Does conduit fill apply to fiber?
Only when the fiber shares the raceway with power. Under NEC 2020 and 2023, section 770.110(B) says the Chapter 3 and Chapter 9 fill requirements do not apply when optical fiber cables are in a raceway without electric light or power conductors. They do apply when nonconductive fiber cable is installed in a raceway with electric light or power conductors.
The 2026 NEC moved the optical fiber rules out of Article 770 into the new Articles 720 to 723. Check which edition your jurisdiction has adopted; many still enforce 2020 or 2023.
So in a fiber-only innerduct or conduit, the percentages are not a code limit. They are still the sensible place to start, because a conduit packed tighter than about 40% gets hard to pull through and leaves no room for the next cable. Long or bendy runs may need less fill than that; the cable's maximum pulling tension and the number of bends usually decide.
How fill is calculated
Fill is the combined cross-section of the cables divided by the inside area of the conduit.
Fill % = Σ (qty × π × (OD ÷ 2)²) ÷ (π × (ID ÷ 2)²) × 100
Worked example
A 144-count loose tube (0.61 in OD) and a 48-count loose tube (0.41 in OD) in 1-1/4" Schedule 40 PVC (1.360 in ID). Cable area is 0.292 + 0.132 = 0.424 in². Conduit area is 1.453 in². Fill is 29.2%, inside the 31% two-cable figure. In 1" PVC (1.029 in ID) the same pair would fill 51%.
Inside diameters
| Trade size | PVC Sch 40 / HDPE | EMT | 40% area, PVC (in²) |
|---|---|---|---|
| 1/2" | 0.602 | 0.622 | 0.114 |
| 3/4" | 0.804 | 0.824 | 0.203 |
| 1" | 1.029 | 1.049 | 0.333 |
| 1-1/4" | 1.360 | 1.380 | 0.581 |
| 1-1/2" | 1.590 | 1.610 | 0.794 |
| 2" | 2.047 | 2.067 | 1.316 |
| 2-1/2" | 2.445 | 2.731 | 1.878 |
| 3" | 3.042 | 3.356 | 2.907 |
| 3-1/2" | 3.521 | 3.834 | 3.895 |
| 4" | 3.998 | 4.334 | 5.022 |
NEC Table 4 lists HDPE conduit on the same row as Schedule 40 PVC. Real HDPE duct is sold by SDR (wall thickness ratio), and an SDR 11 duct's actual inside diameter can differ from the table, so use the manufacturer's ID for a precise figure on HDPE bores.
Why two cables get the lowest limit
Two round cables of similar size can wedge side by side across the conduit and jam during a pull. One cable cannot jam against itself, and three or more tend to settle into a triangle. That is the reason behind 53%, 31% and 40%.
Tips for pulling fiber in conduit
- Leave room for a pull line and a future cable. Fiber routes almost always get a second cable.
- Stay within the cable's maximum pulling tension, usually printed on the spec sheet (often 600 lbf for outside plant loose tube), and use a breakaway swivel.
- Respect the minimum bend radius, typically 20 times the cable's OD while pulling and 10 times at rest unless the manufacturer says otherwise.
- Use innerduct in large conduits so each cable can be pulled and replaced on its own.