Field Tool
Duct Fill
Check percent fill, remaining capacity and jam risk for cables in a duct.
Fill 13.8 percent. Within the 31% NEC reference fill.
Fill
13.8%
NEC Chapter 9 Table 1 percentages (reference), two cables: 31%.
- Duct inside diameter
- 1.9432 in
- Duct area
- 2.966 in²
- Cable area, 2 cables
- 0.408 in²
- Room for 6 more 0.4 in cables within the NEC reference fill, by cross-section area only; jam ratio and pulling tension are not checked.
Show the Math
area = π/4 · d² fill = Σ cable area ÷ duct area jam ratio = duct ID ÷ cable OD (three cables of equal OD)
d is a diameter in inches. "Room for more" is the largest number of extra cables of the last listed OD that keeps the final fill within its limit; the limit is re-read for the new cable count. With one cable the tool also reports the OD as a percentage of the ID.
Status
- Fill at or under 85% of the limit: good.
- Above 85% of the limit, up to the limit: warning.
- Over the limit: critical. A cable OD equal to or larger than the duct ID is an input error.
- The 85% warning band is a FAHMY planning threshold, not a standard; the owner's design rules govern.
Limits and Where They Come From
- NEC Chapter 9 Table 1 percentages (reference), the default basis: 53% for one cable, 31% for two, 40% for three or more.
- NEC does not set fill limits for fiber-only raceways (NEC Article 770) or for utility-controlled OSP (NEC Article 90). Use the owner's limit where one is set.
- Many owners set their own fill rules for OSP duct and innerduct: maximum fill. Choose "Owner spec" and enter it.
- Jam band: three equal cables can wedge in a bend when the ID ÷ OD ratio is between 2.8 and 3.2.
Duct Presets
- HDPE conduit inside diameter at minimum wall: ID = OD × (1 − 2 ÷ SDR), where OD is the standard outside diameter of SDR (outside-diameter controlled) HDPE conduit: 1 in 1.315 in, 1¼ in 1.660 in, 1½ in 1.900 in, 2 in 2.375 in and 4 in 4.500 in. Example: 2 in SDR 11 → 2.375 × (1 − 2/11) = 1.9432 in.
- PVC Schedule 40 inside diameters from pipe dimensions at minimum wall: 2 in 2.067 in and 4 in 4.026 in. NEC Chapter 9 Table 4 lists smaller conduit IDs for these trade sizes; the SME is confirming which values apply.
- Every preset is a minimum-wall ID, the largest possible, so the fill shown is the lowest possible. Actual inside diameters vary by manufacturer. Use the duct spec sheet or choose "Custom inside diameter".
Source
FAHMY tools give planning estimates. The owner's specifications, the authority having jurisdiction and the manufacturer's data govern every job. Values marked "Planning assumption", "Planning default", "Typical maximum", "Rule of thumb" or "Example" are editable starting points, not requirements; "Per owner spec" fields take the owner's value.