Composite Installation
Containment, wiring systems and circuits together
The composite installation combines containment, wiring systems and several circuits in one continuous build, marked against BS 7671, the specification drawing and manufacturer instructions at the same time.
Design decisions come first. Select cable type and cross-sectional area for current-carrying capacity after applying correction factors - grouping, ambient temperature, thermal insulation, protective device type - then check voltage drop is within limits. Choose the wiring system to suit the environment: PVC conduit in a workshop, steel conduit where mechanical protection is needed, trunking where future access matters. Support cables and boxes at the required spacing and keep bends to a sensible radius.
Route cables in permitted safe zones: within 150 mm of a ceiling, a corner or a partition, or in line with an accessory. Anything outside those zones needs earthed metallic protection or 30 mA RCD protection. Keep Band I (SELV, data, communications) and Band II (mains) circuits segregated in shared containment, using separate compartments in trunking. Terminations must be tight and correctly polarised at every accessory, with grommets or glands protecting the sheath. Earthing and main protective bonding conductors are sized from the supply neutral, and every extraneous conductive part that requires bonding is bonded.
Examiner traps: overfilled containment, cable clipped without regard to fixing distances, missing grommets, misaligned screw slots, swarf left inside an enclosure, and omitted circuit labelling. A neat job that fails inspection is still a fail.
Rule of thumb: a cable run 200 mm below a ceiling is not in a safe zone unless it is in line with an accessory - so move it, provide earthed metallic capping or conduit, or fit 30 mA RCD protection.