Gauge Charts
Conduit Fill Calculator
The National Electrical Code limits how much of a conduit cross-section conductors may occupy: 53 percent for a single conductor, 31 percent for exactly two, and 40 percent for three or more. The limits exist so conductors can be pulled without damaging insulation and so heat can escape, which is why the two-conductor case is stricter than the three-conductor case rather than looser.
Conduit fill calculator
Passes 4 × 12 AWG THHN / THWN-2 uses 0.0532 in² of the 0.533 in² inside a 3/4" EMT. With 4 conductors the limit is 40% (NEC Chapter 9, Table 1), so 0.2132 in² is allowed. That conduit takes 16 in total.
Fill is only one of the checks. A raceway that passes fill can still fail on conductor derating once more than three current-carrying conductors share it (NEC 310.15(C)(1)), on box fill, or on bend radius. Areas come from NEC Chapter 9, Tables 1, 4 and 5. Verify against the code edition your jurisdiction has adopted.
Allowable fill area at 40 percent
Total internal areas are NEC Chapter 9, Table 4. The figures below are those areas at the 40 percent limit that applies to three or more conductors, which covers almost all real installations.
| Trade size | EMT | IMC | RMC (rigid) | PVC Sch 40 | PVC Sch 80 |
|---|---|---|---|---|---|
| 1/2" | 0.122 | 0.137 | 0.126 | 0.114 | 0.087 |
| 3/4" | 0.213 | 0.234 | 0.220 | 0.203 | 0.164 |
| 1" | 0.346 | 0.384 | 0.355 | 0.333 | 0.275 |
| 1-1/4" | 0.598 | 0.659 | 0.610 | 0.581 | 0.495 |
| 1-1/2" | 0.814 | 0.890 | 0.828 | 0.794 | 0.684 |
| 2" | 1.342 | 1.452 | 1.363 | 1.316 | 1.150 |
| 2-1/2" | 2.343 | 2.054 | 1.946 | 1.878 | 1.648 |
| 3" | 3.538 | 3.169 | 3.000 | 2.907 | 2.577 |
| 3-1/2" | 4.618 | 4.234 | 4.004 | 3.895 | 3.475 |
| 4" | 5.901 | 5.452 | 5.153 | 5.022 | 4.503 |
Conductor areas
Add up the area of every conductor going into the raceway, including the equipment grounding conductor, and compare the total with the allowable area above.
| AWG | THHN / THWN-2 | XHHW / XHHW-2 |
|---|---|---|
| 1 | 0.1562 | 0.1534 |
| 2 | 0.1158 | 0.1146 |
| 3 | 0.0973 | 0.0962 |
| 4 | 0.0824 | 0.0814 |
| 6 | 0.0507 | 0.0590 |
| 8 | 0.0366 | 0.0437 |
| 10 | 0.0211 | 0.0243 |
| 12 | 0.0133 | 0.0181 |
| 14 | 0.0097 | 0.0139 |
| 1/0 | 0.1855 | 0.1825 |
| 2/0 | 0.2223 | 0.2190 |
| 3/0 | 0.2679 | 0.2642 |
| 4/0 | 0.3237 | 0.3197 |
Common questions
How many 12 AWG THHN wires fit in 1/2 inch EMT?
- Nine. Half-inch EMT has 0.304 square inches of internal area, so the 40 percent limit is 0.1216 square inches. A 12 AWG THHN conductor is 0.0133 square inches, and nine of them come to 0.1197 square inches, which fits. A tenth would take it to 0.1330 and exceed the limit.
Does the ground wire count toward conduit fill?
- Yes. Every conductor in the raceway counts toward fill, including equipment grounding conductors and neutrals. A bare ground counts too, using its bare cross-sectional area rather than an insulated area.
Why is the limit for two conductors lower than for three?
- Because of the jamming risk when pulling. Two round conductors in a round conduit can wedge against each other and the conduit wall in a way that three or more cannot, so the code sets 31 percent for exactly two and 40 percent once there are three or more.