Core Drilling for Electricians: Cable Entry Sizes and Safety Requirements

Electrical cable penetrations through walls are typically the smallest core drilling application — 20mm to 52mm for most domestic and light commercial work. The small diameter does not reduce the safety requirements. Every cable entry through an external wall or partition requires a pre-drill sweep for existing services, correct bit selection, and — where the penetration passes through a fire compartment barrier — fire stopping to Building Regulations Part B.

Core Drill Sizes for Electrical Work

Single Cable Entry — 20mm to 32mm

A single SWA or armoured cable passing through an external wall requires a hole 6–8mm larger than the cable outer diameter. Most 6mm² twin-core SWA cable (used for domestic sub-mains and EV charger circuits) has an OD of approximately 17–20mm — requiring a 25mm core hole. For 16mm² SWA (used for larger EV charger circuits and commercial sub-distribution) the OD is typically 25–28mm, requiring a 32–38mm core hole. Where a conduit sleeve is required, use the conduit OD as the guide.

Multi-Cable Entry and Cable Management — 38mm to 65mm

Where multiple cables need to pass through the same wall penetration, or where a cable management gland plate is to be fitted, a 52mm or 65mm core hole is standard. A 52mm hole accepts a 50mm multi-cable gland or a standard small cable tray entry. For commercial installations with larger cable bundles, a 65mm or 80mm core may be required — check the cable entry device specification before drilling.

EV Charger Cable Routes — 25mm to 38mm

Domestic EV charger installations typically use a 6mm² or 10mm² cable from the consumer unit or distribution board, through the external wall to the charger. A 25mm core hole is sufficient for most 7kW domestic charger installations with a single SWA cable. Where the route passes through an internal wall, floor, or ceiling rather than the external wall, the hole must be fire stopped if it crosses a compartment boundary. The EV charger installer is responsible for ensuring the cable route complies with BS 7671 and Part B.

Data, Comms, and Fibre Entry — 20mm to 25mm

CAT6 data cable (approximately 6mm OD), coaxial cable (9mm OD), and single-mode fibre (3–5mm OD with outer sheath) all fit through a 20mm core hole. Where the cable is to be sleeved or a proprietary wall entry gland is to be fitted, check the gland body OD before drilling. A 25mm hole is generally the practical minimum for a drilled entry with any form of sealing gland.

Pre-Drill Safety — Cable and Pipe Detection

The most serious risk in electrical core drilling is striking an existing embedded cable. In UK domestic buildings, electrical cables are supposed to follow defined routing zones (vertically and horizontally from switch and socket positions) under BS 7671:2018 — but this applies only to installations completed after approximately 1992. Older wiring, extension cables, and non-compliant DIY wiring do not follow zones reliably.

Before drilling any wall penetration:

1. Sweep the full intended drill zone with a cable and pipe detector (combined live wire detector and metal detector)
2. Scan at least 300mm in all directions from the intended hole centre — not just the immediate drill point
3. In buildings with pre-1992 wiring, extend the sweep zone to 500mm in all directions and trace visible cable runs from their source
4. Identify any known service runs through the wall (consumer unit positions, socket/switch locations, radiator pipework) and consider their probable routing before confirming the hole position

Gas pipe strikes require immediate building evacuation and emergency gas network attendance. Electrical cable strikes risk electrocution, fire, or explosion. Both are preventable with a systematic pre-drill sweep. See: diamond core drill safety guide.

Fire Stopping for Cable Penetrations

Building Regulations Part B requires that penetrations through fire compartment barriers are sealed to maintain the fire separation. For electrical cable penetrations, the correct approach is:

• Single cables through compartment floors or walls — intumescent putty pads or proprietary intumescent pillow products applied around each cable at the compartment boundary. The putty pad expands in a fire to close the penetration as the cable burns away.
• Multiple cables through a single core hole — proprietary multi-cable transit systems (MCT) rated to the required fire classification (typically EI60 or EI120 for domestic compartment floors). The transit frame is installed in the core hole; intumescent blocks fill around each cable.
• Conduit penetrations — where conduit passes through a compartment barrier, an intumescent collar or sleeve installed around the conduit at the compartment face maintains the fire rating as the plastic conduit burns away. Metal conduit requires sealant in the annular gap only.

In domestic houses, each floor-to-ceiling separation is a compartment boundary. A cable dropped from a first-floor consumer unit location through the floor into a ground-floor ceiling void crosses a compartment boundary and requires fire stopping at the penetration point. See: core drilling permits and regulations: UK guide.

Bit Selection for Electrical Core Drilling

Small-diameter holes (20–52mm) for cable entries have the widest range of equipment options:

• SDS Plus adaptor with dry diamond core bit — suitable for 20–52mm holes in standard brick and blockwork cavity walls. Rotation-only mode essential — hammer function must be disabled when using a diamond core bit. Cost-effective for occasional single holes. See: best core drilling methods: UK guide.
• Dedicated core drill machine with dry diamond bit — preferred for multiple holes or any material harder than standard cavity brick. Better bit life, more consistent RPM, lower vibration.
• SDS hammer with TCT core bit — acceptable for single occasional holes in standard brick and block. Not suitable for tiles, engineering brick, concrete, or finished surfaces where spalling would be unacceptable.

For concrete or engineering brick external walls (common in commercial buildings), a dedicated core drill machine with a wet bit is required even at small diameters. See: dry vs wet core drilling.

Core Drilling for Electricians: Common Questions