Why two conductors get less conduit fill than three
Every apprentice questions this table the first time they read it. The answer is about pulling, not about heat.
5 min read
What is the maximum conduit fill in Canada?
Under the Canadian Electrical Code, 53% for a single conductor, 31% for exactly two, and 40% for three or more. The two-conductor figure being lower than the three-conductor one is not an error. Two round conductors inside a round conduit can wedge against each other and the wall during a pull — the classic jamming case — and the reduced allowance exists to prevent it.
The numbers do not decrease monotonically
Almost everyone expects the permitted fill to fall steadily as conductors are added. It does not.
One conductor gets 53%. Two get 31%. Three or more get 40%. Adding a third conductor to a two-conductor pull raises the permitted fill by nine percentage points.
Read as a heat problem this makes no sense — more conductors in a raceway means less ability to shed heat, not more. Read as a pulling problem it makes complete sense, which is the clue to what the table is actually for.
Fill limits are about getting the conductors in without damaging them. Heat is handled separately, by ampacity derating.
Jamming, and why two is the worst case
Jamming happens when conductors wedge in a conduit rather than sliding through it. It is most likely when the conductors and the conduit are of similar enough size that two of them can bridge the bore diagonally and lock.
With one conductor there is nothing to wedge against. With three or more, the bundle tends to arrange itself into a triangle that rolls through a bend rather than binding.
Two is the geometry that jams. Hence a lower allowance, giving more clearance for the pull.
This is also why pulling lubricant and correct bend radius matter far more on a two-conductor run than the fill number alone suggests.
Use the real conductor diameter, not the gauge
The gauge describes the metal. The fill calculation cares about the overall diameter including insulation, and that varies considerably between insulation types at the same gauge.
RW90 cross-linked polyethylene, T90 nylon, TECK and armoured cable are all different sizes for the same conductor, and manufacturers differ within a type. Using a generic figure 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.
This is why our calculator asks for the dimensions rather than embedding a table.
Passing the fill check is not compliance
Fill is one requirement among several, and it is the easiest one.
Ampacity derating is usually the binding constraint: once more than three current-carrying conductors share a raceway, permitted ampacity drops, and it keeps dropping as more are added. A pull can sit comfortably within fill limits and still leave conductors undersized for their load.
The number of bends permitted between pull points is separately limited. So are temperature correction and bonding conductor sizing.
And the code has no legal force until a province adopts it, usually with amendments. Check your own jurisdiction's current edition.