Cable Zone and Cable Entry Design: Common Switchboard Mistakes
Cable zone design: Determining cable zone depth, calculating bending radii and detailing cable entries correctly.
ContentsThe cable zone is electrically the simplest and practically the most troublesome part of a switchboard. Projects calculate busbars meticulously and then leave the cable zone as whatever volume is left over. The consequence shows up on site: cables are forced into tight bends, insulation is damaged, terminals carry mechanical stress, and at the next maintenance visit no cable can be freed.
One thing sets the depth: bending radius
The minimum permissible bending radius of a power cable is given as a multiple of its outer diameter. For XLPE insulated multicore cables that multiple is typically 12, and for single core cables it rises to about 15. The cable zone depth has to accommodate that radius, the approach angle into the terminal and the length of the lug together. Otherwise the cable is bent outside its specification and the contact geometry between conductor and insulation is permanently compromised.
| Cable size | Approximate outer diameter | Minimum bending radius | Recommended zone depth |
|---|---|---|---|
| 4×50 mm² | ~35 mm | ~420 mm | 500 mm and above |
| 4×95 mm² | ~45 mm | ~540 mm | 600 mm and above |
| 1×240 mm² | ~30 mm | ~450 mm | 600 mm and above |
| 1×400 mm² | ~38 mm | ~570 mm | 700 mm and above |
| Control cable 1.5 mm² | ~10 mm | ~80 mm | A separate trunking is enough |
Gland plates and sealing
Cable entries are the weakest point of a switchboard IP rating. Using a removable gland plate instead of a fixed bottom panel means the drilling happens under controlled conditions rather than on site. With single core high current cables, magnetic induction can heat a steel plate; in that case a non magnetic plate is used, or all three phases pass through one common opening rather than individual holes. The IP rating of the glands themselves must equal or exceed the rating of the enclosure.
Common mistakes
- Planning the cable zone by width alone and neglecting depth
- Letting the terminal carry the cable weight; the load must always be taken by a separate cleat inside the zone
- Running power and control cables in the same trunking, creating interference and separation problems
- Forgetting to account for plinth height in bottom entry designs
- Labelling cables only at the terminations; every label should be repeated at the zone entry
- Leaving no space on the gland plate for future spare ways
Key takeaways: cable zone design
- One thing sets the depth: bending radius — The minimum permissible bending radius of a power cable is given as a multiple of its outer diameter.
- Gland plates and sealing — Cable entries are the weakest point of a switchboard IP rating.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Does the cable zone affect the separation form?
It does. Form 3b and 4b designs require terminals to be separated from the functional unit, and that separation directly determines the structure and volume of the cable zone. See our article on Form 1 to 4b internal separation for the detail.
Top entry or bottom entry?
If the cable tray runs overhead, top entry simplifies installation but increases the risk of water and dust ingress and complicates the sealing detail. Bottom entry is a cleaner solution but demands adequate plinth height and clearance for the bending radius.
Related articles
- Busbar Trunking or Cable? Choosing the Right Distribution Method
- Cable Sizing and Voltage Drop Calculation
- Sizing the Neutral and Earth Bars in a Switchboard
- All technical articles
At Devpan we size the cable zone starting from the cable schedule and detail gland plates and glands in line with the IP rating. Share your cable schedule and we can fix the enclosure depth in advance so installation is never forced.
