What is Box Fill?

Think of your electrical box like a backpack. You can only fit so many things inside before it won’t close properly… or worse, it becomes a safety hazard!

In the Canadian Electrical Code, we have to calculate how much volume is being used inside the box based on what’s going in it — wires, devices, wirenuts, etc.

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What Counts Toward Box Fill?

Here’s your packing list:

1. Conductors

• Every wire that ENTERS or EXITS the box = 1 count
• 3 ENTER, 3 EXIT = 6 conductors

• If a wire just passes through (enters and exits), it still counts = 1 count
• 3 pass-through = 3 conductors!

• Pigtails don’t count because they don’t enter or exit!
• 3 ENTER, 3 EXIT, 3 PIGTAILS = 6 conductors

Example:
6 wires enter + 3 pass through = 9 conductors total

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2. Devices

Each device counts as if it's taking up 2 conductors:

• Switch = 2 conductors
• Receptacle = 2 conductors
• Fixture stud/hickey = 1 conductor each

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3. Wire Nuts

These are ONLY counted in pairs. So:

• 2 wirenuts = 1 conductor
• 4 wirenuts = 2 conductors
• 5 wirenuts= 2 conductors

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Box Fill Example 1: #8 AWG Wires

• 3 wirenuts = 1 conductor
• 6 wires enter/exit = 6 conductors
• 3 wires pass through = 3 conductors
  Total: 10 conductors

Now go to CEC Table 23 and multiply by the volume for #8 AWG (45.1 ml):

10×45.1=451ml

That’s the minimum box volume you’ll need!

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Example 2: Mixed Wire Sizes

• 2 wirenuts for #8 = 1 conductor
• 6 conductors enter/exit = 6 conductors

Count = 7 total

Volume math:

• #8 AWG = 45.1 ml × 3 = 135.3 ml
• #12 AWG = 28.7 ml × 4 = 114.8 ml

Total = 250.1 ml
(You’d need a box with at least that much space!)

ATTENTION:

Always use the largest wire size to determine the wire nut or device volume!

GROUND WIRES DO NOT COUNT!