The 40% Conduit Fill Rule Explained (3 or More Conductors)
The fill percentages look arbitrary until you know what each one is protecting against. Two conductors, not three, is the tightest limit — and that surprises people.
The three limits
| Conductors | Max fill | Governing concern |
|---|---|---|
| 1 | 53% | Pulling friction only |
| 2 | 31% | Jamming during the pull |
| 3 or more | 40% | Heat dissipation + pulling |
Why two conductors is strictest
When exactly two round conductors are pulled together, they can wedge against the conduit wall at a bend if the conductor-to-conduit diameter ratio falls in a narrow band (the “jam ratio”, roughly 2.8–3.2). The 31% limit keeps the geometry out of that band. With three or more, the conductors bundle in the centre and cannot jam the same way.
Applying it
Conductor areas come from the PEC Chapter 10 dimension tables; conduit internal areas from the raceway tables (EMT, IMC, RMC, PVC each differ slightly for the same trade size).
Worked example
Three 14 mm² THHN (~28 mm² each) = 84 mm². 3+ conductors → 40% limit.
20 mm EMT internal area ≈ 310 mm² → fill = 84 / 310 = 27% ✓
Don’t confuse fill with derating
The 40% fill rule is about physical space. A separate rule — the ampacity adjustment factor for more than three current-carrying conductors — reduces each conductor’s allowable current. Both can apply to the same raceway.
EngEst Pro’s EE Tools conduit-fill calculator picks the right limit for the conductor count, pulls areas from the code tables, and returns the minimum conduit size plus the fill percentage — and separately flags when a bundling derate applies.
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