Pick your conductors and see the actual cross-section drawn to scale, the run profile, and every trade size evaluated at once.
How conduit fill is calculated
Short answer: add the cross-sectional area of every conductor, then compare that total against the internal area of the raceway multiplied by the allowable fill percentage — 53% for one conductor, 31% for two, and 40% for three or more.
- Find each conductor area. NEC Chapter 9, Table 5 lists the area of every conductor by size and insulation type. A 12 AWG THHN conductor is 0.0133 in².
- Add them up. Multiply each area by its quantity and total the result. Equipment grounding conductors count too, insulated or bare.
- Apply the fill limit from Table 1, based on how many conductors are in the raceway.
- Compare against the raceway. Table 4 gives the internal area of each conduit type and trade size. Multiply it by the limit; if the conductor total is at or below that number, the conductors fit.
Worked example: six 12 AWG THHN conductors total 0.0798 in². One inch EMT has 0.864 in² of internal area, and 40% of that is 0.3456 in². The conductors occupy 9.2% of the raceway, so they pass with a wide margin — and the calculator above will tell you that ½" EMT already handles them.
Why the limits exist
The percentages are not arbitrary. They leave room for heat to dissipate and keep pulling tension low enough that insulation does not get damaged during installation. Three or more round conductors cannot pack efficiently inside a round raceway, which is why the allowance drops to 40%.
The 360 degree rule
Separately from fill, a raceway run is limited to 360 degrees of total bend between pull points — four 90° bends. Past that, the pull becomes hard enough to risk damaging conductors, and a pull box or conduit body is required. The run profile above flags this automatically.
Frequently asked questions
What is the maximum conduit fill allowed by the NEC?
53% for a single conductor, 31% for two conductors, and 40% for three or more, per Chapter 9, Table 1. The percentage applies to the internal cross-sectional area of the raceway.
How many 12 AWG THHN conductors fit in 1" EMT?
Twenty-five. One inch EMT has 0.864 in² of internal area; 40% of that is 0.3456 in². Each 12 AWG THHN conductor is 0.0133 in².
Do ground wires count toward conduit fill?
Yes. Equipment grounding and bonding conductors count whether insulated or bare. Use Table 5 for insulated conductors and Table 8 for bare ones.
How many 90° bends are allowed between pull points?
Four — 360 degrees of total bend. Past that you need a pull box or conduit body.
Does EMT or PVC Schedule 40 hold more conductors?
EMT, at the same trade size. Its wall is thinner, so the internal area is larger: 0.864 in² versus 0.832 in² at 1". The comparison table above shows every size where that difference decides whether a pull passes.
Does the drawing show how conductors really sit inside?
The packing is physically plausible and drawn to scale, but the code sizes raceways by area, not by geometric arrangement. Treat the drawing as visualization, not as a jam ratio check.
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