Conduit fill is the share of a conduit's inside area taken up by the wires. The NEC allows 40% for three or more conductors, 31% for two and 53% for one (Chapter 9, Table 1). Six 12 AWG THHN wires plus a 10 AWG THHN ground fill 3/4 inch EMT to 18.9%, well inside the limit, and even 1/2 inch EMT passes at 33.2%. With mixed wire sizes, add up each wire's area and compare the total to the conduit.
How conduit fill is calculated
Every insulated conductor has an approximate cross-section area in Chapter 9 Table 5 (THHN, THWN-2, XHHW-2 and others), bare wires in Table 8, and every conduit type and trade size has an inside area in Table 4. Multiply each wire's area by how many there are, add the rows, and divide by the conduit area.
Fill % = Σ (count × wire area) ÷ conduit area × 100
The fill must not exceed the Table 1 percentage for the total count: 53% for one conductor or cable, 31% for two, 40% for three or more. Note 3 to the tables says ground wires count, at their real size. Note 4 allows 60% in a nipple no longer than 24 inches between boxes.
Worked example: seven wires in EMT
The calculator opens with a common home run: six 12 AWG THHN and one 10 AWG THHN.
- Wire area: 6 × 0.0133 in² + 1 × 0.0211 in² = 0.1009 in²
- 3/4 inch EMT has 0.533 in² inside: 0.1009 ÷ 0.533 = 18.9%, limit 40%
- 1/2 inch EMT has 0.304 in²: 0.1009 ÷ 0.304 = 33.2%, still a pass, so the smallest EMT is 1/2 inch
Six current-carrying wires in one raceway also cut their ampacity to 80% under Table 310.15(C)(1), which the page shows under the result.
How many wires fit: THHN in EMT and PVC
When all wires are the same size, divide 40% of the conduit area by one wire's area. Table 1 Note 7 rounds up a result ending in 0.8 or more. These counts match Informative Annex C for THHN in EMT:
| THHN size | 1/2 in EMT | 3/4 in EMT | 1 in EMT | 1-1/4 in EMT | 3/4 in PVC 40 | 1 in PVC 40 |
|---|---|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 | 21 | 34 |
| 12 AWG | 9 | 16 | 26 | 45 | 15 | 25 |
| 10 AWG | 5 | 10 | 16 | 28 | 9 | 15 |
| 8 AWG | 3 | 6 | 9 | 16 | 5 | 9 |
| 6 AWG | 1 | 4 | 7 | 12 | 4 | 6 |
PVC Schedule 40 has a slightly smaller bore than EMT, and Schedule 80 is smaller again: 3/4 inch Schedule 80 has 0.409 in² against 0.533 in² for EMT. XHHW-2 in small sizes has thicker insulation than THHN (12 AWG: 0.0181 in² against 0.0133 in²), so fewer fit.
Multiple wire sizes
Mixed sizes are where charts stop working and the area method is the only one. Add a row for each size and type, including the equipment grounding wire. The page lists every trade size of the conduit type with its fill, so you can see how much room one size up leaves for a future circuit.
Conduit fill by diameter: cables, Cat 6 and low voltage
A cable counts as one conductor, with its area worked out from the outside diameter (Table 1 Note 9); a flat cable uses its larger dimension as the diameter. Belden lists its 2412 Cat 6 cable at 0.224 inch, which gives 0.0394 in² per cable: five fit in 3/4 inch EMT, three in 1/2 inch. Southwire gives 12-2 NM-B as 0.422 × 0.190 inch, so one cable is 0.1399 in². One fits 3/4 inch EMT at 26%, but two cables fall under the 31% rule and need 1-1/4 inch EMT (1 inch reaches 32.4%). For any other cable, read the outside diameter on its spec sheet and pick "Other cable".
Jamming with three wires
Table 1's Informational Note 2 warns that three conductors can wedge side by side in a bend when the conduit's inside diameter is 2.8 to 3.2 times the conductor diameter. Three 1/0 THHN (0.486 in) in 1-1/4 inch EMT (1.380 in inside) give a ratio of 2.84, inside the jam range, although the fill is only 37%. The calculator flags this for any three same-size conductors or cables.
Fill with derating
More than three current-carrying conductors in one raceway reduce each conductor's ampacity: 80% for 4 to 6, 70% for 7 to 9, 50% for 10 to 20, 45% for 21 to 30, 40% for 31 to 40 and 35% above that. Ground wires never count, and 310.15(E) lets some neutrals drop out. Filling a big conduit with many circuits can pass the fill check and still force bigger wire: check it with the wire size calculator, which takes the conductor count as an input.
Next steps
Size each circuit's conductors with the wire size calculator and the voltage drop calculator for long runs, then match the breaker size. Every electrical tool is on the electrical calculators page.