How many wires can go in an electrical box?

CEC Rule 12-3034 and Table 22 assign each conductor size a volume allowance in millilitres. Count each insulated conductor once, each device twice, and every pair of insulated wire connectors once. Bare bond wires and cable clamps do not count. The required volume must not exceed the box volume.

Do pass-through wires count toward box fill?

Yes — a wire that passes through the box unbroken counts once. Only pigtails that both start and end inside the box are free.

📦 Box Fill

Is your junction or device box big enough? Count what's in it, get a clear fits / doesn't-fit answer.

Box

Plastic boxes have their volume stamped inside (e.g. "18.0 CU IN") — pick Custom and type it. Metal box sizes use the standard code volumes.

Wire size in the box

Most homes: 14 AWG (15 A circuits) or 12 AWG (20 A circuits). Mixed sizes coming — for now pick the size you have most of (or the largest, to be safe).

Insulated wires entering the box

wires

Count every hot and neutral that enters the box — including any that pass straight through unbroken (those count too). Don't count ground wires here (next question), and don't count pigtails that start and end inside the box.

Devices in the box

devices

Switches, receptacles/outlets, dimmers. Each one counts as two wires' worth of space.

Any ground wires in the box?

Bare bond wires are not counted under CEC Rule 12-3034.

Internal cable clamps?

Cable clamps receive no volume allowance under the Canadian counting rule.

Why box fill matters

Cram too many wires into a box and connections get stressed, insulation gets nicked, and heat builds up. The Canadian Electrical Code (Rule 12-3034 with Table 22) assigns every insulated conductor a space allowance in millilitres, and the box must have at least that much room.

The counting rules, in plain English

Canadian counting: each insulated conductor entering the box counts once; bare bond wires and cable clamps do not count. Each device counts twice. Every pair of insulated wire connectors (marrettes) counts once, using the largest conductor connected.