Cable Trunking: Sizes, Fittings, and How to Run It

A homeowner runs a new data cable from the router cupboard to a desk in the corner of the new extension. The cable is left loose along the skirting, gets nicked by a vacuum cleaner six months later, and the run has to be redone. A two-metre length of trunking costing a few pounds would have protected it and looked tidy. Surface cable management is one of those jobs that's cheap to do right and annoying to redo, and trunking is the material that does it.

What it is and what it's for

Cable trunking is a rectangular channel, usually PVC, with a removable lid that snaps or screws on. You fix the back of the channel to a wall, ceiling, or skirting line, lay your cables into it, then close the lid. The result is a neat enclosed run that protects the cables and hides them from view, all without cutting a single chase into your plaster.

That "without cutting a chase" part is the whole point. Chasing means cutting a groove into masonry or plaster to bury a cable, then plastering over it. It's permanent, messy, and creates dust. Trunking is the opposite approach: the cable stays on the surface, fully accessible, and you can lift the lid at any time to add or remove a cable. For anywhere you can't or don't want to chase, like a finished room, a plant cupboard, a garage, or an outbuilding, trunking is the standard answer.

The relevant standard is BS EN 50085, which covers cable trunking and ducting systems for electrical installations. It sets out the construction, classification, and, importantly, the cable fill rules that keep a trunking run safe. Any trunking you buy from a proper electrical merchant or a stockist like Screwfix or Toolstation will be made to this standard. The decorative "cable tidy" strips sold in homeware shops are not, and shouldn't be used for fixed wiring.

People mix up three things constantly, so it's worth nailing the difference early:

Capping catches people out the most. It looks like protection but it isn't. It's a sacrificial cover that stops a trowel nicking a buried cable before the wall is skimmed. It does nothing once the wall is finished. If you want genuine mechanical protection on the surface, that's trunking or conduit, not capping.

Types, sizes, and specifications

Trunking is sold by its internal width and height in millimetres, written as width x height. The size you need is driven by how many cables you're running and how thick they are. Here are the sizes you'll actually meet in UK domestic and light commercial work:

The 16x16mm and 25x16mm sizes are usually called "mini trunking". This is the stuff you run inside a cupboard, behind a desk, or up a wall to feed a single socket or a network point. It's light, cuts with a hacksaw or sharp knife, and is the most common size a homeowner will ever buy. Just don't confuse it with the decorative adhesive cable-tidy channel sold for hiding a TV's wires: that product isn't made to BS EN 50085 and isn't suitable for fixed mains wiring.

Material choice matters too:

• White PVC is the default for surface work in finished rooms. It paints over well and blends into a white wall or skirting.
• Grey PVC is the industrial/garage standard. Same job, less concerned with looking pretty.
• Steel is used where the run needs real mechanical protection (an exposed garage or workshop where something could whack it) or where fire compartmentation is required. It's heavier, costs more, and is cut with a hacksaw and deburred.
• Aluminium turns up occasionally in commercial fit-outs where weight and corrosion resistance matter.

For almost every extension job, white or grey PVC is what you want. Steel only enters the picture if your electrician specifies it for a particular run.

The fill rule, and why it matters

This is the spec that trips up DIYers. You cannot pack a trunking run full of cable. BS EN 50085 sets a maximum cable fill, and in practice electricians work to roughly 40-45% of the internal cross-sectional area. Two reasons. First, cables generate heat when carrying current, and a tightly packed run can't shed that heat, which derates the cable and in the worst case becomes a fire risk. Second, leaving spare capacity means you can pull an extra cable through later without ripping the whole run out.

When routing a Cat 6 data cable plus a power circuit and you think a 25x16mm channel will "probably fit", go up to 50x25mm. The extra cost is pennies per metre and you'll thank yourself when you add a second cable.

Fittings

A run is never just straight lengths. Trunking comes with a family of matching fittings, all sized to clip onto the same profile:

• Internal corner - for an inside angle, like where a wall meets a wall in a recess.
• External corner - for an outside angle, like wrapping around a column.
• Flat tee - where one run branches off another in the same plane.
• Flat bend - a 90-degree turn along a flat surface.
• Lid joiner / coupler - bridges the lid gap where two lengths meet end to end.
• End cap - finishes off the open end neatly.

Buy fittings in the same size and brand as the trunking. Profiles vary slightly between manufacturers, and a Marshall-Tufflex fitting won't always clip cleanly onto an unbranded length. Mixing brands is the fastest way to a run that looks bodged.

How to work with it

Trunking is one of the friendlier building materials to handle. PVC lengths come in 2m or 3m sticks, weigh almost nothing, and cut with a fine-tooth hacksaw or a sharp craft knife scored and snapped. After cutting, run the blade or a file over the edge to take the burr off so it doesn't snag a cable. Steel trunking is a hacksaw job and you must deburr it properly, because a steel edge will strip cable insulation.

To fix it, you've got two routes. Most PVC mini-trunking has a self-adhesive backing strip: peel and press onto a clean, dry, dust-free surface. That's fine for light cable in a cupboard. For anything load-bearing, anything outdoors, or any larger size, screw it. Drill through the back of the channel at regular intervals (every 300-500mm), and fix with screws and wall plugs into masonry or directly into timber. Adhesive alone will let go of a heavy run over time, especially in a cold or damp space.

A few practical points. Keep runs level and plumb, set out with a spirit level, because trunking shows every wobble. Plan your corners before you cut so a fitting lands neatly at each change of direction rather than a hacked mitre. Store PVC flat and out of strong sun before fitting, as long lengths bow if left leaning against a wall in the heat. And remember PVC goes brittle in the cold, so if it's been in an unheated van overnight, let it warm up before you bend or cut it.

White PVC takes emulsion or trim paint well if you want it to disappear into the wall. Wipe it down first and a light key with fine abrasive helps the paint hold.

How much do you need

Trunking is sold by the metre (in fixed-length sticks) plus fittings sold individually. Working out quantities is mostly a measuring job.

Measure the total length of every run with a tape, then add up. Round each run up to the next whole stick length, because you can't buy 1.4m of a 2m stick. Count every corner, tee, and end and buy a matching fitting for each, plus a lid joiner wherever two lengths meet. Then add a wastage allowance of around 10% on the length to cover cutting offcuts and the occasional mistake. Mini-trunking offcuts mount up faster than you'd think once you've trimmed each end of every run.

Worked example. Say you're running a single Cat 6 cable from a router shelf, along the top of the skirting for 4.2m, then up a wall 1.1m to a network point. That's 5.3m of run. You'd need three 2m sticks of 16x16mm (giving 6m, covering the 5.3m plus 10% wastage), one internal corner or flat bend for the turn up the wall, and one end cap. A small, cheap shopping list.

For a busier job, like tidying several circuits around a utility cupboard, measure each leg of the run, total it, add the fittings, add 10%, and you've got your order.

Cost and where to buy

Trunking is inexpensive, which is the good news. PVC mini-trunking in 25x16mm runs £2 – £5 at retail. Step up to the larger profiles and you're looking at £10 – £15 for sizes like 75x50mm or 100x50mm. Fittings are typically under a pound each for mini sizes, a little more for the big stuff. Steel trunking costs several times the PVC equivalent and is bought from electrical wholesalers rather than the sheds.

Screwfix and Toolstation both stock the common PVC sizes and their fittings, usually in self-adhesive mini-trunking multipacks and individual longer lengths. For larger PVC profiles, branded systems (Marshall-Tufflex, MK, Schneider), or any steel trunking, go to an electrical wholesaler such as CEF, Edmundson, or Rexel. The trade brands clip together more reliably and the fitting ranges are more complete.

External resource

Screwfix - Trunking & Conduit

Stocks PVC mini-trunking, larger profiles, and matching fittings in the common UK sizes, with prices and dimensions listed per product.

screwfix.com

Because trunking is light and compact, delivery is rarely an issue: it ships with a normal parcel courier rather than needing a builders' merchant pallet. Order a little extra rather than risk a second delivery for one missing fitting.

Alternatives

Trunking isn't always the right call. Match the method to the run:

• Round or oval conduit is the choice for a single cable, especially one being chased into masonry. Oval conduit in particular is designed to sit in a shallow chase and be plastered over. If your cable is going into the wall, that's conduit territory, not trunking.
• Cable capping covers a chased cable before plastering. It's cheaper than conduit but offers no real protection once the wall's skimmed. Use it only as a trowel guard.
• Burying cable directly in plaster (clipped into the chase, no protection) is common practice for fixed power circuits in new walls, with the cable run in safe zones. That's the standard chased-in approach and it's what most of your extension's wiring will be.

The rule of thumb: surface and multiple cables means trunking, buried and single cable means conduit or chased-in, and capping is just a plastering aid.

Where you'll need this

• First fix electrics - surface runs where cables can't be chased, like cupboards, plant spaces, and feeds to outbuildings
• Kitchen electrical provisions - tidying data and AV cabling behind units and along surfaces

Surface cable management comes up across any extension or renovation project, not just a kitchen build: wherever you're adding circuits or data runs to a finished space, in a garage, a loft, a plant cupboard, or a feed out to a garden room, trunking is how the run stays neat and protected.

Common mistakes

The classic error is overfilling. People size the trunking to just fit today's cables, cram it full, and leave no room for heat to escape or for a future cable. Size up so you're at no more than about 45% fill.

The second is mixing brands of trunking and fittings. The profiles differ just enough that an off-brand corner won't seat properly on a length from another maker, and the run ends up looking rough at every junction. Buy the lengths and the fittings as a matched set.

The last one is relying on the self-adhesive strip for everything. It's fine for a light data cable in a warm cupboard. For a heavier run, an outdoor or cold location, or any larger profile, screw and plug it to the wall. Adhesive lets go over time, and a trunking run sagging off the wall with cables spilling out is exactly the untidy result you bought trunking to avoid.