What's Conduit Fill Anyway?

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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
  

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Table 6

• 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 is at the bottom of the table!
  

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Table 8

• Most conductors are NOT lead-sheathed, but that pesky C of Q wants to trip you up. So 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!
  

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Table 9

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

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Same-Size Conductors

Let’s say you’re stuffing 3 identical wires into a rigid metal conduit:

Example:

• 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
  

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Now Try Eight Big Wires

• 8 × #3 AWG TW in EMT
  
• Table 6J: each #3 = 73 mm²
  
• Total: 8 × 73 = 584 mm²
  
• 40% fill = Table 9G → 53 Trade Size
  
• That's one chunky conduit!
  

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Mixed-Size Conductors

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

Example 1: EMT with Two Sizes

• 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 3 Sizes

• 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!