Drill into the wrong part of a wall and you can strike a live cable. Cable zones exist to lower that risk. Under BS 7671, the IET Wiring Regulations, cables concealed in walls must either follow set routes or be protected in other prescribed ways, so anyone working on that wall can reasonably predict where the wiring runs. For electricians and apprentices, knowing these routes thoroughly is part of installing safely and signing off compliant work. Get them wrong and you put people at risk and fail inspection.
What Cable Zones Actually Are
These permitted routes, also called safe zones or prescribed zones, are set out in Section 522.6 of BS 7671. The thinking behind them is simple: keep hidden cables in predictable lines, so a person fixing a shelf or adding a socket can anticipate where wiring is likely to run. The zones reduce the chance of a nail, screw or drill bit catching a cable that nobody can see.
Cable zones apply to concealed cables, meaning anything buried in plaster, blockwork or a plasterboard partition rather than run on the surface. The rules matter most when a cable sits close to the face of the wall, which is where most domestic wiring ends up.
The Prescribed Safe Zones Under BS 7671
BS 7671 gives three positions where concealed cables are allowed to run. These govern almost every domestic layout, so they’re worth committing to memory:
- Along the Top of the Wall: Within 150mm of the top of the wall or partition, where it meets the ceiling.
- In Each Corner: Within 150mm of an angle formed by two adjoining walls or partitions.
- In Line With an Accessory: Running horizontally or vertically, straight to a socket, switch or other point or accessory on that wall.
On a wall or partition 100mm thick or less, a zone formed on one side extends to the reverse side, but only where the position of the accessory can be determined from that reverse side. There’s no zone along the skirting line or across the open middle of a wall, and diagonal runs fall outside the zones altogether, which is where a lot of non-compliant runs are found.
The 50mm Depth Rule and RCD Protection
The zone rules apply when a cable is concealed at a depth of less than 50mm from the surface of the wall or partition, measured from either side. At that depth the cable is within reach of fixings, so BS 7671 requires one of the following:
- Prescribed Zone Plus RCD: Run the cable inside a safe zone and provide additional protection by a 30mA RCD.
- Earthed Metallic Covering: Use a cable with an earthed metal sheath or armour that meets the requirements for a protective conductor, such as steel wire armoured or mineral-insulated cable.
- Earthed Conduit or Trunking: Enclose the cable in earthed steel conduit, trunking or ducting that is suitable as a protective conductor.
- Mechanical Protection: Guard the cable with protection able to stop penetration by nails, screws and the like. Ordinary plastic or light steel capping does not count, since it only shields the cable from the plasterer’s trowel.
- SELV or PELV: The cable forms part of a SELV or PELV circuit.
For most domestic installs, the first option wins on cost and time. A cable in a safe zone at less than 50mm deep needs 30mA RCD additional protection in any type of installation; the old exemption for installations under the supervision of skilled or instructed persons was removed in 2015.
Amendment 4 (2026) reorganised these requirements into a table (Table 52.1) to make them easier to follow, and the IET has indicated that the substance is unchanged. Check the regulation numbers against your own copy, as they may be cited differently in older guidance.
Bury the cable 50mm or more from both faces and the wall-depth rule no longer applies, though other RCD duties, such as those for socket-outlet circuits, still stand. That depth is also rarely workable in a standard 100mm leaf.
Metal Studs and Partition Walls
Walls built with metal studs, or containing metal parts such as noggins and fixing plates, change the picture. Where the internal construction of a wall or partition includes metallic parts, other than fixings such as nails and screws, a concealed cable needs 30mA RCD protection regardless of its depth, unless it uses one of the alternatives above: an earthed metallic covering, earthed conduit or trunking, suitable mechanical protection, or a SELV or PELV circuit. Where the cable is also less than 50mm deep, it must still follow the prescribed zones. Where cables pass through holes in steel studs, fit grommets or bushes to protect them from sharp edges.
Timber stud partitions need care too. Two habits keep cables clear of screws driven into the board:
- Drill the Centre Line: Pass cables through holes bored in the middle of the stud depth, away from both faces. In a typical stud wall the cable will still be within 50mm of a surface, so the zone and RCD rules still apply.
- Fit Protection Plates: Where a cable has to sit close to the face of a stud, cover it with a steel protection plate before boarding over. This reduces the risk of a strike, but only counts as mechanical protection under BS 7671 if it is capable of preventing penetration by nails and screws.
Chasing Cables Into Walls
Cutting a chase removes material, so there are limits that protect the structure as much as the cable. Approved Document A sets maximum chase depths for masonry walls:
- Vertical Chases: No deeper than one third of the wall or leaf thickness.
- Horizontal Chases: No deeper than one sixth of the wall or leaf thickness.
Good practice adds two more habits:
- Straight Runs Only: Keep a chase vertical or horizontal from start to finish, never diagonal. A diagonal run would also take the cable out of the prescribed zones.
- No Back-to-Back Cuts: Don’t cut chases directly opposite each other on the two faces of a wall.
On a standard 100mm block, that works out at roughly 33mm vertically and 16mm horizontally, measured into the block rather than the plaster. Horizontal cuts weaken a wall more than vertical ones, so keep them short and in line with an accessory wherever the layout allows. Take extra care with hollow blocks and follow the manufacturer’s guidance. Walls built for sound or fire separation carry their own requirements under Approved Document E and Approved Document B, so check those before you cut.
Why Cable Zones Still Call for Caution on Site
Cable zones tell you where compliant wiring should sit, not what’s actually buried in an old wall. Plenty of properties have runs installed before the current rules, or added by someone who ignored them. So treat the zones as a firm design rule for your own work, and only as a rough guide when you’re cutting into someone else’s:
- Isolate First: Switch off, isolate and prove the circuit dead before you cut or drill.
- Use a Detector: Scan the wall with a cable and pipe detector, and treat a weak or odd reading as a reason to stop.
- Expect the Unexpected: Assume an older installation may have cables outside the zones until you’ve proven otherwise.
Skipping the detector because the zone looks clear is how cables get struck. The habit of scanning first separates a clean job from a dangerous one.
Where These Rules Sit in UK Regulation
Cable routing requirements sit within BS 7671, the national standard for electrical installations, currently BS 7671:2018+A4:2026. Alongside that, Part P of the Building Regulations covers electrical safety in dwellings in England, with Wales applying its own version, and some work, such as new circuits, consumer unit replacements and work in special locations like bathrooms, is notifiable to building control. Scotland and Northern Ireland have their own building standards, so check what applies where you’re working.
Fixed electrical work belongs with a competent electrician, and registration with a competent person scheme such as NICEIC or NAPIT lets notifiable work be self-certified rather than notified to building control job by job. For background on the wall rules, the IET’s guidance on cables concealed in walls is a useful reference to keep to hand.
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Réflexions finales
Learn the three zones, respect the 50mm line, add the RCD where the Regulations call for it, and prove dead before cutting. That’s the difference between wiring that passes inspection and wiring that comes back as a callback, or worse.
Calculators for this job
Free Tradefox calculators that go with this guide:
- Voltage drop using mV/A/m – Check the run once you know the cable route.
- Cable rating correction factors – Apply correction factors for how and where the cable is installed.
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