Conduit and Raceway Fill Calculation per PEC
Overfilled conduit traps heat, damages insulation during pulling, and fails inspection. The fill rules are simple arithmetic once you have the areas.
The maximum fill percentages
| Number of conductors | Maximum fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
The 31% two-conductor case is the strictest — it guards against the conductors jamming during a pull.
Conductor areas
Use the total cross-sectional area including insulation (from PEC Chapter 10 tables). Approximate THHN areas:
| Size (mm²) | Approx. area incl. insulation (mm²) |
|---|---|
| 3.5 | ~9 |
| 5.5 | ~13 |
| 8.0 | ~17 |
| 14 | ~28 |
| 22 | ~39 |
Worked example
Four 14 mm² THHN (3 phase + 1 neutral) plus one 8 mm² ground:
3+ conductors → raceway must be ≥ 129 / 0.40 = 323 mm² internal area
25 mm (1") EMT has an internal area of about 510 mm² → fill = 129 / 510 = 25%. A 20 mm EMT (~310 mm²) would be over 40% and is not allowed. Select 25 mm EMT.
Also check
- Bend count: no more than 360° total between pull points.
- Derating: more than three current-carrying conductors triggers an ampacity adjustment factor — separate from the fill rule.
EngEst Pro’s EE Tools conduit-fill calculator applies the 40% rule (and 53/31% for one or two conductors) automatically — enter the conductor schedule and it returns the minimum conduit size and the resulting fill percentage for the BOM.
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