Phase Academy
Phase Academy Interactive Power Systems Lab

Conduit Fill Calculator

NEC CH. 9 · EDUCATIONAL

What this tool does — and does not — do. It performs a conduit fill calculation from verified NEC Chapter 9 tables. It does not verify ampacity adjustment, conductor sizing, EGC sizing, voltage drop, raceway permitted uses, installation conditions, pull tension, jamming, bends, or AHJ amendments.
Fill Gauge
Fill gauge
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Fill in selected conduit
Configure inputs
Conduit Cross-Section
Illustrative — not to geometric scale
Quick Metrics
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Conductor Area
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Allowable Fill
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Fill %
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Min Raceway
Conductor Area Breakdown
Insulation Type Comparison

Base conductor size & quantity, per insulation column.

Raceway Size Ladder

All trade sizes for your raceway type. Red = over limit, blue = fits, highlighted = minimum compliant.

Educational Disclaimer
Conduit fill calculation support based on verified NEC Chapter 9 tables. This is not a substitute for engineering judgment, plan review, or AHJ interpretation. Always verify against the adopted NEC edition in your jurisdiction.
Minimum Raceway Size
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Configure inputs to see results.
Next Larger Raceway
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EGC 250.122 Check
Enter an OCPD rating in the sidebar to check EGC size against NEC 250.122. Skipped otherwise.
Detailed Results
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Total Conductor Area
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Allowable Fill Area
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Fill %
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Conductors & Cables
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Fill Rule
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Remaining Area
Your Conduit Size Test

Selection from the left panel compared here:

Select a specific conduit size (not Auto) to test your design choice.
Common Mistakes
  • ✗ Forgetting EGCs count toward fill
  • ✗ Using bare (Table 8) area for an insulated conductor
  • ✗ Wrong fill % — 1, 2, and 3+ have different limits
  • ✗ Ignoring raceway type differences for the same trade size
  • ✗ Applying 60% nipple fill to a full conduit run
Step-by-Step Calculation Trace

Every number is traceable to a Chapter 9 table.

Assumptions
    Area Breakdown
    Design Challenge Mode

    Given a scenario, pick the correct minimum conduit size. Your left-panel inputs set the scenario.

    Change any input to generate a fresh scenario.

    Challenge: Pick the Minimum Conduit Size

    Current Configuration

    Based on your current inputs, what is the minimum compliant conduit size?

    Challenge: Estimate the Fill Percentage

    For the minimum compliant conduit, what fill percentage do you expect?

    30%
    Challenge: Which Insulation Fits Best?

    For the current conductor set, which insulation type gives the smallest total area?

    NEC Reference Panel

    Paraphrased for education. NEC text is copyrighted by NFPA. Section numbers update with the selected edition where they changed.

    Ch.9, Table 1 — Percent Fill 1 conductor 53%, 2 conductors 31%, over 2 (3+) 40%. Applies to all Chapter 9 raceways.
    Ch.9, Table 1, Note 4 — Nipples A nipple ≤ 24 in. between enclosures may be filled to 60%; ampacity adjustment need not apply.
    Ch.9, Table 4 — Raceway Dimensions Internal area per trade size and raceway type. Same trade size, different type = different area.
    Ch.9, Table 5 — Insulated Conductor Area Cross-sectional area incl. insulation. Used for fill — not Table 8 bare areas.
    Ch.9, Table 5A — Compact Conductors Nominal areas for compact copper/aluminum building wire (dimensions from industry sources).
    Ch.9, Table 8 — Conductor Properties Bare/overall areas. Used here only for bare EGCs (stranded values — conservative for solid).
    Ch.9, Notes to Tables, Note 7 — Same-size round-up Where all conductors are the same size (total cross-sectional area including insulation), the next higher whole number of conductors applies when the calculation results in a decimal of 0.8 or larger.
    NEC Edition Notes
    Edition caution Chapter 9 fill tables (1, 4, 5, 5A, 8) as implemented here are stable across 2017/2020/2023, with two edition notes: the 2020 cycle renumbered the ampacity-adjustment reference cited in the nipple note, and the 2023 cycle added 5" and 6" EMT (358.20(B)) which this tool does not include — EMT is shown through 4". 2026 values are not independently validated in this tool.
    Calculation Method
    1. Look up insulated area (Table 5 / 5A), bare area (Table 8), or use a custom/cable dimension.
    2. Multiply by quantity for each group.
    3. Sum all areas (conductors, EGC, spares, custom, cables).
    4. Determine fill % from Table 1 by total conductor/cable count (or 60% nipple).
    5. Allowable fill = internal area × fill %.
    6. Smallest trade size where allowable ≥ total area (full precision).
    7. Same-size sets: apply Ch.9 Note 7 — the next higher whole number of conductors when the division yields ≥ 0.8. Mixed sets use the strict area sum (Note 6).

    Allowable is computed as internal area × fill % at full precision. For all-same-size conductor sets this tool applies the Note 7 ≥ 0.8 round-up (Ch.9 Notes to Tables, Note 7), which is why its counts match NEC Annex C.

    Data Provenance
    DatasetSourceValues
    Conduit Fill Basics
    What is conduit fill? The ratio of total conductor cross-sectional area to raceway internal area. NEC limits it to allow heat dissipation and safe installation/removal.
    Why different percentages? 1 wire 53%, 2 wires 31%, 3+ 40%. The lower two-wire figure reflects the pulling geometry of a conductor pair. Jamming is a separate phenomenon: per Ch.9 Informational Note No. 2 it involves three conductors when the raceway ID ÷ conductor OD ratio is between about 2.8 and 3.2.
    Why size grows quickly Area scales with the square of diameter. Doubling conductor count often means jumping a trade size or two.
    Conductor Size & Insulation
    Size is nonlinear Larger AWG number = smaller wire. Below 1 AWG: 1/0, 2/0, 3/0, 4/0, then kcmil.
    Insulation type THHN has thinner insulation than THW or RHH (with covering). It can change your conduit size. RHH values without the outer covering (RHH*) are smaller and are a distinct Table 5 column.
    Fitting "in your head" isn't fitting in the conduit. The tables account for practical pulling realities.
    Common Mistakes — Expanded
    Bare vs. insulated area Table 8 = bare, Table 5 = insulated. Fill of insulated conductors uses Table 5. Example: bare 12 AWG ≈ 0.005 in² (Table 8) vs insulated THHN 12 = 0.0133 in² (Table 5).
    Forgetting EGCs They take space and must be included in fill.
    Raceway type differences 3/4" EMT ≠ 3/4" RMC internal area. EMT has thinner walls = more room; PVC Sch 80 has less.
    Misusing the nipple rule 60% fill applies only to a nipple ≤ 24 in. between enclosures — not a full run.
    Why Fill Rules Matter
    Pulling friction Higher fill = more friction = harder pulls and insulation-damage risk.
    Heat buildup Tight packing prevents heat dissipation and degrades insulation over time.
    Future maintenance Overfilled conduit is impractical to modify. Fill limits support serviceability.
    Educational Disclaimer
    Conduit fill calculation support based on verified NEC Chapter 9 tables. This tool does not verify ampacity adjustment, conductor sizing, EGC sizing, voltage drop, raceway permitted uses, installation conditions, pull tension, jamming, or AHJ amendments. Not a substitute for engineering judgment, plan review, or AHJ interpretation.