Preflex vs. Open Wiring: Which Cable Method for Your Project?
Preflex conduit or XVB installed directly? Comparison for new builds and renovations in Belgium, with the AREI/RGIE rules on conduits, fire classes and installation zones.
AuthorStephan BehmIn new builds, Preflex with VOB wires is the Belgian standard: the conduits go onto the slab or into the walls during the shell phase, and the wires stay replaceable later. In renovations, XVB cable without conduit is usually the faster option, in a wall chase, surface-mounted or in a cavity. The AREI/RGIE permits both methods; it governs how you carry them out.
What is Preflex?
In Belgium, Preflex is the common name for flexible corrugated plastic electrical conduit. It comes empty or with factory-pulled VOB wires. In new builds, it is laid during the shell phase on the concrete slab, in the screed or in wall chases, and then cast in or plastered over.
Open wiring here means cables without conduit, usually XVB: directly in a chase under plaster, surface-mounted, on a cable tray or in building cavities.
Preflex and open wiring compared
| Criterion | Preflex with VOB wires | Open wiring with XVB |
|---|---|---|
| Conductor type | Single-core wire (H07V-U/-R) in conduit | Sheathed cable |
| Typical use | New build, major renovation | Renovation, basement, plant room, extension |
| Replacing wires later | yes, wires must remain pullable at all times (5.2.9.3) | only by opening up or using a new route |
| Surface mounting | only the conduit, not the bare wire (5.2.9.5) | yes (5.2.9.5) |
| Directly in a chase | not applicable | yes, within the installation zones (5.2.9.10) |
| Material | conduit plus each wire separately | one cable with all cores |
What the AREI requires for conduits
Subsection 5.2.9.3 governs installation in electrical conduits:
- Diameter: conduits and fittings sized so that conductors can easily be pulled in and removed after installation (letter d).
- Bending radius: at least eight times the outside diameter for flexible thermoplastic conduit (letter e).
- No joints in the conduit: connections belong in pull or junction boxes on a suitable terminal block (letter f).
- Combustible conduit: permitted only if embedded at least 3 cm deep in non-combustible materials (letter f).
- Heat: no thermoplastic conduit where the temperature can exceed 60 °C in normal operation (letter f).
- Embedded conduit: no tees or elbows; conductors remain accessible in the boxes (letter i.2).
The AREI sets no fill limit as a percentage. The test is that conductors can be pulled in and removed at any time.
What the AREI requires for open wiring
Wiring embedded without conduit must be cable; conductors with basic insulation only are prohibited there (5.2.9.10). In the walls of rooms, installation zones apply: horizontal runs 25 to 35 cm below the ceiling or above the floor, vertical runs as close as possible to a corner or 10 to 20 cm beside door frames, and at least 0.4 cm of plaster cover. In concrete or cement, you need at least 3 cm of cover.
Which fire class do you need?
What counts is the CPR class of the cable, not of the conduit:
- Installed singly: at least F1 or class Eca (5.2.7.2).
- In bundles or flat formation: at least F2 or class Cca (5.2.7.3).
- In building cavities: also F2 or Cca (4.3.3.5 b and 5.2.9.9). A timber-frame or plasterboard stud wall is a building cavity.
Cables and conduits embedded at least 3 cm deep in non-combustible cladding are exempt (5.2.7.1 no. 3). If the Preflex conduit sits at least 3 cm deep in concrete or screed, this exemption applies. In a cavity, by contrast, the class of cable and conduit counts (5.2.9.9 with 5.2.7.3 and 5.2.7.4).
Which method when?
- New build: Preflex with VOB wires, cast into or plastered into floors and walls.
- Renovation with chases: XVB directly in the chase, within the installation zones.
- Basement, garage, plant room: XVB surface-mounted or on a cable tray.
- Timber frame, drywall, suspended ceiling: cable rated at least Cca or a suitable conduit system.
- Partial renovation: combine both; new circuits in Preflex, existing wiring kept after inspection.
For the cable types, see XVB cable or VOB wire; for the complete system, Installation methods under AREI.
Related articles
- What Changes with the AREI 2026?
- Bathroom Electrics: Zones and Cable Types
- Old Buildings and AREI Part 8
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Regulatory basis: AREI/RGIE Book 1, version 06, subsections 4.3.3.5, 5.2.7.1–5.2.7.3, 5.2.9.3, 5.2.9.5, 5.2.9.9 and 5.2.9.10. Official text: FPS Economy.