How many #10 THHN wires can you fit in 3/4 inch EMT?

Ten. NEC 2023 Annex C, Table C.1 allows ten 10 AWG THHN conductors in 3/4 inch EMT. The math behind it: Chapter 9 Table 4 gives 3/4 inch EMT a total area of 0.533 square inches, and with three or more conductors Table 1 limits fill to 40 percent, which is 0.213 square inches. Chapter 9 Table 5 lists 10 AWG THHN at 0.0211 square inches per conductor, and 0.213 divided by 0.0211 comes out just past ten.

Fill is not the real limit, derating is

Before you stuff ten current-carrying conductors in one raceway, run 310.15(C)(1). With 10 or more current-carrying conductors you take a 50 percent adjustment on the 90 degree column. THHN 10 AWG starts at 40 amps, so ten CCCs drops you to 20 amps, and now your 30 amp circuits do not work. Keep it to nine or fewer CCCs and the factor is 70 percent, which leaves 28 amps, still short of 30. Four to six CCCs at 80 percent gives 32 amps, which supports a 30 amp circuit. Neutrals that carry only unbalanced current of a multiwire circuit and EGCs do not count toward the CCC total.

What fits alongside a ground

A common real pull is three or four 10 AWG THHN circuits plus one 10 AWG equipment ground. Mixed or not, total the areas from Table 5 and stay under 0.213 square inches. Eight 10 AWG THHN plus one 10 AWG ground is 0.19 square inches, and that passes fill.

Field notes

Ten conductors pass the table but pull hard around bends, and jam ratio is worth a thought on long runs. If the plan calls for maxing fill, upsize to 1 inch EMT for a few cents more per foot: 16 of the same conductor fit and the pull goes from a fight to a walk. Fill tables assume new conductors with intact insulation, so old runs with added conductors deserve a fresh count.