EMT Conduit Fill Chart: Areas & 40% Fill

The 40% rule and how to use it

Conduit fill limits how many conductors you may pull into a pipe so they can be installed without damage and can shed heat. The governing figure for the common case — more than two conductors in a raceway — is 40% of the conduit's internal cross-sectional area, per NEC Chapter 9. This table lists, for each EMT trade size, the total internal area, that 40% usable area, and the resulting maximum number of #12 and #10 THHN conductors as a ready reference for the most common branch-circuit wiring.

To size a mixed pull yourself, add up the actual cross-sectional area of every conductor (each insulation type has its own area in NEC Chapter 9, Table 5) and pick the smallest conduit whose 40% area is at least that sum. Note the special cases the 40% figure replaces: a single conductor is allowed 53% fill and exactly two conductors only 31%, the tightest ratio, because two round conductors leave the most awkward gaps. Trade size is a nominal name, not the real bore, which is why the area column — not the label — is what you compute against.

The Conduit Fill calculator does the arithmetic for any conductor count and gauge. For the reasoning, read NEC conduit fill explained. Overfilling makes conductors hard to pull and traps heat; when in doubt, go one trade size up.