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Box Fill Explained (CEC Rule 12-3034)
An overfilled box is a common reason for failed inspections. Cramming too many conductors into a box also makes the wiring harder to work in and can cause overheating. Rule 12-3034 sets how much room everything inside a box takes up. Here’s how it works in practice.
Try it: Free box fill calculator
Step 1: Count the conductors (Subrule 1)
- A conductor that runs through the box without a connection counts as one.
- A conductor that enters or leaves the box and connects to a terminal or connector counts as one.
- A conductor that doesn’t leave the box isn’t counted.
- No. 18 and No. 16 AWG fixture wires in a luminaire box aren’t counted.
Step 2: Add the deductions (Subrule 2)
Some items count as extra conductors of the largest size in the box:
| Item | Counts as |
|---|---|
| Each fixture stud or hickey | 1 conductor |
| Each pair of connectors with insulating caps (wire nuts) | 1 conductor |
| Each flush-mounted device on a single strap | 2 conductors |
Step 3: Check the volume
Table 23 gives the maximum number of conductors for standard boxes, but only when every conductor is the same size. For mixed sizes, or for boxes not listed in Table 23, add up the volume each conductor needs from Table 22 and compare it to the box volume (Subrule 4):
| AWG | Space per conductor |
|---|---|
| 14 | 24.6 mL |
| 12 | 28.7 mL |
| 10 | 36.9 mL |
| 8 | 45.1 mL |
| 6 | 73.7 mL |
Deep devices: if a device has more than 2.54 cm between the mounting strap and its back, subtract 32 cm³ for every centimetre of device depth from the box’s usable volume (Subrule 3).
Ganged boxes and extension rings: use the total volume of the assembled sections (Subrule 6).
Don’t count locknuts, bushings, box connectors or clamps (Subrule 5).
Example
A 4 × 1-1/2 octagonal box (245 mL) with a fixture stud and four #14 conductors needs five conductors’ worth of space, which is 5 × 24.6 = 123 mL. That fits comfortably. Swap the wire to #6 and the same box tops out at three conductors per Table 23.
When to upsize
If the calculated fill is more than the box holds, use a larger box, such as a 2-1/8 deep or a 4-11/16 square, or reduce the number of splices. Do this planning at rough-in. At trim, your options are limited and you’ll often have to redo work.