Conduit fill calculator
You supply the conduit and conductor areas; the tool only compares their ratio to the percentage limit. It is not the Canadian Electrical Code, your local amendments or a qualified person — verify the areas and the applicable fill rule against your own book, and remember pulling tension and bend count can matter as much as the percentage.
Enter the conduit's internal area and the area of each conductor going in, set how many conductors there are, and the tool reports the fill percentage and whether it clears the limit for that number of conductors. Mixed sizes? Total their areas and enter that as one figure with a count of one.
Worked example
Three conductors at 20 mm² each fill 60 mm² of a raceway whose internal area is 200 mm². That is 60 ÷ 200 = 30% fill. The limit for three or more conductors is 40%, so this passes with room to spare. Add a fourth conductor and the fill rises to 40% — right on the line.
The formula
fill % = (Σ conductor areas ÷ conduit internal area) × 100
Fill is simply the fraction of the pipe's cross-section the wires occupy. The accepted ceiling depends on how many conductors share the raceway — 53% for one, 31% for two, 40% for three or more — because more conductors need more room to be pulled without damage.
Questions
- How do I calculate conduit fill?
- Add up the cross-sectional area of every conductor going into the raceway, divide by the conduit’s internal area, and turn it into a percentage. This tool does that division once you enter the conduit’s internal area and the conductors’ areas — both of which you take from your own tables or datasheets.
- What is the fill limit?
- The accepted maximum is a percentage of the conduit’s internal area: 53% for a single conductor, 31% for two, and 40% for three or more. Those percentages are rules, not table data, so they are stated here. The conductor and conduit dimensions behind them are yours to supply.
- Where do I get the conductor and conduit areas?
- From your conductor datasheets and conduit specifications, or by computing them — a round conductor’s area is π times its radius squared. The tool holds no dimension tables; it works on the areas you enter, which keeps it valid across insulation types and code editions.
- Does the tool tell me which conduit size to use?
- No — it tells you whether the fill you describe passes the percentage limit. Choosing a trade size is your call against your own tables. Enter a candidate conduit’s internal area and see if your conductors fit under the limit; if not, try a larger internal area.
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