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Conduit Fill Calculator

Conductor area and count against conduit area to percent fill and max conductors.

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Conduit fill

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40% fill assumes 3+ conductors

NEC Chapter 9 tables use different fill limits depending on how many conductors are in the raceway: 53% for one conductor, 31% for two, and 40% for three or more. This calculator defaults to the common 40% case; adjust the fill limit field if you have exactly one or two conductors.

Conduit fill limits exist so conductors can be pulled through without damaging the insulation, and so heat generated by current flow can dissipate rather than building up inside a packed conduit. The percentage is simply the total cross-sectional area of all conductors divided by the conduit's internal usable area.

NEC Chapter 9, Table 1 sets the standard fill limits: 53% for a single conductor, 31% for exactly two, and 40% for three or more conductors in one raceway. Table 5 in the same chapter lists the actual cross-sectional area for each wire gauge and insulation type, which varies by more than you'd expect between THHN, XHHW, and other insulation types.

This calculator handles the fill-percentage math; the specific area values for your wire gauge and conduit size and type should come from the current NEC tables or your electrical supplier, since insulation type changes the numbers.

Where these numbers come from

These are planning estimates only. Electrical work must meet the National Electrical Code and your local code, and should be installed or inspected by a licensed electrician. This is not a substitute for a permit or a licensed electrician's judgment.

Frequently asked questions

Why does the fill limit change based on conductor count?

With very few conductors there's more room per wire relative to the conduit, so code allows a higher fill percentage for one or two conductors (53% and 31%) than for three or more (40%), where a lower limit leaves room for heat dissipation and pulling.

Where do I find the area for my specific wire and conduit?

NEC Chapter 9, Table 5 lists cross-sectional area by wire gauge and insulation type (THHN, XHHW, etc.), and Table 4 lists usable internal area by conduit type and trade size. These values change between NEC code cycles, so check the current edition adopted in your area.

What happens if I exceed the fill limit?

Overfilled conduit makes pulling wire difficult and can damage insulation, and it fails inspection. This calculator is a planning estimate only; a licensed electrician or your local inspector should confirm the specific fill limit and wire areas that apply to your installation.

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