Corrugated conduit (concealed in chases) and trunking (exposed, screwed to the wall) mechanically protect the electrical cabling and are mandatory under regulations: cable must never run loose inside the wall.
Metres of conduit per home
As a practical rule, the length of corrugated conduit usually runs at around 1.2 to 1.5 times the length of cable in the installation, because several cables can share the same conduit on common runs. In a 90 m² home with a high electrification level, it's usual to need between 150 and 220 metres of conduit.
Diameters by circuit
The diameter of the conduit depends on how many cables run inside and their cross-section: 16 mm for a light-point cable, 20 mm for general-purpose sockets, and 25–32 mm for larger cross-section circuits such as the kitchen, oven or the consumer unit circuit itself. Pushing in cables of more cross-section than the conduit takes is a common infringement that complicates any later fault.
Trunking: when to use it instead of concealing
Surface trunking is used when you can't or don't want to chase the wall (commercial premises, extensions to finished work, temporary installations). It is calculated by linear metres of visible run and usually also includes surface-mounted accessory boxes, which are more expensive than recessed ones.
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