Above about 100 mm, coring stops being a hand tool operation. The bit mass, the torque and the core weight all scale up, and the failure modes change from "slow" to "dangerous".

Sizing the setup

Core diameter	Rig	Typical power
25-50 mm	Hand-held acceptable	1-1.5 kW
50-100 mm	Small stand, anchored	1.5-2.2 kW
100-200 mm	Anchored stand, mechanical feed	2.2-3 kW
200-350 mm	Heavy stand, two anchors, wet only	3-5 kW

Pilot first

Drill a 12-20 mm pilot through the wall. It fixes the centre, tells you where the rebar is, and gives the large bit a track. On walls over 300 mm thick, the pilot also confirms the wall is as drawn before you commit a 250 mm bit.

Staging and core removal

• For horizontal cores over 150 mm, plan how the core will be held: a wedge, a second operator, or a core catcher.
• Break the core loose with the bit still running slowly, then stop and remove it; do not let it drop onto the bit.
• On deep walls, core in two passes from both sides to meet in the middle; this avoids a long slug and reduces wander.

Water and slurry

Large cores need real flow, often 4-8 litres per minute, with a vacuum collar. On a wall, slurry runs down the face: set a catchment tray and a dam at the base before starting, and protect the floor finish.

Safety items that get skipped

• Two anchors minimum; a single anchor on a large core is a kickback waiting to happen.
• Anchor pull-out test on old or cracked walls before mounting the rig.
• Never stand in line with the bit axis; if the core grabs, the rig rotates around the anchor.
• No coring through a wall without knowing what is behind it: scan for conduit, pipe and tendon before the pilot.

Rig and anchor selection by diameter

Large holes in walls fail on anchoring more often than on cutting. The reaction force of a 200 mm core is substantial, and a rig that is merely "big enough" will walk, twist or pull its anchors out of a wall that was never designed to hold it.

Core diameter	Typical rig class	Anchoring	Watch for
100-150 mm	Medium, 2-3 kW	Two anchors plus a brace	Anchor pull-out in hollow block
150-250 mm	Heavy, 3-5 kW	Four anchors, stand or wall bracket	Wall flexing as the core loads up
250-350 mm	Heavy with stand	Through-bolted plate or threaded rod both sides	Vibration loosening the anchor nuts
350 mm+	Track rig or chain-clamped stand	Mechanical fix through the wall	Wall thickness versus core length

The wall itself is part of the rig. A 100 mm block wall carrying a 250 mm core needs the load spread over a plate, ideally with a rod through to the far side. Anchoring into the mortar joint rather than the block is the quickest way to lose a rig mid-cut.

Staged drilling: pilot, step-out and the last 20 mm

• Pilot at 20-30 mm first, all the way through where access allows. It gives the core a track and tells you where the far side breaks.
• Step out in stages for anything over 200 mm: 50 mm, then 100 mm, then final. Each stage clears slurry and lets you check the hole is still on line.
• Cut the last 20 mm at reduced feed. The core drops when it breaks through, and an unbraced rig takes the shock.
• Support the core before breakthrough on a wall: a wedge, a strap or a second operator. A 20 kg plug falling out of a wall is a hazard and a repair.
• Withdraw the core with the rig, not by hand. Pulling a loaded core by hand is how fingers and wrists get injured.

Two intermediate checks matter more than any other: after the pilot, confirm position and plumb; after the first stage, confirm the hole is not tapering. A taper of 3 mm over 200 mm is invisible on the wall and fatal to the pipe that has to pass through.

Planning the hole before the rig arrives

Large-diameter work is decided in the office, not at the wall. Five questions answered in advance remove most of the improvisation that turns a 250 mm hole into a two-day job.

• Does the core have somewhere to go? A 300 mm plug weighs 15-25 kg and needs a trolley, a cradle or two people at the far side.
• Is there access on both faces for anchoring and for catching the core? Single-sided access means a through-rod and a stand, not a hand-held rig.
• What is behind the wall, within the core's length? A drilled hole that breaks into a riser, a duct or a neighbour's space is a different job.
• Is the wall structural or infill? Load-bearing walls need an engineer's sign-off before the layout is fixed.
• Where does the water go, and who takes the slurry? At 250 mm the volume is measured in tens of litres per metre of depth.

Answering these takes a twenty-minute site walk and prevents the three most common large-hole failures: a core that falls through into a finished space, an anchor that pulls out of a hollow wall, and a hole that has to be moved after it was cut.

The cost of a large core is mostly setup. An extra 30 minutes on anchors and piloting is cheaper than a bent bit or a damaged wall.