Conduit Fill Calculator
Size the conduit, before the pull.
Pick the right trade-size conduit by NEC Chapter 9 fill limits: 53% for a single wire, 31% for two, 40% for three or more. Visual fill gauge for EMT, RMC and PVC.
- 1 wire max
- 53%
- 2 wires
- 31%
- 3+ wires
- 40%
- Chapter
- NEC 9
Selection method
Three steps, one conduit size
Add up the cross-sectional area of all conductors, apply the NEC fill percentage, then pick the smallest trade size whose internal area clears the total.
Use NEC Table 5 for insulated wire areas by AWG and insulation type (THHN, XHHW, etc.).
F = 0.53 for 1 wire, 0.31 for 2 wires, 0.40 for 3+ wires. Gives required internal conduit area.
Look up NEC Table 4 for internal area per conduit trade size. Round up to next standard size.
Worked example
Six 12 AWG THHN → which EMT size?
Scenario: pulling 6 × 12 AWG THHN (4 phases, 1 neutral, 1 ground) through EMT — 3+ wires, 40% fill limit.
- Step 1 · Wire area: 12 AWG THHN = 0.0133 in² each
- Step 1 · Total: 6 × 0.0133 = 0.0798 in²
- Step 2 · Fill limit: 0.0798 / 0.40 = 0.1995 in² required
- Step 3 · EMT 3/4": 0.213 in² available → passes ✓
- Step 3 · EMT 1/2": 0.122 in² → fails
- Verdict: Use 3/4-inch EMT for 6 × 12 AWG THHN
NEC Reference
Conduit internal areas (NEC Table 4)
| NEC Fill percentages | |
| 1 wire | 53% of conduit cross-section |
|---|---|
| 2 wires | 31% of conduit cross-section |
| 3 or more wires | 40% of conduit cross-section |
| EMT · Internal area (in²) | |
| 1/2" | 0.304 · 40% = 0.122 |
| 3/4" | 0.533 · 40% = 0.213 |
| 1" | 0.864 · 40% = 0.346 |
| 1-1/4" | 1.496 · 40% = 0.598 |
| 1-1/2" | 2.036 · 40% = 0.814 |
| 2" | 3.356 · 40% = 1.342 |
| Rigid / IMC | |
| RMC 1/2" | 0.314 · 40% = 0.125 |
| RMC 3/4" | 0.549 · 40% = 0.220 |
| RMC 1" | 0.887 · 40% = 0.355 |
| PVC Sch 40 | |
| PVC 1/2" | 0.285 · 40% = 0.114 |
| PVC 3/4" | 0.508 · 40% = 0.203 |
| PVC 1" | 0.832 · 40% = 0.333 |
| THHN wire area (Table 5) | |
| 14 AWG | 0.0097 in² per conductor |
| 12 AWG | 0.0133 in² per conductor |
| 10 AWG | 0.0211 in² per conductor |
| 8 AWG | 0.0366 in² per conductor |
| 6 AWG | 0.0507 in² per conductor |
Frequently Asked Questions
Conduit fill FAQ
01What’s the difference between the three fill percentages?
NEC allows a higher fill with one wire because it can’t jam on itself in a pull. Two wires tend to “bind” against each other — the limit drops to 31%. Three-plus wires find random packing, so 40% is safe.
02Does this apply to cables (like MC or NM) too?
The fill percentages are the same, but use the actual outer diameter of the cable to compute area, not individual conductors. NEC 300.17 also requires 40% for cables in conduit.
03Does temperature derate affect conduit sizing?
Yes — when more than 3 current-carrying conductors share a conduit, NEC 310.15(B)(3)(a) requires ampacity derate; the conduit still sizes to 40% fill, but the wires may need to be upsized first.
04Is this a substitute for a licensed electrician?
No — use this for planning, estimating, and prototype work only. Final installation sizing should be reviewed by a licensed electrician against local code amendments.
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