How conduit fill is calculated
Conduit fill compares the total cross-sectional area of the conductors to the internal area of the raceway. Overfilled conduit is hard to pull, damages insulation and traps heat. The NEC sets the limits in Chapter 9, Table 1:
| Number of conductors | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
Fill % = Σ (conductor area × quantity) ÷ conduit area (100%) × 100
Conductor areas come from Chapter 9 Table 5 (for example, 12 AWG THHN = 0.0133 in²) and conduit areas from Table 4 (3/4 in EMT = 0.533 in² total). Two conductors get the lowest limit because two round wires can jam against each other in a bend.
Worked examples
Two 20 A circuits plus a ground in 3/4 EMT
Six 12 AWG THHN plus one 10 AWG ground: 6 × 0.0133 + 0.0211 = 0.1009 in². Divided by 0.533 in² gives 18.9%, well under 40%. It would also fit in 1/2 EMT (0.304 in², 33.2%). Six current-carrying conductors in one raceway require an 80% ampacity adjustment.
60 A feeder in EMT
Three 6 AWG THHN (0.0507 in² each) and a 10 AWG ground total 0.1732 in². In 1/2 EMT that is 57%, which fails. In 3/4 EMT it is 32.5%, which passes.
200 A feeder in PVC
Four 3/0 THHN (0.2679 in²) and one 4 AWG ground (0.0824 in²) total 1.154 in². The smallest Schedule 40 PVC that passes is 2 inch (3.291 in², 35.1%).
Maximum THHN conductors in EMT
Identical conductors, 40% fill with Note 7 rounding. These match NEC Annex C, Table C.1.
| THHN size | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" |
|---|---|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 AWG | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 AWG | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 AWG | 1 | 4 | 7 | 12 | 16 | 26 |
| 4 AWG | 1 | 2 | 4 | 7 | 10 | 16 |
| 3 AWG | 1 | 1 | 3 | 6 | 8 | 13 |
| 2 AWG | 1 | 1 | 3 | 5 | 7 | 11 |
| 1 AWG | 1 | 1 | 1 | 4 | 5 | 8 |
| 1/0 AWG | 0 | 1 | 1 | 3 | 4 | 7 |
Conduit areas (NEC Chapter 9 Table 4, total in²)
| Trade size | EMT | PVC Sch 40 | RMC |
|---|---|---|---|
| 1/2" | 0.304 | 0.285 | 0.314 |
| 3/4" | 0.533 | 0.508 | 0.549 |
| 1" | 0.864 | 0.832 | 0.887 |
| 1-1/4" | 1.496 | 1.453 | 1.526 |
| 1-1/2" | 2.036 | 1.986 | 2.071 |
| 2" | 3.356 | 3.291 | 3.408 |
Tips from the field
- Fill is a maximum, not a target. Long pulls with several bends go much easier at 25–30%.
- Count the grounds. Equipment grounding conductors count toward fill.
- Watch derating. More than three current-carrying conductors trigger the 310.15(C)(1) adjustment. Check it with the ampacity derating calculator.
- Limit bends to 360° total between pull points (358.26 for EMT, 352.26 for PVC, 344.26 for RMC).
- Pick the right raceway. PVC Schedule 80 has less internal area than Schedule 40, so the same conductors may need a larger size.
Need the wire size first? Use the wire size calculator. Working in Canada? Use the CEC conduit fill calculator.