Free tool · NEC Chapter 9

Conduit Fill Calculator

Enter your wire size, conductor count, and conduit size to get the fill percentage against the NEC's 53% / 31% / 40% limits — with every step shown, the same way you'd work it on the journeyman exam.

Conduit fill

17.5%

4 × #12 THHN area0.0532 in²
1/2" EMT internal area0.304 in²
NEC max fill (4 cond.)40%
Fill17.5%

Compliant — under the 40% NEC maximum for 4 conductors.

How to calculate conduit fill (NEC Chapter 9)

  1. 1

    Find each conductor's area

    Look up the cross-sectional area of your wire in NEC Chapter 9, Table 5 (for THHN). Multiply by the number of conductors of that size.

  2. 2

    Find the conduit's internal area

    Look up the total internal area of your conduit in NEC Chapter 9, Table 4 (e.g. 1/2" EMT = 0.304 in²).

  3. 3

    Divide and compare to the limit

    Fill % = total conductor area ÷ conduit internal area × 100. Compare to the Chapter 9, Table 1 maximum: 53% for one conductor, 31% for two, 40% for three or more.

The NEC fill limits (Chapter 9, Table 1): 53% for one conductor, 31% for two conductors, and 40% for three or more. Going over means heat and a wire-pull that can damage insulation — so the exam (and the inspector) hold you to it.

Conduit fill — frequently asked questions

What is the maximum conduit fill allowed by the NEC?

NEC Chapter 9, Table 1 sets the maximum fill at 53% for a single conductor, 31% for two conductors, and 40% for three or more conductors. These percentages account for heat dissipation and the ability to pull wire without damage.

How do you calculate conduit fill?

Multiply each conductor's cross-sectional area (NEC Chapter 9, Table 5) by how many of that conductor you have, add them up, then divide by the conduit's internal area (Table 4) and multiply by 100. Compare the result to the 53/31/40% limit for your conductor count.

Which NEC tables are used for conduit fill?

Chapter 9, Table 1 (max fill percentages), Table 4 (conduit/tubing dimensions and internal area), and Table 5 (conductor dimensions and cross-sectional area). For bare or compact conductors, see Table 5A and Chapter 9 notes.

Does this calculator work for the journeyman exam?

Yes — conduit fill is one of the most common calculation questions on the journeyman electrician exam. This tool uses the same Chapter 9 tables you'll use on the open-book exam, and shows the worked steps so you learn the method, not just the answer.

Conduit fill is one of dozens of calcs on the exam.

See how you'd score today — take the free 8-question journeyman diagnostic and get the exact topics you'd fail, in 3 minutes. No account needed.

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