. Inputs used

Conduit inside diameter.
Outside diameter and quantity of each conductor or cable.

3. Formulas and assumptions

Fill % = Σ (π/4 · dᵢ² · nᵢ) / (π/4 · D²) × 100 Jam ratio = D / d (three equal conductors)

RokBench provides no conduit or conductor dimension tables. The result is only as good as the diameters you enter.
A multiconductor cable counts as a single conductor.

4. What is NOT checked

Pulling tension, sidewall pressure and bend radius.
Ampacity adjustment for more than three current-carrying conductors.
Nipples (short sections) and other special cases.

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

NEC 2023 Chapter 9, Table 1 and Notes to Tables
NEC 2023 344.22, 358.22 (and the corresponding section for each raceway type)

6. Validation cases

Hand-calculated cases run on every change. See Validation.

7. Known issues and changes

See the Changelog. Try the tool: Conduit Fill.