Picking a box for a job comes down to four things: what the box is made of, how deep it is, how it mounts, and how many conductors and devices it has to hold. Get those settled and the box type usually picks itself. The rest is matching the box to the location, indoors or out, dry or wet.
Pick the box from the job, not the shelf
Start with the cable or conduit you are running. Armoured cable and EMT conduit point you toward metal. Non-metallic sheathed cable and retrofit work often point toward plastic. Then look at depth. A shallow box in a thin wall leaves no room for conductors, and a deep box behind a tile or siding profile may not sit flush.
Mounting comes next. New work, old work, stud mount, ceiling mount, surface mount. Each has a box or bracket made for it, and forcing the wrong one costs time on the rough-in.
Last is box fill, which is covered below.
Metal boxes: 4-inch square and device boxes
Metal 4-inch square boxes and metal device boxes, the switch and receptacle boxes, are the usual pick with armoured cable and EMT conduit. They are also used anywhere a grounded metal enclosure is wanted, and where the box has to take some abuse on a commercial job.
A device box holds one device. A 4-inch square box gives you more room and takes a cover or ring sized for one or more devices. See the metal boxes collection for shapes and depths.
Big metal boxes matter when a run has a lot of conductors coming together. If you are landing several cables in one spot, a larger junction box gives you the volume to do it legally. The metal junction boxes collection is the place to look.
What size junction box do I need?
Box size comes down to volume. The box has to hold the conductors, the devices, and any clamps inside it, and the Canadian Electrical Code sets out how that is counted. Your electrician or inspector will use the current Canadian Electrical Code tables to do the math. The volume is marked on the box, so read the stamp or the label before you buy.
Two things trip people up. First, a box can be big on the outside and short on volume inside once a device and a clamp are in it. Second, a splice with several cables needs more room than a single cable passing through. If the count is close, go up a size. It is cheaper than a redo.
Do I need a junction box?
If you are joining conductors, yes. Splices and terminations belong inside an accessible enclosure, not taped up in a wall cavity or buried behind drywall. That applies to a small extension as much as to a full pull.
The box has to stay reachable. If the only way to get at it later is to cut open a finished wall, the splice is in the wrong place. Ask your inspector before you close anything in.
For plastic options on the same jobs, see the plastic boxes collection.
Wrap-around electrical boxes
A wrap-around box is shaped to fit over a siding or masonry profile so an outdoor fixture or receptacle sits flat and weathers properly. The profile is the whole point. Lap siding, shakes and brick all need different shapes, and a box made for the wrong one leaves a gap you will be sealing for years.
Check the product page for the exact profile before you order. Take a scrap of the siding with you if you are unsure.
Arlington box styles
Arlington-style boxes are a recessed-mounting family. The box sets back and the device or fixture sits in a snap-in or supported frame, which gives a clean finish and room for the wiring behind the surface. Styles vary a lot by application, so read the product listing for the exact style and what it mounts to.
Weather resistant boxes and covers
Outdoors, the box, cover and gasket all have to suit the location. A wet location needs a cover that keeps water out, and if the device stays in use, you need an in-use cover that lets the cord stay plugged in with the lid closed.
The gasket is part of the assembly. A missing or pinched gasket turns a weather resistant box into a leak. Confirm the cover rating for the location with your inspector. More on covers and enclosures is on the boxes and enclosures page.
Low voltage junction boxes and brackets
Doorbell, data, thermostat and other low voltage wiring runs on its own boxes and brackets, kept separate from line voltage boxes. Use the box or bracket made for the job. A low voltage ring or bracket mounts to the stud and holds the cable and the device plate, and it is a different part from a line voltage box.
Do not mix the two in one enclosure. Keep the separation and check the rules with your inspector. Browse the junction boxes collection for the low voltage options.
Box by job
| Job | Usual box type | What to check |
|---|---|---|
| Switch or receptacle in a wall | Metal or plastic device box | Depth for the device and conductors, mounting for new or old work |
| Ceiling fixture | Ceiling box, or 4-inch square with a fixture ring | Fixture support, volume for conductors |
| Outdoor light on siding | Wrap-around box matched to the siding profile | Profile fit, gasket, cover |
| Wet location receptacle | Weather resistant box with in-use cover | Cover rating, gasket, whether the device stays plugged in |
| Splice or junction | Metal or plastic junction box, sized by fill | Volume against conductor count, accessibility |
| Low voltage | Low voltage ring, bracket or box | Separation from line voltage, plate fit |
FAQ
What size junction box do I need for a splice? Count the conductors, devices and clamps, then match that to the volume marked on the box. Your electrician or inspector will apply the current Canadian Electrical Code tables.
Do I need a junction box if I am just extending a wire? Yes. A splice belongs in an accessible enclosure, even on a short extension. Confirm the setup with your inspector.
Can I use a metal box outdoors? Only with a cover and gasket rated for the location, and an in-use cover if the device stays plugged in. Confirm the setup with your inspector.
Is an Arlington box always metal? No. Arlington-style is a recessed mounting style, and it comes in different materials. Read the product listing for the exact style and material.
Need a hand matching boxes to a job? Send us a quote request with the details.
Sources
Check box fill and location rules against the current Canadian Electrical Code and with your inspector. Technical Safety BC: https://www.technicalsafetybc.ca/technologies/electrical