Internal Panel Wiring: Conductor Size, Colour Code and Trunking Fill Ratio
internal panel wiring: Three rules govern internal panel wiring: conductor size is chosen from the rating of the protective device with a derating factor for the internal temperature, the colour code must follow IEC 60204-1, and the trunking fill ratio should stay below 60 percent to keep the wiring bundle cool.
Contents
No matter how good the components are, an assembly will not operate reliably if its internal wiring is careless. Conductors sized without calculation, bundles crammed into trunking and arbitrary colour choices lead to thermal problems and to serious mistakes during maintenance. This article covers internal wiring under the headings of conductor sizing, colour code, fill ratio, segregation and labelling, within the framework of IEC 61439-1 and IEC 60204-1.
How to Size an Internal Conductor
Internal conductor size follows the continuous current it will carry and the ambient temperature around it. Cable tables usually give current capacity for free air at 30 degrees Celsius, whereas the temperature inside an enclosure can reach 55 to 70 degrees Celsius, which means a reduction of 20 to 40 percent in capacity. Bundled conductors inside trunking also require a grouping factor. As a result, even when a table suggests 10 mm² for a 63 ampere outgoing way, using 16 mm² inside the panel is frequently the correct engineering decision.
Colour Code and Functional Identification
IEC 60204-1 defines colour use by function: green-and-yellow for the protective conductor, light blue for the neutral, black for alternating current power circuits, generally dark blue for direct current circuits, and orange for circuits fed from an external source that stay live when the main switch is open. Red is widely used for control circuits. This discipline lets a maintenance technician understand the function of every conductor at a glance, and the orange rule in particular acts as a life-saving warning. The colour code must be applied consistently across the project and documented in the technical file.
Trunking Fill Ratio and Heat Build-up
The full cross section of a wiring duct cannot be used: the combined cross sectional area of the conductors should not exceed 60 percent of the internal area of the duct. In practice, staying between 40 and 50 percent is safer in ducts dominated by power cables, because a compressed bundle cannot release heat and the insulation temperature rapidly approaches its limit. If the cover no longer closes easily, that is the simplest sign the fill ratio has been exceeded. The position of the duct on the mounting plate should also avoid regions where hot air collects.
Segregating Power and Signal Cables
| Circuit type | Colour (IEC 60204-1) | Wiring note |
|---|---|---|
| Protective conductor | Green-and-yellow | Reserved exclusively for PE |
| Neutral | Light blue | Insulated from the enclosure after the split |
| AC power circuit | Black | Apply derating when bundled in trunking |
| DC power circuit | Dark blue | Polarity labels mandatory |
| Externally supplied circuit | Orange | Stays live with the main switch open |
| AC control circuit | Red | Fed from the control transformer |
Power cables must not share a duct with analogue signal, communication and encoder cables; where they have to run together, a metallic divider is required and a clearance of at least 100 millimetres should be maintained. The output cable of a variable frequency drive is the most aggressive noise source and needs its own route with the screen terminated over 360 degrees. Crossings, where unavoidable, are made at right angles, and long parallel runs must be prevented entirely. This segregation eliminates the root cause of many problems, from noisy analyser readings to phantom signals on PLC inputs.
Labelling, Termination and Cable Management
Every conductor should be permanently marked at both ends with labels that will not rotate or fade during maintenance. Stranded conductors are never clamped directly into a terminal; they are terminated with a correctly sized insulated ferrule or lug and tightened to the manufacturer torque. Cables feeding devices on the door are selected as flexible types able to survive an unlimited number of opening cycles, and are guided in spiral wrap or a flexible conduit. If spare conductors are provided their ends must be insulated, and at least 20 percent spare capacity should be planned in the terminal rows.
Key takeaways: internal panel wiring
- Size conductors for the internal temperature — Selecting a size straight from the catalogue without a derating factor is the most common mistake.
- Apply the orange conductor rule — Wiring externally fed circuits in orange is critical for the safety of maintenance work.
- Keep the fill ratio under 60 percent — A compressed bundle cannot release heat, so 40 to 50 percent is safer for power wiring.
- Label every conductor at both ends — Permanent, fade-resistant labels noticeably shorten fault finding time.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Why does derating matter so much for internal wiring?
Current carrying tables are normally given for an ambient of 30 degrees Celsius, yet the temperature inside a closed enclosure can exceed 60 degrees. The same conductor may then carry only 60 to 80 percent of the tabulated value. Without derating, the insulation ages prematurely, terminal connections loosen and the joint eventually shows up as a hot spot on a thermal image.
How is the trunking fill ratio calculated?
Calculate the internal cross sectional area of the duct, add up the areas of all conductors based on their outer diameter and take the ratio. The result should remain below 60 percent. A practical check is whether the cover closes without force; if the cover is under pressure, the ratio has almost certainly been exceeded.
Why should a stranded conductor never be clamped directly?
Under screw pressure the individual strands spread sideways, the contact area shrinks and the joint loosens over time. Contact resistance rises, the joint heats up and the terminal is eventually damaged. The correct practice is to terminate the conductor with a properly sized insulated ferrule or lug and tighten it to the specified torque.
Related articles
- Terminal and Cable Lug Connections: Tightening Torque and Thermal Reliability
- Cable Sizing and Voltage Drop Calculation
- Switchboard Heat Load Calculation and Choosing a Cooling Solution
- All technical articles
In Devpan assemblies, wiring sizes are determined by derating calculation, the colour code and labelling follow the standard, and fill ratios are checked during production. Let us define the wiring standard for your assemblies together. Related Devpan solution: electrical panel design services.
