How CEC Rule 12-3034 works
Rule 12-3034 of the Canadian Electrical Code, Part I (CSA C22.1:24) requires every box to have enough usable space for all the insulated conductors in it. It gives two ways to check:
- Table 23 (conductor count). For boxes of the nominal sizes listed in Table 23 where every conductor is the same size and no device is deeper than 2.54 cm, you can compare a conductor count with the table, after the deductions below (Subrule 2).
- Table 22 (volume). For any other box, any box with conductors of different sizes, or any box with a deep device, you add up the usable space each conductor needs from Table 22 and compare it with the box volume (Subrule 4).
The volume method works for every box, which is why this calculator uses it. Most boxes sold in Canada are marked with their volume in mL or cm³ (the two are the same unit). Only the space for conductors counts as filled, and the usable space is the box's internal volume (Subrule 5).
What counts and what doesn't
| Item in the box | Counts as | Rule |
|---|---|---|
| Insulated conductor entering or leaving and connected to a terminal or connector | 1 conductor of its size | 12-3034(1)(b) |
| Insulated conductor passing through with no connection | 1 conductor of its size | 12-3034(1)(a) |
| Insulated conductor that never leaves the box (pigtail) | 0 | 12-3034(1)(c) |
| No. 18 or 16 fixture wires of a luminaire mounted on the box | 0 | 12-3034(1)(d) |
| Bare bonding conductors | 0 (only insulated conductors count) | 12-3034(1) |
| Each fixture stud or hickey | 1 conductor, largest size in the box | 12-3034(2)(a) |
| Every pair of wire connectors with insulating caps (marrettes) | 1 conductor, largest size under the connectors | 12-3034(2)(b) |
| Each flush-mounted device on a single strap | 2 conductors | 12-3034(2)(c) |
| Device deeper than 2.54 cm, strap to back | 32 cm³ × depth in cm | 12-3034(3) |
| Locknuts, bushings, box connectors, internal clamps | 0 | 12-3034(5) |
| Marked plaster ring, extension ring or raised cover | Adds its marked volume to the box | 12-3034(6) |
An insulated (green) bonding conductor is still an insulated conductor, and the rule text has no exception for it, so count it unless your inspector tells you otherwise.
Formula
Required space = Σ (conductors × Table 22 space) + deductions for studs, connector pairs and devices ≤ box volume + marked ring volume
Usable space per conductor (CEC Table 22)
| Conductor size | mL (cm³) | in³ equivalent |
|---|---|---|
| 14 AWG | 24.6 | 1.50 |
| 12 AWG | 28.7 | 1.75 |
| 10 AWG | 36.9 | 2.25 |
| 8 AWG | 45.1 | 2.75 |
| 6 AWG | 73.7 | 4.50 |
These are smaller than the NEC Table 314.16(B) allowances (2.00 in³, or 32.8 mL, for 14 AWG), but the CEC adds space for marrettes and the NEC adds space for grounds and clamps, so the two codes can give different answers for the same box.
Worked examples
1. Switch loop in a 3 × 2 × 2-1/2 device box
A single-pole switch fed by one 14/2 NMD90 cable with a second 14/2 to the light: four insulated 14 AWG conductors, the bare bonds, one flush switch and three marrettes (bonds and neutrals). The box is 204 mL.
Conductors: 4 × 24.6 = 98.4 mL
Switch: 2 × 24.6 = 49.2 mL
Marrettes: 3 → 1 pair → 1 × 24.6 = 24.6 mL
Total = 172.2 mL ≤ 204 mL ✓
The bare bonding conductors add nothing. In conductor terms that is 7 against the 8 the box holds.
2. Replacing the switch with a smart switch
Same box, but the new switch measures 3.7 cm from strap to back. Subrule 3 applies, so the switch takes its own volume instead of two conductors:
Device: 32 × 3.7 = 118.4 mL
Total = 98.4 + 118.4 + 24.6 = 241.4 mL > 204 mL ✗
The box is overfilled. A 3 × 2 × 3 device box at 245 mL passes, at 99%. This is the scenario the Régie du bâtiment du Québec uses in its published interpretation of Rule 12-3034 (it rounds the factor to 32.3 cm³ per cm and gets 243 mL, with the same result).
3. Kitchen counter receptacle with three 12/2 cables
Feed in, two runs out: six insulated 12 AWG conductors, one receptacle, and three marrettes (hots with a pigtail, neutrals with a pigtail, bonds). The pigtails themselves never leave the box, so they don't count.
Conductors: 6 × 28.7 = 172.2 mL
Receptacle: 2 × 28.7 = 57.4 mL
Marrettes: 3 → 1 pair → 1 × 28.7 = 28.7 mL
Total = 258.3 mL
That doesn't fit a 245 mL device box. Use a box marked 258.3 mL or more, or a 4 in square box with a marked device ring so the combined volume is at least that. Add a fourth marrette and the connectors become two pairs, pushing the total to 287.0 mL, so plan the splices before you pick the box.
4. The same switch box under the NEC
Example 1 under NEC 314.16: four 14 AWG conductors, the switch (2), all grounds (1) and internal clamps (1) make 8 allowances × 2.00 in³ = 16.0 in³, or about 262 mL. The CEC count is 172.2 mL because bare bonds and clamps take no space while marrettes do. Don't carry a US box fill result over to a Canadian job, or the other way round. The NEC box fill calculator handles US work.
Maximum conductors by box volume
Box volume divided by the Table 22 space, rounded down, for conductors of one size with nothing else in the box. The two device box volumes are the nominal values used in the RBQ interpretation. The others are common marked volumes, not code values. Take off two conductors for each flush device, one for each pair of marrettes and one for each stud or hickey.
| Box | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|---|
| 3 × 2 × 2-1/2 in device box (204 mL) | 8 | 7 | 5 | 4 | 2 |
| 3 × 2 × 3 in device box (245 mL) | 9 | 8 | 6 | 5 | 3 |
| Marked 300 mL | 12 | 10 | 8 | 6 | 4 |
| Marked 350 mL | 14 | 12 | 9 | 7 | 4 |
| Marked 400 mL | 16 | 13 | 10 | 8 | 5 |
| Marked 500 mL | 20 | 17 | 13 | 11 | 6 |
CEC vs NEC box fill at a glance
| Item | CEC Rule 12-3034 | NEC 314.16 |
|---|---|---|
| 14 AWG allowance | 24.6 mL | 2.00 in³ (32.8 mL) |
| Bare grounds / bonds | Not counted | 1 allowance for all |
| Wire connectors | 1 per pair | Not counted |
| Internal cable clamps | Not counted | 1 allowance |
| Device on a strap | 2 conductors | 2 allowances |
| Deep device | 32 cm³ per cm of depth if over 2.54 cm | No depth rule; 2 allowances per gang if wider than a 2 in device box |
| Loops 300 mm or longer | No separate rule | 2 allowances |
Tips from the field
- Measure deep devices. Smart switches, dimmers, timers and GFCI or AFCI receptacles are often deeper than 2.54 cm. Check the depth on the spec sheet before reusing an old box.
- Reduce marrettes. Each pair costs a conductor's worth of space. Combining splices where the connector is rated for it can free up room.
- Use marked rings. A raised cover or extension ring with a marked volume adds to the box under Subrule 6, often the easiest fix on a 4 in square box.
- Sizing the circuit first? Use the Canadian wire size calculator, then check long runs with the CEC voltage drop calculator and raceways with the CEC conduit fill calculator.
What this calculator does not cover
It does not reproduce the Table 23 conductor counts, which you can use instead for listed box sizes when all conductors are the same size. It does not size pull boxes or junction boxes for 4 AWG and larger conductors, and it uses the largest conductor in the box for every deduction, which can overstate the space needed by a little. Check provincial amendments, such as Quebec's Chapter V, and your inspector's requirements.