Conduit Fill Calculator

The Conduit Fill Calculator works out the NEC conduit fill percentage for any run of wire. Enter your conductors — any mix of insulation types and sizes — pick a conduit, and the tool sums the conductor areas, compares them against the National Electrical Code fill limits (53% for one wire, 31% for two, 40% for three or more), and tells you instantly whether the run passes. It also draws a to-scale cross-section of the packed conduit and can auto-find the smallest legal trade size.

What is Conduit Fill?

Conduit fill is the fraction of a conduit's interior cross-sectional area occupied by the conductors running through it. The NEC caps this fraction so that wires can be pulled without stripping insulation and so that the heat the conductors generate can escape. Overfilled conduit is a top code-violation on inspections — it makes pulls difficult and can cause conductors to overheat.

Conduit Fill Formula

The calculation has three short steps rooted in NEC Chapter 9.

where \( n_i \) is the number of conductors of type \(i\) and \( a_i \) is that conductor's area from NEC Table 5.

NEC Maximum Conduit Fill Limits

The allowable fill depends only on how many conductors share the conduit (NEC Chapter 9, Table 1):

The single-conductor value is highest because a lone wire is easy to pull; two conductors get the lowest limit because they tend to lie side by side and jam. The 40% three-or-more case covers almost every real branch circuit and feeder.

Common Conductor Areas (NEC Chapter 9, Table 5)

Approximate cross-sectional area in square inches for the most common building-wire insulations:

Conduit Interior Areas (NEC Chapter 9, Table 4)

Total interior area in square inches by trade size for the supported conduit types:

How Insulation Type Changes the Answer

Two wires of the same copper gauge can need very different amounts of conduit space depending on their jacket. THHN/THWN-2 is the thin, dual-rated workhorse used in most commercial and residential raceways. XHHW-2 has cross-linked polyethylene insulation and is common on feeders and in wet locations. THW/THHW carries a thicker thermoplastic jacket, so it eats more room — a 6 AWG THW conductor is over 40% larger in area than 6 AWG THHN. Always match the insulation in the calculator to what you are actually installing.

Why the To-Scale Cross-Section Matters

Because the fill limit is really about area, this calculator draws every conductor as a circle sized to its true area relative to the conduit bore. When the picture looks crowded, the conduit is genuinely close to its limit — the drawing and the fill percentage always agree. It is an intuitive gut-check on top of the numbers, and it makes it obvious why a couple of large feeder conductors can eat a conduit faster than a bundle of small ones.

How to Use This Calculator

1. Choose the conduit: Select EMT, PVC Sch 40/80, RMC, or IMC and a trade size — or leave it on Auto to have the tool find the smallest legal size.
2. Add your conductors: For each group of wires, choose the insulation type and wire size, then set the quantity. Use Add conductor for different sizes in the same conduit.
3. Optional nipple: Tick the nipple box if the run is a short piece of conduit no longer than 600 mm (24 in) to apply the 60% allowance.
4. Click Calculate Fill: Read the pass/fail verdict, the to-scale cross-section, the fill gauge against the NEC limit, and the full step-by-step area breakdown.

Frequently Asked Questions

What is conduit fill?

Conduit fill is the percentage of a conduit's interior cross-sectional area that is taken up by the conductors inside it. The National Electrical Code (NEC) limits this percentage so that wires can be pulled without damage and so heat can dissipate. The limits are 53% for one conductor, 31% for two, and 40% for three or more.

How is conduit fill calculated?

Add up the cross-sectional area of every conductor using NEC Chapter 9 Table 5, then divide by the interior area of the conduit from Table 4 and multiply by 100. Compare the result against the allowable fill percentage for the number of conductors. Fill percent = total conductor area ÷ conduit area × 100.

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. A short conduit nipple no longer than 600 mm (24 inches) is allowed up to 60% fill.

Does wire insulation type affect conduit fill?

Yes. The same copper size has a different overall diameter depending on its insulation. THHN/THWN-2 is the thinnest common building wire, while THW and RHW have thicker jackets and take more room. Always use the area for the exact insulation you are installing from NEC Table 5.

What is the 40% fill rule?

When three or more current-carrying or grounding conductors share a conduit, the total conductor area may not exceed 40% of the conduit's interior area. This is the most common case for branch circuits and feeders and is set by NEC Chapter 9, Table 1.

How do I find the right conduit size for my wires?

Total up your conductor areas, then pick the smallest conduit whose interior area times the allowable fill percentage is still larger than that total. This calculator does it automatically when you leave the conduit size on Auto, showing the smallest legal EMT, PVC, RMC, or IMC size.

Additional Resources

• Electrical Conduit - Wikipedia