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

\text{Fill}\,\% = \dfrac{\sum_i \frac{\pi}{4} d_i^2\, n_i}{\frac{\pi}{4} D^2} \times 100

d conductor outside diameter, n count, D conduit inside diameter.

Jam ratio

J = \dfrac{D}{d}

Three equal conductors.

Limit

\text{Fill}_{\max} = 53\% \ (1),\ 31\% \ (2),\ 40\% \ (\ge 3),\ 60\% \ (\text{nipple} \le 24\ \text{in})

Symbols

$d_i$	Outside diameter of conductor type i	in or mm
$n_i$	Number of conductors of type i
$D$	Conduit inside diameter	in or mm
$J$	Jam ratio for three equal conductors

Conduit cross-section with three conductors; shaded conductor area compared with the conduit inner area and the 40 percent limit. — **Figure 1.** Conduit fill: conductor areas against the usable conduit area.

Two sketches of three conductors in a conduit: triangular configuration and a jammed side-by-side configuration. — **Figure 2.** Jamming: three conductors lining up across the conduit.

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.
Limits: 53 % for one conductor, 31 % for two, 40 % for three or more, 60 % for a nipple (Chapter 9 Table 1 and Note 4).

4. What is NOT checked

Pulling tension, sidewall pressure and bend radius.
Ampacity adjustment for more than three current-carrying conductors.
Nipples: the 60 % limit is applied when you mark the raceway as a nipple (24 in or less between enclosures); the ampacity adjustment exemption for nipples is not part of this tool.

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

No worked example with a published answer is registered for this page yet. Have one we should add? Use the Feedback link. All cases: Validation.

7. Known issues and changes

See the Engineering Notes. Try the tool: Conduit Fill.