Where the 40% conduit fill rule comes from, and when it is not 40%
40% is not a universal limit — it is the value for three or more conductors. Chapter 9, Table 1 gives three different percentages depending on how many conductors are in the raceway, and a note raises the limit for short nipples. The one that surprises people is two conductors, which is 31% — lower than the 40% they were expecting, not higher.
The four cases
| Conductors in the raceway | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| More than 2 | 40% |
| Any number, in a nipple 24 inches or shorter | 60% |
The 60% nipple allowance comes from a note to that same table and applies to a raceway no longer than 24 inches between boxes or enclosures. It is a real allowance, not a shortcut — but check the length before you use it.
Do not compute the allowed area yourself
It is tempting to take the raceway's total internal area, multiply by 0.40, and compare. That will occasionally disagree with the code by a hair, because the printed tables carry their own rounding. Chapter 9 Table 4 already lists the allowable area for each percentage, for each trade size, for each raceway type. Use the printed column.
Same principle on the conductor side: Chapter 9 Table 5 gives the approximate area for each insulation type and size. THHN and XHHW of the same AWG are not the same area, and the difference compounds fast across a bundle.
Going the other direction: how many will fit
When you have the raceway and want a count, divide the allowable area by the conductor area — then apply Chapter 9 Note 7 rather than normal rounding. If the result lands at 0.8 of a conductor or more, you round up. So 12.83 conductors is 13, but 12.79 is 12.
The jam ratio — the failure the percentages will not catch
Three same-size conductors in a raceway can wedge against each other at a bend and lock, even when the fill percentage is comfortably legal. Fill is an area calculation; jamming is a geometry problem, and the two are unrelated.
The usual check is the ratio of raceway inside diameter to conductor outside diameter, with a 1.05 factor to account for the raceway going slightly oval at a bend:
| Jam ratio = 1.05 × raceway ID ÷ conductor OD | Interpretation |
|---|---|
| Below 2.5 | No jamming concern |
| 2.5 – 2.8 | Low risk |
| 2.8 – 3.2 | Jamming is possible — this is the band to avoid |
| Above 3.2 | No jamming concern |
Checklist before you commit to a raceway size
- 1
Count the conductors first
The count decides which percentage you are allowed, and it is not always 40%.
- 2
Check the length
24 inches or under between enclosures unlocks the 60% nipple allowance.
- 3
Use the printed area columns
Chapter 9 Table 4 for the raceway, Table 5 for the conductors. Do not recompute from percentages.
- 4
If it is exactly three of the same size, check the jam ratio
Legal fill and a jammed pull are entirely compatible states.
One more thing worth knowing, since it comes up when people size large runs: EMT does not stop at trade size 4. Article 358.20(B) allows up to trade size 6, and Chapter 9 Table 4 lists the 5 and 6 inch dimensions in both the 2023 and 2026 editions.
Related guides
- How to calculate box fill, item by itemBox fill is six separate counts added together, not one. The volume table, what counts as a conductor, why device yokes are doubled, and the three different meanings of 'largest conductor' that make this easy to get wrong.
- What actually changed in the NEC 2026A working electrician's list of the NEC 2026 changes that move numbers on the job: Table 310.16 gained rows, 250.122 letters shifted, and one renumbered article now points somewhere else entirely.
Run the numbers
AmpRule is not affiliated with or endorsed by NFPA. NEC and National Electrical Code are trademarks of the National Fire Protection Association. Always verify against the edition adopted by your AHJ.