How to Calculate Conduit Fill

Conduit fill is how much of a conduit's interior space the conductors occupy. The NEC limits it so wires can shed heat and be pulled without damage. Here's how to calculate it.

The 53 / 31 / 40 rule

NEC Chapter 9, Table 1 sets the maximum fill by number of conductors: 53% for one conductor, 31% for two, and 40% for three or more.

The three tables you need

Table 1 — the allowable fill percentage (above).
Table 4 — the interior cross-sectional area of each conduit type and size.
Table 5 — the cross-sectional area of each conductor by insulation and size.

How to calculate it

Add up the area of all conductors (Table 5). Multiply the conduit's total interior area (Table 4) by the Table 1 percentage to get the allowable fill. If the conductor total is at or below the allowable area, it passes.

Worked example

Three 12 AWG THHN conductors (0.0133 in² each = 0.0399 in²) in 1/2" EMT (0.304 in² total): allowable = 0.304 × 0.40 = 0.1216 in². 0.0399 is well under 0.1216, so it passes — about 13% fill.

Does insulation type matter?

Yes. THHN/THWN-2 and XHHW have different diameters for the same wire size, so they take up different amounts of space. Always use the area for the actual insulation you're pulling.

Open the free Conduit Fill Calculator →

Frequently asked questions

How do you calculate conduit fill?

Add the cross-sectional area of all conductors (NEC Chapter 9, Table 5), then compare it to the conduit's interior area (Table 4) times the Table 1 limit for the number of conductors.

What is the maximum conduit fill percentage?

Per NEC Chapter 9, Table 1: 53% for one conductor, 31% for two, and 40% for three or more conductors.

Does conduit fill depend on wire insulation type?

Yes — different insulations have different diameters for the same wire size, so they occupy different space. Codewire accounts for the insulation type you select.

All Codewire calculators · Verify against the NEC edition adopted by your AHJ.