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Conduit Fill Calculator

Calculate maximum conduit fill capacity per NEC Chapter 9. Easy tool for finding how many wires fit in EMT, PVC, and Rigid conduits.

NEC Chapter 9 Table 1, Table 4

Raceway & Conductors

13.1% fillPasses 40% · 1/2" EMT
Quick Start

Following the smallest size that fits. Turn off to pick your own.

Grounds and unloaded neutrals don't count. Above 3, ampacity derating kicks in.

Conductors in the Raceway

Conduit Fill Results

Fill in 1/2" EMT13.1%Passes

Limit applied: 40% — 3 or more conductors

1/2" EMT cross-section — 3 conductors, 13.1% fill

Schematic — relative conductor areas at true scale. Not a pull plan; actual arrangement varies.

Area Breakdown (Chapter 9 Table 5)

3 × #12 THHN @ 0.0133 in²0.0399 in²
Total conductor area (3 conductors)0.0399 in²
Allowable at 40%0.1220 in²
Headroom remaining0.0821 in²

1/2" EMT holds up to 9 #12 THHN conductors — the published NEC Annex C count.

Calculations based on NEC 2023 guidelines. Results are for estimation purposes — verify against local code before installation.

Verified against NEC 2023 · Chapter 9 Table 1, Table 4, Table 5

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How this works

Conduit fill is a physical fit question: how much room does a raceway need to hold your conductors without the kind of jamming that damages insulation on the pull. NEC Chapter 9, Table 1 sets the limit at 40% of the conduit's cross-sectional area for three or more conductors, 31% for two, and 53% for a single conductor, and this calculator checks your wire count and sizes against those numbers to give you the smallest conduit that legally fits.

Every wire counts toward that number, including the neutral and the ground. Skipping the ground because "it's just a ground" is one of the more common ways a conduit that looks fine on paper ends up overfilled in the field.

Fitting isn't the same question as capacity. Once you're past three current-carrying conductors in a raceway, NEC 310.15(C)(1) also cuts the ampacity of each wire, which is a separate calculation this tool doesn't cover.

Worked example

A 20A branch circuit (hot, neutral, and ground) wired in #12 THHN conductors, run in 3/4" EMT.

3/4" EMT allowable fill area at 40% (3+ conductors)NEC Chapter 9, Table 10.213 in²
#12 THHN conductor areaNEC Chapter 9, Table 50.0133 in²
16 conductors × 0.0133 in²NEC Annex C0.2128 in² (fits)
17 conductors × 0.0133 in²NEC Annex C0.2261 in² (over the limit)

3/4" EMT holds up to 16 #12 THHN conductors; a 17th does not fit, matching the maximum Annex C publishes for this combination.

Frequently Asked Questions

What is the maximum allowable conduit fill?↓

Per NEC Chapter 9, Table 1, the maximum fill for a conduit containing three or more conductors is 40% of the conduit's total cross-sectional area. For two conductors, it's 31%, and for a single conductor, it's 53%.

Do ground wires count towards conduit fill?↓

Yes, all wires inside the conduit, including equipment grounding conductors and neutral wires, must be included in the total cross-sectional area calculation for conduit fill.

Does conduit fill affect ampacity derating?↓

Conduit fill itself determines if wires physically fit without damage. However, if you have more than 3 current-carrying conductors in a raceway, you must also apply ampacity adjustment factors (derating) per NEC 310.15(C)(1).

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