Conduit Fill Calculator
Check NEC conduit fill compliance: calculate fill percentage, compare to code limits, and find maximum conductor count.
About this calculator
Pulling too many conductors into a conduit traps heat and makes future wire pulls nearly impossible, so NFPA 70 (the National Electrical Code) caps how much of a conduit's cross-section can be filled with wire, in Chapter 9 of the code itself. This calculator looks up the conduit's internal area (in mm²) from a trade-size table and the individual conductor's cross-sectional area from a THHN or XHHW outer-diameter table — the two insulation types have different areas because XHHW's jacket is built differently even at the same AWG size, which is why conductor type changes the result even with an unchanged wire gauge. Multiplying the per-conductor area by how many conductors you're pulling gives the Total Fill Area, and dividing that by the conduit's internal area gives the Fill Percentage.
That percentage is checked against NEC Chapter 9's tiered limits: 53% is allowed for a single conductor, 31% for exactly two (since two conductors pack less efficiently than three or more), and 40% for three or more — the calculator applies whichever tier matches your conductor count. It also works the fill limit backward: dividing the maximum allowable fill area for that tier by the single-conductor area, then rounding down, gives the Max Conductors Allowed for that conduit and wire combination, useful for quickly checking headroom on an existing run. Note the compliance check and max-conductor figure both assume all conductors are the same size and type — a mixed bundle of different gauges needs the areas summed by hand rather than run through this calculator directly.
Inputs
NEC Chapter 9: 1 conductor = 53% fill; 2 conductors = 31%; 3+ conductors = 40% max fill
NEC Table 1: 1 conductor = 53% fill; 2 = 31%; 3+ = 40% max of conduit inside area
Results
Fill Percentage
9.1%
NEC Compliant?
1 (1=yes, 0=no)
Figures current as of 2026. Source: National Fire Protection Association, NFPA 70, National Electrical Code (NEC), 2026 Edition, Chapter 9
How to Use This Calculator
- Select the Conduit Trade Size in mm: 16 mm = 1/2", 21 mm = 3/4", 27 mm = 1", 35 mm = 1-1/4", 53 mm = 2".
- Enter the Conductor Size in AWG and the Conductor Type (THHN or XHHW) — conductor OD varies by insulation type.
- Enter the Number of Conductors to be pulled through the conduit.
- Review the Fill Percentage — NEC Chapter 9 allows 40% fill for 3+ conductors, 31% for 2, and 53% for 1 conductor.
- Check the NEC Compliant? output — a value of 1 means the fill is within limits; 0 means you must upsize the conduit.
- Note the Max Conductors Allowed to quickly assess how many more wires could be added to an existing conduit.
How the result changes with Number of Conductors
| Number of Conductors | Fill Percentage | NEC Compliant? |
|---|---|---|
| 1.5 | 4.5% | 1 (1=yes, 0=no) |
| 2.25 | 6.8% | 1 (1=yes, 0=no) |
| 4.5 | 13.6% | 1 (1=yes, 0=no) |
| 7.5 | 22.6% | 1 (1=yes, 0=no) |
What each input means
- Conduit Trade Size
- Conduit trade size per NEC Chapter 9 Tables. Common: 16 (1/2"), 21 (3/4"), 27 (1"), 35 (1-1/4"), 41 (1-1/2"), 53 (2"). Fill limits per NEC Table C1–C12 based on conduit type.
- Conductor Size (AWG)
- AWG wire size: 14, 12, 10, 8, 6, 4, 3, 2, 1, 0. Smaller number = larger wire.
- Number of Conductors
- Total number of conductors to be installed in the conduit. NEC Chapter 9 Table 1 limits fill to 40% for 3+ conductors to ensure proper heat dissipation and installation ease.
- Conductor Type
- Conductor insulation type per NEC Article 310. THHN and XHHW have different outer diameters per NEC Chapter 9 Table 5, which directly affects conduit fill calculations.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersConduit Trade Size = 27, Conductor Size (AWG) = 12, Number of Conductors = 3, Conductor Type = 1 = 4 input(s) provided
- Calculate Fill PercentageFill Percentage9.1 = 9.1
- Calculate NEC Compliant?NEC Compliant?1 = 1
- Calculate Conduit Internal AreaConduit Internal Area387 = 387
- Calculate Area per Conductor11.68 = 11.68
Figures and sources
- NEC Chapter 9, Table 1 (conduit fill limits: 53% for one conductor, 31% for two, 40% for three or more) and Table 5 (conductor dimensions by insulation type) (2026) — National Fire Protection Association, NFPA 70, National Electrical Code (NEC), 2026 Edition, Chapter 9
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the same AWG size have a different area depending on THHN vs XHHW?
The calculator looks up conductor area from two separate tables, thhnAreas and xhhwAreas, indexed by AWG size — for example, #12 AWG is 11.68 mm² under THHN but 14.52 mm² under XHHW. The two insulation types have different outer-diameter construction even at the same wire gauge, so switching Conductor Type changes the Total Fill Area calculation even though you didn't touch the wire size.
Why is the fill limit 31% for two conductors but a more generous 40% for three or more?
This is a direct NEC Chapter 9 rule the calculator encodes as a step function: conductorCount === 1 gets 53%, exactly 2 gets 31%, and 3+ gets 40%. Two conductors pack less efficiently inside a round conduit than three or more do (three round wires nest more evenly against a round conduit's wall), so the code recognizes fewer usable percentage points for exactly that in-between case.
How does the calculator arrive at 'Max Conductors Allowed'?
It takes the conduit's total area, multiplies by the fill-percentage tier that applies to your conductor count (53%, 31%, or 40%), and divides that allowable area by the single-conductor area, rounding down. Because the fill-percentage tier itself depends on how many conductors you're pulling, this figure is really 'how many more of this exact wire and count-tier could fit' rather than an absolute maximum independent of your current conductor count.
Can I use this calculator to check a conduit with a mix of different wire gauges?
Not directly — the tool multiplies one singleConductorArea value by conductorCount, assuming every conductor pulled is the same AWG size and insulation type. For a mixed bundle, you'd need to look up each gauge's area separately, sum them by hand, and compare that hand-summed total against the conduit's internal area and the appropriate NEC fill percentage yourself.
Where does the NEC get the 53%/31%/40% fill percentages from?
Those figures come directly from NFPA 70's Chapter 9, Table 1 — the National Electrical Code's own table of allowable percentage fill for conduit and tubing. Conductor and conduit cross-sectional areas used to apply that percentage come from the companion Table 4 (conduit dimensions) and Table 5 (conductor dimensions), which is why switching between THHN and XHHW at the same AWG size changes the result — the two insulation types have different tabulated areas in Table 5.
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