Video reference. John Ward (jwflame) covers the BS 7671 rules for concealed cable zones in his video "Zones for Concealed Cables in Walls, BS7671 Wiring Regulations", hosted on his Flameport channel. John is a UK-based qualified electrician whose content covers UK-specific electrical theory, regulations and practice. This video draws directly from BS 7671 and is worth watching if you plan any chasing or cable work on your property.
1. What are prescribed cable zones?
When an electrician conceals a cable inside a wall, floor or ceiling, BS 7671 requires it to run within defined areas called prescribed zones. The idea is simple: if cables are always run in the same predictable areas, anyone who drills into the wall later knows where to look and where to be careful.
In earlier editions of the wiring regulations these were called "safe zones." The 18th edition, which came into force in January 2019, renamed them "prescribed zones," though the principle is the same. The zones apply to cables concealed within the structure of the building, not to surface-run cables clipped to the wall or contained in trunking or conduit.
2. The vertical zones
There are two types of vertical prescribed zone:
- Within 150 mm of the edge of a wall or partition. A cable concealed in a wall must run within 150 mm of a corner, angle or edge. This includes internal corners where two walls meet, and the edges of partition walls.
- Directly above or below any electrical accessory. A cable running to or from a socket, switch, joint box or other accessory should run vertically in line with that accessory -- straight up to the ceiling or straight down to the floor. If you see a socket, the cable feeding it is expected to run directly above or below it.
These two vertical zones cover most of the areas where you would expect to find wiring in a typical room.
3. The horizontal zones
Horizontal prescribed zones are strips running along the full length of a wall:
- Within 150 mm of the top of a wall (just below the ceiling line). Cables are expected to run horizontally here, often looping between lighting outlets and switches along the top of the wall cavity.
- Within 150 mm of the bottom of a wall (just above the floor). Cables can also run along the bottom of the wall, particularly where they drop down from a switch or socket to go under the floor.
For a floor or ceiling, the prescribed zone is within 50 mm of the surface on either side -- so a cable buried in a concrete floor should be within 50 mm of the floor surface or within 50 mm of the ceiling above.
4. Why older properties are different
The prescribed zones only apply to work done under the 18th edition (July 2018 onwards). In properties built before that date, cables may run in any direction at all -- diagonals, curves, wherever the original electrician found convenient. There is no guarantee that the zones apply.
In Victorian and Edwardian properties, the wiring is often surface-run in metal conduit or inside skirting voids, and the walls themselves may have no concealed cables at all. In homes from the 1960s and 1970s, twin and earth cable was often buried at whatever angle suited the run, with no zone requirement in force at the time.
In any property, old or new, use a cable detector. It is the only way to be certain.
5. What happens if a cable must go outside the zones
If an electrician absolutely must route a cable outside a prescribed zone -- for example, because the layout of the building makes an in-zone route impractical -- the cable must either be enclosed in earthed metallic conduit, or be provided with equivalent mechanical protection. This means that even if someone does drill into the cable, the metal provides a protective barrier and the damaged conduit or armour becomes obvious rather than invisible.
Regulation 522.6.6 of BS 7671 sets out the full requirements. An EICR inspection will check that visible cable runs comply, and any non-compliant run is likely to generate a code depending on the risk it presents.
6. What this means before you drill
Before fixing a TV bracket, putting up shelves, fitting a new radiator, or drilling through an internal wall for any reason, check the wall first with a combined cable, metal and stud detector. Scan the area vertically within 150 mm of each corner, horizontally within 150 mm of the ceiling and floor, and vertically in line with any switches or sockets on the wall.
If the detector indicates a cable, mark its approximate position and avoid it. If you are not sure what you have found, call an electrician before drilling. It takes a few minutes and costs nothing compared with the alternative.
When to call us
If you need cable traced before building or renovation work, or if an EICR has flagged concerns about concealed cable runs, Richard covers Sandwich and east Kent for inspection and testing work. Ring or send a message to arrange a visit.
Not sure where cables run in your walls?
Richard can trace cable routes before building or renovation work starts, and carries a cable detector on every visit as standard. Covering Sandwich and east Kent.
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