How this works
The calculator adds up the cross-section of every cable, treating each one as a circle of its outside diameter, and compares it with the conduit's internal area for its trade size.
cable area = π ÷ 4 × diameter² × number of cables fill = total cable area ÷ conduit area
The limit depends on how many cables there are, per NEC Chapter 9, Table 1:
| Cables in the conduit | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
Two cables get the tightest limit because two round cables side by side can jam across the conduit's diameter. A jacketed multi-pair cable, such as Cat6 or 18/2, counts as one conductor with its overall diameter (Table 1, Note 9). For a flat or oval cable, use its larger dimension. Internal areas come from NEC Chapter 9, Table 4.
Use the real diameter
Cable diameter varies by manufacturer, construction and jacket rating: the Cat6 cables in the list run from 0.224 in to 0.238 in, and a plenum cable is often a different size from the riser version. The typical values are from specific datasheets, named in the list, and are a starting point. The diameter on your cable's datasheet is the one to use.
Is fill a code requirement for low-voltage cable?
Not always. NEC 800.110(B) (2017) says the Chapter 9 fill requirements do not apply to communications cables in raceway. Many specifications, inspectors and designers still hold low-voltage cable to the Table 1 limits because a conduit filled past them is hard to pull and hard to add to later. Check the edition and the specification that apply to your job.
What this does not account for
- Bends and pulling tension. A long run with several bends can be hard to pull well below the fill limit. Some guides reduce the cable count for each bend beyond two; that rule is not in the NEC or TIA-569, so it is not applied here.
- Jamming when three cables of nearly equal size sit in a conduit about three times their diameter; Table 1's informational note explains it.
- Short nipples (24 in or less), which Table 1 Note 4 allows to 60%.
- Cable tray and J-hooks, which follow different rules.
Common questions
- How many Cat6 cables fit in 3/4 in EMT?
- Five, at the 40% limit with a 0.224 in cable (Belden 2412). A fatter Cat6 fits fewer; enter its diameter.
- How many in 1 in EMT?
- Eight of the same 0.224 in Cat6.
- Why does adding a second cable fail when one fit easily?
- Because the limit drops from 53% for one cable to 31% for two. It rises to 40% for three or more.
Sources
- NFPA 70, National Electrical Code, Chapter 9, Table 1 and Notes, and Table 4. Values checked against the 2017 edition's text.
- NFPA 70, 800.110(B) (2017 edition): raceway fill for communications cables.
- Cable diameters: Belden datasheets 1583A, 2412, 3612, 3613, 10GX13, 6320UL, 5502UE, 5300FE, 1694A and 1505A; West Penn AC251822B.
Every source used on this site, and how each number is used.