Conduit fill calculator
Enter the dimensions from your own conductor data sheet and conduit. The code limits are applied automatically, including the 31% case for exactly two conductors.
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These are the figures we verified on 2026-08-01. Change them to work with a historic period, or with a new rate that has been announced but which we have not published yet.
Defaults to the code limit for the conductor count you entered — 53% for one, 31% for two, 40% for three or more. Override only if a provincial amendment or a specific rule applies.
What is the maximum conduit fill in Canada?
Under the Canadian Electrical Code the limit depends on how many conductors are in the raceway: 53% for a single conductor, 31% for exactly two, and 40% for three or more. The 31% figure is the one people miss — it is lower than the three-conductor allowance, because two round conductors in a round conduit leave an awkward gap and carry a real jamming risk during the pull. Compare the total cross-sectional area of the conductors against the conduit's internal area.
The 31% rule is the one that catches people
Three fill percentages apply, and their order is counter-intuitive: 53% for one conductor, 31% for exactly two, and 40% for three or more.
So going from two conductors to three raises the permitted fill. Almost everyone expects the limit to fall monotonically as conductors are added, and it does not.
The reason is geometric rather than thermal. Two round conductors inside a round conduit can wedge against each other and against the wall during a pull — the classic jamming case — and the lower allowance is there to keep that from happening.
The calculator applies the correct percentage for the count you enter, and flags the two-conductor case explicitly.
Why the dimensions are not built in
Most conduit fill calculators embed conductor and conduit dimension tables. This one asks you for them, and that is deliberate.
Conductor outside diameter varies with insulation type — RW90 XLPE, T90 nylon, TECK and armoured cable are all different at the same gauge, and manufacturers differ within a type. Conduit internal diameter varies between EMT, rigid, and PVC schedules. The CSA tables are also copyright material.
Using a generic number where the actual conductor is fatter is how a pull that passes on paper fails in the trench.
Take the outside diameter from the conductor's own data sheet and the internal diameter from the conduit specification. Those are the numbers an inspector will hold you to.
Fill is only one of several limits
Passing the fill calculation does not mean the run is compliant. Several other requirements apply independently and are not checked here.
Ampacity derating is the big one: once you have more than three current-carrying conductors in a raceway, their permitted ampacity drops, and it keeps dropping as you add more. A pull can be well within fill limits and still leave conductors undersized for their load.
Bend count between pull points is separately limited, and exceeding it makes a pull physically difficult even where it is legal.
Treat this as one check among several, not as a compliance verdict.
Provincial amendments exist
The Canadian Electrical Code is published by CSA, but it has no legal force of its own. Each province and territory adopts it, usually with amendments, and it is the adopted version that applies to your work.
Ontario, Quebec, Alberta and British Columbia all publish their own amendments, and adoption of a new code edition happens at different times in different jurisdictions. A rule that changed in the national code may not yet apply where you are working, or may apply in modified form.
Check your provincial authority's current adopted edition rather than assuming the national code applies unmodified.
The calculation
Compare conductor area against conduit area, then check it against the code percentage.
conduit area = π × (internal diameter)² ÷ 4conductor area = π × (outside diameter)² ÷ 4total fill area = Σ (count × conductor area)fill % = total fill area ÷ conduit area × 100code limit, CEC Rule 12-910 and Table 8:1 conductor 53%2 conductors 31%3 or more 40%compliant when fill % ≤ code limit
Note that the areas are computed from outside diameters — the whole conductor including insulation, not the copper. A conductor's gauge describes the conductor; the fill calculation cares about the overall dimension.
Where conductors of different sizes share a raceway, sum each group's area separately and add them, which is what the two conductor rows above let you do.
Worked example: four conductors in 27 mm conduit
Four RW90 conductors of 9.4 mm outside diameter in a conduit with a 26.6 mm internal diameter.
- Conduit area = π × 26.6² ÷ 4555.7 mm²
- One conductor = π × 9.4² ÷ 469.4 mm²
- Four conductors = 4 × 69.4277.6 mm²
- Code limit for 3 or more40%
- Permitted area = 555.7 × 0.40222.3 mm²
- Fill used = 277.6 ÷ 555.750.0%
Over the limit at 50% against a 40% allowance — this pull needs a larger conduit.
Reduce to three conductors and the fill drops to 37.5%, which passes. Note that going the other way, from three conductors to two, would tighten the limit to 31% rather than loosening it.
What this calculator does not check
- Ampacity derating for more than three current-carrying conductors
- Ambient temperature correction factors
- Number of bends permitted between pull points
- Bonding and grounding conductor sizing
- Provincial or territorial amendments to the national code
- Whether the raceway type is permitted in that location at all
Terms on this page
- CEC
- Canadian Electrical Code, published by CSA as C22.1. It has no legal force until a province or territory adopts it, usually with amendments.
- Fill
- The proportion of a raceway's internal cross-sectional area taken up by conductors, including their insulation.
- Jamming
- The failure mode where conductors wedge against each other and the conduit wall during a pull. The reason the two-conductor limit is lower than the three-conductor one.
- Derating
- The reduction in permitted conductor ampacity when several current-carrying conductors share a raceway and cannot shed heat independently.
- AHJ
- Authority having jurisdiction — the provincial or municipal body that adopts the code, issues permits and inspects the work.
Common questions
What is the maximum conduit fill in Canada?
53% for a single conductor, 31% for exactly two, and 40% for three or more, under the Canadian Electrical Code. The two-conductor figure being lower than the three-conductor one surprises most people; it exists because two round conductors in a round conduit can jam during a pull.
Why is the limit for two conductors lower than for three?
It is a geometry and pulling problem rather than a heat problem. Two round conductors inside a round conduit can wedge against one another and the wall — the classic jamming case — so the code allows less fill to keep the pull practical.
Do I use the conductor gauge or the outside diameter?
The outside diameter, including insulation. The gauge describes the metal; the fill calculation cares about the full physical dimension, which varies with insulation type between RW90, T90 and others at the same gauge.
Does passing the fill check mean my installation is compliant?
No. Fill is one requirement among several. Ampacity derating for multiple current-carrying conductors, temperature correction, bend counts between pull points and bonding conductor sizing all apply separately and are not checked here.
Does the national code apply directly in my province?
Not on its own. Each province and territory adopts the CEC with its own amendments and on its own timetable, and it is the adopted version that has legal force. Check your provincial authority's current edition.
Where these numbers come from
- CSA Group — C22.1 Canadian Electrical Code, Part I The code itself, including Rule 12-910 and Table 8. A paid standard, which is why the dimension tables are not reproduced here.
- Electrical Safety Authority (Ontario) — Ontario Electrical Safety Code An example of a provincial adoption with amendments, and the permit and inspection requirements that go with it.
Last verified 2026-08-01 Rates on this page are checked against the sources above at least once a year, and whenever the governing authority announces a change. Spotted something out of date? Tell us and we will fix it.
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