PVC Cable Trunking Guide: Sizing Slotted Duct for Panel Wiring

PVC cable trunking is the grey plastic duct that runs along the inside of a control panel and keeps the wiring in order. It is the least glamorous item on a switchgear bill of materials and one of the few that an electrician will form an opinion about within a minute of opening the door. Chosen well it makes a panel that can be worked on for the next fifteen years. Chosen badly it makes one that cannot accept an additional circuit without being rewired.

What slotted trunking is for

The duct is a U-section channel with a snap-on or slide-on lid, and its side walls are cut into teeth — the slots. Wire leaves the duct between those teeth and goes to a terminal on the component beside it. That is the whole idea: the wiring runs bundled inside the duct and exits exactly where it is needed, instead of being tied in a loom that has to be cut open every time a wire is added.

Two consequences follow. First, the panel can be modified: a new circuit is a new wire fed through the slot nearest its terminal. Second, the trunking has to be sized for the future, not for the wires that happen to be there on installation day. This is the mistake that shows up years later.

Reading the size code

Trunking is designated by two dimensions — height and width of the channel cross-section, in millimetres. A duct marked H45 × W25 is 45 mm tall and 25 mm wide. The height is the internal depth into which wires are laid; the width is the internal width across the channel. Both matter, and they matter for different reasons:

• Height limits how many layers of wire can be stacked before the lid will not close. It also sets the clearance between the duct and the panel.
• Width through which the wires pass, and therefore how many wires of a given cross-section the duct can carry side by side.

Because wires are round and ducts are rectangular, a duct never holds its area in conductor. Plan on filling roughly half the cross-section and leave the rest for routing around bends and for additions later.

Sizing: the fill ratio

A working method for a panel:

• Count the wires that will pass through the busiest section of the duct, by cross-section.
• Multiply each wire's overall diameter (including insulation, which is noticeably larger than the conductor) to get an approximate occupied area.
• Total it, double it for routing inefficiency, and pick the next standard size up.
• Then add at least one size for growth if the panel is a prototype or is expected to be extended.

Where a duct must be filled heavily, remember that bundled conductors de-rate. A duct packed solidly with current-carrying wire traps heat and reduces the current each conductor may carry, which is a separate calculation from whether the lid will close.

The material properties that matter

On our PVC trunking the published figures give a working temperature of 100 °C to 105 °C, with the material described as fire retardant, halogen free, heat resistant, rigid and UV resistant, in a slotted H45 × W25 section for electric wire installation. Those are the properties to compare against the ambient inside your enclosure, and against any building requirement that applies to the area the panel sits in.

Where each type is used

• Slotted duct inside panels. The classic application: control panels, drive cabinets, PLC panels, machine control enclosures.
• Surface trunking in buildings. Carrying circuits along a wall or skirting where cables cannot be buried; here the duct is a finished surface and the lid is part of the appearance.
• Larger wall-mounted runs. Wider sections used as a distribution route between enclosures, with the wire leaving at box positions.

Installation points

• Cut square and de-burr. A ragged cut at a slot root becomes a crack.
• Leave the teeth where the terminals are. Plan the slot positions against the component layout before cutting the duct to length.
• Do not over-bend. PVC will follow a panel corner, but a tight radius at a slot stresses exactly the weakest point of the profile.
• Keep duct clear of live terminals. It is insulating, not arc-rated; it is not a barrier between a person and a live part.
• Label at both ends of a run. The advantage of trunking is that a circuit can be traced later; that only works if it is identified.

Frequently asked questions

What is PVC cable trunking used for?

Routing and protecting electrical wiring, most often inside control panels where slotted duct lets wires leave the bundle exactly at each terminal, and on surface runs in buildings where cable cannot be buried.

How do I read a trunking size such as H45 x W25?

It is the channel cross-section in millimetres: 45 mm tall and 25 mm wide. The height limits how many layers of wire can be stacked under the lid; the width limits how many wires pass through the channel.

Why does halogen-free matter for trunking?

Because halogenated plastics release acidic and toxic smoke when they burn. In an occupied building or a confined space, halogen-free material reduces the hazard to people even though it does not change the electrical performance.

Is slotted trunking suitable outdoors?

Only with UV-resistant material, because ordinary PVC embrittles in sunlight. Check the published UV rating rather than assuming the duct is suitable for an outdoor run.

If you are buying trunking for a panel build or a surface run, tell us the wire count and sizes in the busiest section, the ambient temperature inside the enclosure, whether the installation is indoors or outdoors, and the length of the run. We will confirm the section and the material rather than leaving you to over-size it.