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Conduit Fill Calculator (NEC Chapter 9)

Conduit fill percentage check. User enters cross-sectional areas — verify against NEC Ch 9 Tables 1, 4, 5.

warning

ESTIMATE ONLY — not a stamped engineering design. Verify with a licensed PE or licensed electrician before procurement or construction. This tool contains no NEC table values. You enter the conduit interior cross-sectional area (NEC Ch 9 Table 4) and each conductor area (NEC Ch 9 Table 5) yourself.

Conduit

Informational only.

Informational only.

See NEC Chapter 9 Table 4 for the interior cross-sectional area of your conduit type and trade size. Enter the total area (100% column) here.

See NEC Chapter 9 Table 1 for the maximum fill percentage based on the number of conductors. The widely-applied default for >2 conductors is 40% (the "40% rule"). Single-conductor and two-conductor installations, plus short nipples (≤24 in), use different limits — consult Table 1 directly and enter the appropriate value here.

Conductors

For each conductor type/size used in this conduit, add a row. See NEC Chapter 9 Table 5 for the cross-sectional area of each conductor (insulation included). Enter that area in the row.

Fill Status

OK

0.00% fill (at or below 40% target)

Remaining Capacity

0.0000

sq in available

Conduit interior area0.0000 sq in
Total conductor area (Σ count × area)0.0000 sq in
Allowable area at 40% fill0.0000 sq in
Actual fill %0.00%

Methodology

Total conductor area = Σ (per-conductor cross-sectional area × count) across every row. Allowable area = conduit interior area × target fill % ÷ 100. Actual fill % = (total conductor area ÷ conduit area) × 100. Reference standard: NEC Chapter 9 — Tables 1 (Percent of Cross Section of Conduit and Tubing for Conductors), 4 (Dimensions and Percent Area of Conduit and Tubing), and 5 (Dimensions of Insulated Conductors and Fixture Wires). No NEC Chapter 9 table values are hardcoded in this tool — you supply the conduit interior area, each conductor area, and the target fill percentage yourself.

Frequently Asked Questions

What is the 40% fill rule?
NEC Chapter 9 Table 1 limits the maximum allowable conduit fill based on the number of conductors. For more than two conductors, the limit is widely known as the "40% rule" — the most common case in practice. Lower-conductor-count and single-conductor allowances are different; consult Table 1 in your adopted NEC edition for the exact percentages. These limits leave room for pulling without damaging the insulation and for heat dissipation.
What happens if I overfill a conduit?
Overfilled conduit is a Code violation that will fail inspection. Physically, overfilling leads to damaged insulation during the pull (jamming, abrasion, stretched jackets), reduced heat dissipation that can derate the conductors and cause overheating, and difficulty in pulling additional conductors later. It also increases the chance of insulation breakdown over the life of the installation. Re-pull into a larger trade size if you are over fill.
Do I need to apply conductor ampacity adjustment (bundling) factors?
Yes — separately. NEC 310.15(C)(1) requires ampacity adjustment when more than three current-carrying conductors are bundled in a single raceway or cable. The adjustment factors are independent of the fill calculation. A conduit can pass the 40% fill check but still require ampacity derating of the conductors inside. The grounding conductor (EGC) is not counted in the current-carrying conductor count for adjustment purposes, but it does count toward the fill area.
Does the equipment grounding conductor (EGC) count toward fill?
Yes. The EGC takes up physical space in the conduit, so its cross-sectional area must be included in the total conductor area for the fill calculation. Add a row for the EGC at its correct size (per NEC Table 250.122, sized to the overcurrent device protecting the circuit) with the area from NEC Chapter 9 Table 5 (insulated) or Table 8 (bare). The EGC does not, however, count as a current-carrying conductor for ampacity adjustment under 310.15(C)(1).