Electrical Conduit Fill

Free Electrical Conduit Fill calculator with clear step-by-step results.

About Electrical Conduit Fill

Conduit fill compares the total cross-sectional area of the conductors against the internal area of the conduit. The permitted percentage depends on how many conductors are present - 53% for one, 31% for two and 40% for three or more - because the limits exist to allow pulling without damaging insulation.

Formulas

Fill percentage

Fill % = conductors x pi x (OD/2)^2 / (pi x (ID/2)^2) x 100

Permitted fill

One conductor 53%, two conductors 31%, three or more 40%

Worked example

With the default values below, Electrical Conduit Fill returns Conduit fill of 9.97%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Conduit internal diameteraccepts 5–200 mm: 20.9 mm
Number of conductorsaccepts 1–100: 4
Conductor outside diameteraccepts 0.5–60 mm: 3.3 mm

Results

Conduit fill: 9.97% %
Permitted fill for this conductor count: 40% %
Verdict: Within the permitted fill
Maximum conductors of this size: 16

Step by step

1Values used
2Fill percentage
3Permitted fill
4Conduit fill= 9.97 %
5Permitted fill for this conductor count= 40 %
6Verdict= Within the permitted fill
7Maximum conductors of this size= 16

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Conduit fill, bend radius, derating and conductor selection are governed by electrical codes that differ by jurisdiction. Installations must be designed and inspected by a licensed electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Conduit fill, bend radius, derating and conductor selection are governed by electrical codes that differ by jurisdiction. Installations must be designed and inspected by a licensed electrician.

Frequently Asked Questions

Why is two conductors the strictest limit?

Two round conductors in a round conduit jam against each other and the wall in a way that makes pulling disproportionately hard, so the code allows less fill than for one or for three.

Does fill affect current-carrying capacity?

Yes, separately. More conductors in a raceway means mutual heating, which triggers ampacity derating on top of the geometric fill limit.