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    Calculates locally in your browser · conduit fill to Chapter 9
    Electrical / raceways

    Conduit fill

    The calculation is trivial and the rule is not: Chapter 9 Table 1 allows 53% of the raceway for one conductor, 31% for two and 40% for more, because conductors in a raceway cannot shed heat the way conductors in free air can. This applies Table 4, Table 5 and Table 1 together and tells you the smallest trade size that will actually pass.

    VerdictPassesRacewayEMTAllowance40%
    Fill
    7.5%
    40% allowed
    Area used
    0.040 in²
    of 0.213 in² available
    Conductors it holds
    16
    at 12 AWG
    Smallest size that passes
    1/2 in
    EMT
    The fill calculation
    Chapter 9 Table 5 for the conductor area, Table 4 for the raceway, Table 1 for the allowance.
    7.5%
    The fill calculation
    StepValueSource
    Raceway0.533 in²Table 4, EMT 3/4 in
    Conductor area0.0133 in²Table 5, 12 AWG THHN
    Conductors3as entered
    Total conductor area0.0399 in²count × area
    Allowance40%Table 1
    Allowed area0.2132 in²raceway area × allowance
    Fill7.5%within the allowance
    Maximum conductors16floor(allowed / conductor area)
    Recommended size1/2smallest that passes

    The 40% limit is about heat, not packing: conductors in a raceway cannot dissipate heat as freely as conductors in free air, so the code derates the usable volume. A 24-inch nipple is exempt because it has no appreciable heat build-up.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Conductors held, by trade size
    The step function is the point: one size up can double the capacity.
    Conductors held, by trade size0.000104.55209.10313.64418.19522.741/21/23/43/4111-1/41-1/41-1/21-1/2222-1/22-1/2333-1/23-1/244Conductors
    Fill against conductor count
    Fill rises linearly; the allowance steps down from 53% to 31% and back to 40%.
    Fill against conductor count0.0006.7113.4120.1226.8333.541122334455668810101212Fill %
    Canonical equations
    The exact relationships implemented by this engine.

    n × A_conductor · Table 5 area per conductor.

    A_raceway × allowance · Table 4 × Table 1.

    used / total · Reported as a percentage.

    floor(allowed / A_conductor) · Conductors of one size.

    engineering methodology

    A clear path from inputs to results.

    The calculation is trivial and the rule is not: Chapter 9 Table 1 allows 53% of the raceway for one conductor, 31% for two and 40% for more, because conductors in a raceway cannot shed heat the way conductors in free air can.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Choose the raceway type and trade size.
    2. Pick the conductor size and how many there are.
    3. Say whether it is a 24-inch nipple.
    4. Read the fill, the capacity and the smallest size that passes.

    The formula

    fill=n×AcondAraceway≤0.40\text{fill} = \frac{n \times A_{cond}}{A_{raceway}} \le 0.40

    Reference: NFPA 70 Chapter 9 Table 1 (allowance), Table 4 (raceway area) and Table 5 (conductor area); the nipple exception is §310.14(A)(2).

    Worked example

    Three 12 AWG conductors in 3/4-inch EMT

    Inputs
    • 3/4-inch EMT, internal area 0.533 in².
    • Three 12 AWG THHN conductors at 0.0133 in² each.
    • Normal run, not a nipple.
    Calculation path
    1. 1

      The conductor area is 3 × 0.0133 = 0.0399 in².

    2. 2

      Three conductors allow 40%, so the usable area is 0.533 × 0.40 = 0.2132 in².

    3. 3

      Fill is 0.0399 / 0.533 = 7.5%, far inside the limit.

    4. 4

      The raceway would hold 16 conductors of this size.

    5. 5

      Half-inch EMT allows 0.1216 in², which is still more than 0.0399, so it passes as well.

    Result

    7.5% fill · 16 conductors maximum · 1/2 inch passes.

    Interpreting the result

    The 40% limit is about heat, not packing. Conductors in a raceway are surrounded by other conductors and by the raceway wall, so the heat they generate cannot escape as freely as it would in free air. That is why a raceway that looks half empty can still be at its limit.

    Assumptions & limitations

    • Conductor areas are Chapter 9 Table 5 for THHN/THWN-2.
    • Raceway areas are Table 4 for EMT, PVC Schedule 40 and rigid metal.
    • All conductors are the same size; mixed sizes must be summed individually.
    • The nipple exception applies to raceways 24 inches or less between enclosures.

    Frequently asked questions

    Does the ground wire count?

    Yes — every conductor in the raceway counts toward fill, including the equipment grounding conductor. It is only current-carrying conductors that matter for ampacity derating.

    What about different insulation types?

    Each has its own area in Table 5. THHN is thinner than XHHW, so mixing types changes the total and the calculation has to be done conductor by conductor.

    Can I run different circuit voltages in one conduit?

    Yes, subject to §300.3(C), provided all conductors are rated for the highest voltage present. Fill is calculated the same way.

    Why is a nipple allowed 60%?

    Because a short raceway has no appreciable heat build-up, so the thermal reason for the 40% limit does not apply.

    Does fill affect ampacity?

    Not directly — ampacity depends on the number of current-carrying conductors, not on how full the raceway is. Both requirements have to be satisfied separately.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.