CEC Box Fill Calculator
Size your junction boxes right the first time. Built on CEC Rule 12-3034 and Tables 22-23—no overfilled boxes, no failed inspections.
Switch to NEC1Conductors Entering Box
Count each conductor per CEC 12-3034(1). Pigtails originating inside the box do NOT count.
Current-Carrying Conductors
Equipment Grounding Conductors (EGC)
All EGCs count as ONE allowance based on the largest EGC
2Devices (12-3034(2)(c))
Each flush-mounted device counts as 2× the largest conductor connected to it.
3Other Fill Items
4Select Box (Table 23)
Calculation breakdown
How We Calculate Box Fill (CEC)
This calculator uses conductor volumes from CEC Table 22 and box capacities from Table 23. Fill rules come from Rule 12-3034.
Per Rule 12-3034(2), reduce conductor count for:
- (2)(a) Fixture stud/hickey — 1× largest conductor
- (2)(b) Conductor connectors — 1× largest conductor per pair
- (2)(c) Flush-mounted device — 2× largest conductor connected
CEC vs NEC: Key Differences
Metric Units
CEC uses millilitres (mL) / cubic centimetres (cm³) for volumes. NEC uses cubic inches. The conversion is approximately 1 cu in = 16.4 mL. This calculator handles the math automatically.
Table References
CEC uses Table 22 (conductor volumes) and Table 23 (box capacities). NEC uses Table 314.16(A) and (B). The values are equivalent when converted between metric and imperial.
Deep Device Rule
CEC 12-3034(3) has a specific calculation for devices deeper than 2.54 cm: reduce usable space by 32 cm³ × device depth. NEC has a similar concept but uses different thresholds.
CEC Box Fill: What Canadian Electricians Should Know
What's the difference between CEC and NEC box fill rules?
The CEC (Rule 12-3034) and NEC (314.16) box fill rules are very similar in concept but have key differences:
- Units: CEC uses millilitres (mL) / cm³, NEC uses cubic inches
- Tables: CEC uses Table 22 (conductor volumes) and Table 23 (box capacities), NEC uses Table 314.16(A) and (B)
- Rules: Fill rules for devices, clamps, and grounding are nearly identical
The underlying physics is the same—both codes aim to prevent overheating and provide working space for connections.
How do conductor connectors affect box fill under CEC?
Per CEC 12-3034(2)(b), conductor connectors with insulating caps require one conductor volume allowance for every pair of connectors.
This is based on the largest conductor size terminated under the connector. For example, if you're using wire nuts to connect 14 AWG and 12 AWG wires, you'd use the 12 AWG volume (28.7 mL) for each pair of connectors.
This accounts for the physical space the connectors occupy in the box.
What conductors don't count in CEC box fill calculations?
Per CEC 12-3034(1), several conductors don't count toward box fill:
- Pass-through conductors (12-3034(1)(a)) — count as ONE conductor, not two
- Pigtails (12-3034(1)(c)) — conductors originating inside the box that don't leave don't count at all
- Fixture wires (12-3034(1)(d)) — No. 18 and 16 AWG wires supplying a luminaire on the box don't count
This is why pigtails are often the solution when a box is getting tight—they don't add to the fill calculation.
How does CEC handle deep devices in boxes?
CEC 12-3034(3) has a special rule for deep devices—those with more than 2.54 cm (1 inch) between the mounting strap and the back of the device.
For these devices, the total usable space is reduced by:
This accounts for GFCI receptacles, smart switches, and other devices that extend further back into the box than standard devices.
What units does CEC use for box fill?
CEC uses millilitres (mL) which equals cubic centimetres (cm³).
Table 22 conductor volumes:
- 14 AWG = 24.6 mL
- 12 AWG = 28.7 mL
- 10 AWG = 36.9 mL
- 8 AWG = 45.1 mL
- 6 AWG = 73.7 mL
Conversion: 1 cubic inch ≈ 16.4 mL
This calculator shows both units to help electricians familiar with imperial measurements.
How do equipment grounding conductors count in CEC box fill?
CEC treats equipment grounding conductors (EGCs) similarly to NEC—all EGCs in a box count as a single conductor volume allowance based on the largest EGC present.
So if you have four 14 AWG grounds and two 12 AWG grounds, you only add 28.7 mL (the 12 AWG volume) to your calculation, not six separate conductors.
Common mistake: Counting each EGC separately. Always use the largest EGC size for the single allowance.
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