What is Conduit Fill?

Conduit fill is like packing a suitcase. You’ve got to fit your clothes (wires) into your suitcase (conduit), but you can’t overstuff it — or the trip (a.k.a. your wiring job) won’t go smoothly.

The Canadian Electrical Code (CEC) says:

  • Only 40% of the conduit’s inside area can be filled by conductors (unless special rules apply).

To check if everything fits, you’ll use:

  • Table 6: Area of the conductors
  • Table 8: Max fill percentage (usually 40%)
  • Table 9: Conduit types and their total space
  • Sometimes Table D5

Table 6 Tips

  • 💡 Pay attention to the VOLTAGE and whether or not the conductor is jacketed or unjacketed!
  • Assume stranded conductor unless the question says otherwise
  • Solid conductors are at the bottom of the table!

Table 8 Tips

  • ⚠️ Most conductors are NOT lead-sheathed, but that pesky C of Q wants to trip you up. If you see a cable you do not recognize, head over to Table D1 to see what the conductor is made out of.
  • Don’t assume 40% fill — ALWAYS double-check!

Table 9 Tips

  • Make sure you pick the right table based on the fill percentage!

SAME-SIZE CONDUCTORS

Example: Stuffing 3 identical wires into a rigid metal conduit

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3 × #6 AWG RW90XLPE unjacketed

  • Table 6A: each #6 = 38 mm²
  • Total: 3 × 38 = 114 mm²
  • Table 8 (max fill): 40%
  • Table 9G: Find a conduit with at least 114 mm² capacity at 40% fill

Result: Choose 21 Trade Size (TS) rigid metal conduit

Now Try EIGHT Big Wires

8 × #3 AWG TW in EMT

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  • Table 6J: each #3 = 73 mm²
  • Total: 8 × 73 = 584 mm²
  • 40% fill = Table 9G → 53 Trade Size

💡 That’s one chunky conduit!

MIXED-SIZE CONDUCTORS

When you mix wire sizes, just add up their individual areas.

Example 1: EMT with Two Sizes

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  • Six #12 AWG RW90XLPE unjacketed = 69.5 mm²
  • Four #8 AWG T90 nylon = 94.7 mm²
  • Total: 69.5 + 94.7 = 164.2 mm²

Then go to Table 9G, check what size EMT conduit allows 164.2 mm² at 40% fill.

Example 2: PVC with Three Sizes

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  • Four #10 AWG TW75 solid = 53.1 mm²
  • Three #6 AWG T90 nylon = 98.1 mm²
  • Two #2 AWG RW90 jacketed = 148 mm²
  • Total = 299.2 mm²

Head to Table 9G, look for Rigid PVC conduit that allows at least 299.2 mm² at 40% fill. Boom. You’ve cracked it.

Fun Fact!

💡 The C of Q Exam will often mix up the Trade Size, Metric, and Imperial designators on purpose!

There is a table in the beginning of the CEC (even before the Table of Contents) which shows you Trade Size to inch conversion.

You will absolutely need to know about that!

⚠️ Also, just to be tricky, if the question doesn’t mention a specific TYPE of cable… Go to Table D5!