Conduit Fill Calculator (Cables in a Conduit or Trunking)
How full a conduit is with a given number of same-size cables, the fill the code allows for that number of cables, and the most cables that fit.
Results
What this tool does
A conduit is not meant to be packed: cables need room to be pulled in and to shed heat. The US National Electrical Code (Chapter 9, Table 1) allows 53 % of the cross-section for a single cable, 31 % for two and 40 % for three or more — the figure drops for two because they tend to wedge. Fill is the ratio of areas, so it goes with the square of the diameters: a cable only 10 % wider takes 21 % more room. Other countries and standards set their own limits, and bundled cables may need derating, so check the rules that apply to your installation.
Formula
fill = n × (cable diameter ÷ conduit diameter)² × 100 · limits (NEC Chapter 9): 1 cable 53 %, 2 cables 31 %, 3 or more 40 %
Variables
| Symbol | Meaning | Unit |
|---|---|---|
dd | Inside diameter of the conduit | mm |
in2 | Number of cables | — |
cc | Outside diameter of each cable (insulation included) | mm |
FL | Share of the conduit area filled | % |
AL | Fill allowed for that many cables | % |
MX | Most cables of that size that fit | — |
OK | Result | — |
Worked example
- Inside diameter of the conduit25 mm
- Number of cables4
- Outside diameter of each cable (insulation included)6.5 mm
- Share of the conduit area filled27.0 %
- Fill allowed for that many cables40 %
- Most cables of that size that fit5
- ResultYes — within the fill limit
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.