You can’t fill a conduit to the brim — the NEC caps it so the wires pull without stripping their insulation and the heat can escape. The limit depends on how many conductors are inside, and the check is a comparison of areas. Here is the whole method.
The fill limits
| Conductors in the conduit | Max fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more | 40% |
These are NEC Chapter 9, Table 1. The 40% case (three or more) is by far the most common.
The method
Add up the cross-sectional areas of your conductors (NEC Table 5), and compare that with the allowed percentage of the conduit’s internal area (NEC Table 4). It fits when the conductor total is at or under the limit. Example: a ¾″ EMT has 0.533 in² inside; 40% of that is 0.213 in². A 12 AWG THHN conductor is 0.0133 in², so 0.213 ÷ 0.0133 = 16 conductors.
Conduit Fill Calculator
Add your conductors for the smallest EMT, rigid, IMC or PVC conduit that fits, with the fill % and a pass/fail.
Insulation type changes the count
The same gauge of wire has different diameters in different insulations. THHN/THWN-2 is thin, so more fit; older THW and some XHHW are fatter, so fewer do. Always use the area for the insulation actually printed on your wire — it can change the conduit size by a whole trade size.
Count the ground (and neutral)
Every conductor in the raceway counts toward fill — the hots, the neutral and the equipment grounding conductor. The ground takes up space even though it carries no current in normal operation. (It does not count as a current-carrying conductor for the separate ampacity-derating rule.)
Fill is not derating
Two different rules get confused here. Fill is whether the wires physically fit — this page. Derating is NEC 310.15: once you have more than three current-carrying conductors in a raceway, their ampacity is reduced (4–6 conductors to 80%, and so on), which can force a bigger wire. Size the wire and check derating first, then use the fill rule to pick the conduit.
Same-size wire? Use Annex C
When a conduit is full of one wire size, NEC Annex C lists the exact count directly — handy as a quick chart. For a mix of sizes (the usual real job), there is no chart: you add the areas and apply the percentage, which is exactly what the calculator does. A short nipple (24 in or less) is allowed 60% fill and skips derating.