6 min read
In panel building, trunking is one of the smallest budget items — which is why it is the first place people economise. Yet its cross-section and fill ratio determine both the panel's operating temperature and the time taken by every change made over the next ten years.
Why fill should not exceed 60%
Cables inside trunking heat one another. Every cable alongside another reduces its neighbour's ability to shed heat, which is why current-capacity tables carry a correction factor for the number of cables grouped together. With trunking packed full, that factor drops sharply and the cables run hotter than calculated.
- 60% fill — the practical ceiling; leaves room for heat and for adding cables later.
- 40–50% fill — preferred in panels likely to be modified.
- Above 80% — adding a cable becomes impossible, the lid is forced shut and cables are crushed.
Separating power and signal
Drive output cables should not share trunking with sensor and communication cables. The drive's high-frequency switching couples noise into the signal line alongside; analogue values drift, encoder counts slip and communications drop. Where separate trunking is impossible, a metal divider is used between them and crossings are made at right angles.
Marking: done once, pays for ten years
Finding a fault in an unmarked panel means tracing every wire from both ends. Marking takes a few hours during the build; without it, every intervention adds hours. The correct method is that the number in the terminal schedule appears identically at both ends of the wire.
| Marker type | Durability | Where it suits |
|---|---|---|
| Heat-shrink (thermal printed) | High — does not rub off | Permanent panel wiring |
| Clip-on ring markers | Medium | Connections that change often |
| Adhesive labels | Low — lifts in oil | Temporary marking, terminal rail |
| Terminal markers | High | Terminal numbering |
Two things overlooked when choosing trunking
First, slot spacing: narrow slots let you bring a cable out where you want, wide ones force you to lengthen it. Second, height: taller trunking takes more cable but eats panel depth and reduces the space between mounting plate and door. Both should be settled while the panel layout is drawn.
Slotted panel trunking, PVC cable trunking, conduit and Konsan marking systems ship on one delivery note. Send your panel layout and we will work out the trunking size and fill together.
Frequently asked questions
What is done when the trunking in an existing panel is full?
Three options. First, replace the trunking with a taller profile — changing the body and lid without removing the cables is often possible. Second, add a second run of trunking; where the mounting plate has room, this is the most practical. Third, remove dead cables: in most panels, wires abandoned over the years take up a noticeable share of the trunking.
Which numbering should wire markers follow?
The terminal number on the drawing. The most common and most useful system is for the wire to carry, at both ends, the number of the terminal it lands on; you can then read from one end where it goes. Numbering the wires themselves in sequence (1, 2, 3) does not match the drawing and is useless in maintenance.
Is there a colour standard for wiring inside a panel?
Common practice is: black or brown for phases, light blue for neutral, green-yellow for protective earth, dark blue for DC control, red for AC control, and orange for circuits fed from an external source that stay live with the main switch off. Orange matters especially: it warns whoever is working that the main switch does not isolate that line.
What is used to calculate the fill ratio?
Compare the trunking's internal cross-sectional area with the sum of the outside-diameter areas of the cables that will run in it. The calculation uses the cable's outside diameter with insulation, not its conductor size — the difference between the two is large at small sizes. In practice, the area calculated from the cable list should not exceed 60% of the trunking section.