Conduit can only be filled to 40% with three or more conductors, to allow heat dissipation and pulling. This chart gives the max number of THHN conductors for common EMT sizes.
| EMT conduit | 12 AWG THHN | 10 AWG THHN |
|---|---|---|
| 1/2 in | 9 | 5 |
| 3/4 in | 16 | 10 |
| 1 in | 26 | 16 |
| 1-1/4 in | 45 | 28 |
FAQ
Why does conduit fill percentage matter? Conduit fill limits (commonly 40 percent for three or more conductors) leave room to pull wires without excessive friction or damage to insulation, and allow some airflow for heat dissipation, both of which get worse as fill percentage increases.
See the Electrical Box Fill Chart and Wire Ampacity Chart.
The fill percentage
The maximum fill of conduit depends on the number of conductors. The code allows 53 percent for a single conductor, 31 percent for two, and 40 percent for three or more, which is the figure this chart is based on. The limits leave room for heat to escape and for the wires to be pulled through without damage.
Planning the run
Fill gets harder on long runs with many bends, so leave some spare capacity, and the total of the bends between pull points is normally limited to 360 degrees. When wires of different sizes share a conduit, add up the cross sectional areas rather than counting wires. Derating for more than three current carrying conductors also applies, so check the code for your area.