Maximum number of insulated conductors in a box (see Appendix B)

1) Boxes shall be of sufficient size to provide usable space for all insulated conductors contained in the box, subject to the following:

a) an insulated conductor running through a box with no connection therein shall be considered as one insulated conductor;
b) each insulated conductor entering or leaving a box and connected to a terminal or connector within the box shall be considered as one insulated conductor;
c) an insulated conductor of which no part leaves the box shall not be counted; and d) No. 18 and No. 16 AWG fixture-wires supplying a luminaire mounted on the box containing the fixture-wires shall not be counted.

Δ 2) Except as specified in Subrule 3) and subject to the details given in Subrule 1), boxes of the nominal dimensions given in Table 23 shall not contain more insulated conductors of a given size than permitted by the Table, and the number of conductors shall be reduced for each of the following conditions as applicable:

a) one insulated conductor, of the largest size in the box, for each fixture stud or hickey within the box;
b) one insulated conductor, of the largest size terminated under the conductor connector, for every pair of conductor connectors with insulating caps; and
c) two insulated conductors for each flush-mounted device on a single strap within the box.

3) Where a box contains a device having a dimension greater than 2.54 cm between the mounting strap and the back of the device, the total usable space shall be reduced by the space occupied by the device, calculated as 32 cm3 multiplied by the depth of the device in centimetres.

4) Subject to the details given in Subrules 1) and 3), boxes having nominal dimensions or volume other than those shown in Table 23, or any box containing insulated conductors of different sizes, shall have the amount of usable space per insulated conductor as specified in Table 22, but the number of insulated conductors so calculated shall be reduced for each of the conditions of Subrule 2) as applicable with the exception of Subrule 2 b), provided that such exception is based on the size of the largest insulated conductor that is included with every pair of conductor connectors.

5) The total usable space in a box considered under Table 22 shall be considered to be the internal volume of the box and shall disregard any space occupied by locknuts, bushings, box connectors, or clamps.

6) Where sectional boxes are ganged, or where plaster rings, extension rings, or raised covers are used in conjunction with boxes, ganged or otherwise, and are marked with their volume measurement, the space in the box shall be the total volume of the assembled sections.

Source: Rule 12-3034, CSA C22.1:21, Canadian Electrical Code, Part 1 (25th Edition), Safety Standard for Electrical Installations. © 2021 Canadian Standards Association. Please visit