PVC Conduit: Oval vs Round, When You Need It, and the Cable-First Rule Your Electrician Knows

Your builder finishes chasing the walls and your electrician runs the cables. Six months later, you hang a picture, drive a screw into the wall, and hit a live cable. The RCD trips. You're lucky. Without RCD protection, that screw through the cable could have killed you. Conduit is the thin PVC tubing that sits in the wall chase around your cables, giving them a physical barrier against exactly this kind of accident. It costs pennies per metre and takes minutes to install. Skipping it is a false economy that no competent electrician would consider.

What it is and what it's for

Conduit is PVC tubing that protects electrical cables from physical damage. In a domestic extension, you'll encounter two types: oval conduit for cables buried in wall chases, and round conduit for cables run on the surface of walls or ceilings. Both are made from high-impact uPVC (unplasticised polyvinyl chloride, the same rigid plastic used for waste pipes and guttering) and both serve the same purpose: stopping nails, screws, and other fixings from piercing the cable inside.

The distinction matters. Oval conduit has a flat, shallow cross-section that fits neatly into a narrow wall chase without requiring deep cutting into the masonry. Round conduit has a circular cross-section and is used where the cable runs along the surface of a wall, typically held in place with saddle clips. Your electrician will use oval conduit for 99% of the cable runs in a standard extension, because almost all domestic wiring is chased into the blockwork before plastering.

Why not just bury the cable directly?

You can. BS 7671 (the IET Wiring Regulations, 18th Edition) doesn't mandate conduit for every concealed cable. What it does mandate, under Regulation 522.6.202, is that any cable concealed in a wall at less than 50mm depth must be within a "prescribed zone" (the defined safe areas around accessories, corners, and ceiling/floor lines) AND protected by a 30mA RCD at the consumer unit. Meet both conditions and the cable can be buried directly in plaster or bonding compound without conduit.

So why bother with conduit at all? Two reasons.

First, protection during construction. A bare cable sitting in an open chase is vulnerable. The plasterer's trowel catches it. A labourer drops a lump of mortar on it. Someone treads on it while the wall is still open. Conduit prevents all of this.

Second, and this is the argument that pays for itself decades later: rewirability. A cable inside conduit can be pulled out and replaced without hacking off the plaster. One electrician on a rewire job found that a previous installer had used oval conduit to a switch position. He pulled the old cable out and fed the new one through in minutes. Without conduit, that same job would have meant chasing the wall open, replastering, and redecorating. On a full rewire with 30+ cable runs, conduit saves thousands in making-good costs.

Types, sizes, and profiles

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Oval conduit

This is the domestic workhorse. The flat, shallow profile means your electrician can chase a narrow slot into a block or brick wall and the conduit sits flush without needing excessive depth. It's white PVC, sold in 3m lengths (sometimes supplied as two 1.5m pieces joined together), and comes in three sizes:

The price range on 25mm reflects a real split between own-label brands (Toolstation's Profix at around ~£2 per 3m length) and premium brands (Univolt at over £5+ per 3m). For domestic wall chases, the cheaper option does the same job.

You cannot bend oval conduit into tight radiuses. Its flat profile makes it inherently rigid in one plane. It will flex gently to follow minor wall imperfections, but it won't go around a 90-degree corner. For direction changes, you cut the conduit and start a new straight run from the corner.

Round conduit

Round PVC conduit is used for surface-mounted cable runs: exposed wiring in garages, workshops, utility rooms, or anywhere you can't or don't want to chase into the wall. It comes in 20mm and 25mm diameters (plus 32mm for commercial installations), in black or white, and uses a complete system of fittings: couplers, bends, tees, inspection elbows, terminal boxes, and saddle clips.

Round conduit is more expensive per metre than oval (around £1 – £1 per metre for 20mm round, versus £0 – £1 for 20mm oval). The real cost difference is in the fittings. A surface-mounted round conduit system with bends, tees, junction boxes, and saddle clips every 300mm adds up quickly.

For a domestic extension where all the wiring is buried in wall chases, you're unlikely to need round conduit at all. Your electrician will use oval conduit in the chases and may use short sections of round conduit for any surface-mounted runs (a cable drop in the garage, a surface feed to an external light).

Flexible conduit

Corrugated flexible conduit (the black ribbed tubing) is used for awkward transitions: from a surface conduit run into a back box, around obstacles, or where vibration is a factor (near a boiler or mechanical plant). It costs around £1 – £2 per metre (a 10m roll is about ~£13 from Screwfix) and shouldn't be used as a substitute for rigid conduit on long runs. It's a connector, not a cable management system.

Steel conduit

You'll see galvanised steel conduit on commercial sites and in older industrial buildings. For domestic extensions, steel conduit is only required in specific circumstances: cables outside prescribed zones without RCD protection, cables in fire-rated walls or floors, or cables passing through walls containing metalwork (metal stud partitions fall under Regulation 522.6.203). Steel conduit that is properly earthed satisfies BS 7671 Regulation 522.6.204 as mechanical protection, meaning it can legally substitute for the prescribed-zone-plus-RCD requirement. PVC conduit cannot make that same claim.

Unless your electrician or building control officer specifically calls for steel conduit, PVC is standard for domestic work.

How to work with it

This section covers what your electrician does with conduit during first fix. You're not expected to install conduit yourself (all electrical work in an extension falls under Part P of the Building Regulations and must be done or certified by a qualified person). But understanding the process helps you spot problems and have informed conversations with your electrician.

The cable-first rule

This is the single most important thing to know about conduit installation, and no published guide aimed at homeowners explains it.

Feed the cable into the conduit before fixing the conduit in the chase.

Your electrician cuts the conduit to length, threads the cable through it on the floor or a workbench, then carries the whole assembly to the wall and fixes it into the prepared chase. Trying to pull cable through conduit that's already fixed in a wall chase, especially around any bends or through multiple joined sections, ranges from difficult to impossible. The cable jams, the conduit pulls away from its fixings, and the whole job takes twice as long.

Chase cutting

Chases in masonry walls must be vertical or horizontal. Never diagonal. BS 7671 prescribed zones assume cables run vertically from accessories or horizontally between accessories at the same height. A diagonal cable run is invisible to anyone who later drills or fixes into the wall, because they can't predict its path.

Structural limits apply to chase depth. For a 100mm block (standard internal wall thickness), the maximum vertical chase depth is one-third of the wall thickness: 33mm. The maximum horizontal chase depth is one-sixth: just 16mm. These are structural limits set by building regulations, not electrical rules, and they apply regardless of what's being chased in. An oval conduit at roughly 10mm deep fits comfortably within both limits. A round conduit at 20mm diameter needs a deeper chase and may breach the horizontal limit on a 100mm block wall.

This is another reason oval conduit dominates domestic chasing. It physically fits within the structural depth limits that round conduit may not.

Fixing methods

Oval conduit in a chase is held in place with one of three methods:

• Galvanised clout nails driven either side of the conduit (not through the middle). Fast, cheap, and the traditional trade method. Works well in aerated blocks and standard aggregate blocks.
• Plastic conduit clips (crampets) that grip the conduit. These need a slightly deeper chase because the clip adds a few millimetres of height. Available in 20mm and 25mm sizes (around £1.10 for a 5-pack from Screwfix).
• Adhesive (grab adhesive or similar) works on smooth PVC in dry conditions. Less common but useful on hard brick where nailing is difficult.

For hard engineering brick or dense concrete, drill and plug first (use red wall plugs), then nail into the plugs. For aerated blocks like Thermalite, nails go straight into the block face without any plugging.

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Filling and making good

After the conduit and cables are fixed, the plasterer fills the chases. The standard method: PVA the chase to seal the block surface, apply bonding compound (Thistle Bonding or similar) as bulk fill leaving a 2-3mm gap below the wall surface, then finish with the skim coat that covers the entire wall. In damp areas (bathrooms, utility rooms), use a cement-based filler instead of gypsum-based bonding.

If the conduit sits proud of the wall surface because the chase was cut too shallow, the plasterer has to build out around it. This creates a visible ridge in the finished wall that no amount of skimming will fully disguise. Chase depth matters. Get it right the first time.

The regulatory picture

You don't need to memorise the wiring regulations. Your electrician does. But understanding the logic behind conduit requirements helps you make sense of what's happening during first fix and why your electrician makes certain decisions.

BS 7671:2018+A2:2022 (the 18th Edition of the IET Wiring Regulations) governs all of this. Three regulations matter for conduit:

Regulation 522.6.200 covers cables under floors. A cable passing through a timber joist must be at least 50mm from the top or bottom of the joist, or enclosed in metallic conduit, trunking, or ducting. This is directly relevant if your extension includes an island unit with a floor-level electrical supply.

Regulation 522.6.202 covers cables concealed in walls. Any cable buried at less than 50mm depth must be in a prescribed zone and protected by a 30mA RCD. "Prescribed zones" are the predictable routes: vertically above or below an accessory, horizontally between accessories at the same height, within 150mm of a ceiling, floor, or wall corner.

Regulation 522.6.204 lists the alternative protection methods that can substitute for the prescribed-zone-plus-RCD requirement: earthed metallic sheath, earthed steel conduit, earthed steel trunking, or mechanical protection sufficient to prevent penetration by nails and screws.

In practical terms for your extension: your electrician will run cables in prescribed zones, protect them with 30mA RCDs at the consumer unit, and use oval conduit in the wall chases for construction-phase protection and future rewirability. That combination satisfies the regulations. The conduit isn't doing the regulatory heavy lifting (the prescribed zone and RCD are), but it's doing the practical heavy lifting that prevents damage during the build and makes life easier for whoever rewires the house in 25 years.

How much do you need

Conduit quantities depend on your electrical layout. A typical single-storey rear extension (20-25m2) with a kitchen might have 15-25 cable runs from the consumer unit to individual accessories (sockets, switches, cooker connection, extractor fan, island supply). Each cable run that passes through a masonry wall chase needs conduit for the chased section.

Not every run needs the same length of conduit. A vertical drop from ceiling level to a socket is about 700mm. A vertical drop to a light switch is about 900mm. A horizontal run between two sockets at the same height might be 1-2m. The total conduit needed for a typical extension electrical installation is usually 15-30 metres.

Add 10-15% for waste (cut-offs from matching lengths to chase positions) and buy a couple of spare 3m lengths. Conduit is cheap enough that having leftovers is better than sending someone back to the supplier mid-install.

For a 20m2 extension, budget for around 10-12 lengths of 3m oval conduit (30-36 metres). At current retail prices that's £13 – £24 for 20mm or £20 – £60 for 25mm depending on brand. It's one of the cheapest materials on the entire job.

Cost and where to buy

Conduit pricing is straightforward and consistent across the major UK retailers. The material cost is trivial compared to the labour cost of installing it.

Electrical wholesalers (CEF, Edmundson, your local independent) stock the same products, often at lower trade prices, but don't publish pricing online. If your electrician buys through a wholesaler, the conduit cost on his materials invoice should be lower than retail.

The premium brands are Marshall-Tufflex (manufactured from 99% recycled PVC, their Supertube range) and Univolt (Dietzel-Univolt, compliant with BS 4607). The own-label brands at Toolstation (Profix) and Screwfix (Deta TTE) are perfectly adequate for domestic chasing. Your electrician will have a preference; it's rarely worth arguing about.

Don't forget fittings if you're pricing a surface-mounted round conduit run. Couplers are about 45p each, inspection bends and elbows about £1.20 each, and terminal boxes about £2 each. A surface run with four direction changes and two junctions can easily cost more in fittings than in conduit.

Alternatives

Cable capping (new builds)

In new-build construction on bare blockwork (before any plaster), the standard practice is to use plastic or metal cable capping instead of conduit. Capping is a flat strip nailed over the cable after it's been laid in the chase. It's faster to install than conduit because there's no threading of cable required.

The trade rule of thumb: capping in new builds, conduit in rewires and extensions. In a new build, the electrician is working on bare block before any plaster, and the cable routes are clear and predictable. In a rewire or extension where existing plaster may be involved, oval conduit makes more sense because it protects the cable during the messy process of chasing through existing finishes.

Metal capping is preferred over plastic in situations where the wall will be finished with wet plaster (bonding coat plus skim), because plaster adhesion to plastic surfaces is unreliable. Your Building Control officer may raise this if plastic capping is used under wet plaster.

Remember: capping is not classified as mechanical protection under BS 7671 Regulation 522.6.204. It's construction-phase protection only.

Service void

An alternative to chasing entirely: fix 25mm timber battens to the wall surface, run cables horizontally behind them, then plasterboard over the top. This creates a service void that avoids weakening the masonry with chases, makes future cable access trivial, and costs nothing in conduit. The trade-off is that you lose 25-30mm of room width on every battened wall, and the plasterboard fixing is more involved than plastering directly onto block.

This approach is more common in Scotland (where internal wall construction traditions differ) and in energy-efficient builds using thicker insulation layers that already create a service void.

Armoured cable

For cables that must run outside prescribed zones and without 30mA RCD protection (rare in domestic work, but possible in outbuildings or detached garages), steel wire armoured (SWA) cable provides its own mechanical protection without needing separate conduit. SWA is significantly more expensive than standard twin-and-earth plus conduit, but it eliminates the conduit system entirely.

Where you'll need this

• First fix electrics - oval PVC conduit protecting cables run through chases in masonry walls before plastering
• Electrical layout planning - identifying which cable routes need conduit, particularly floor runs to an island unit

Conduit appears at the first fix stage of any extension or renovation project where cables are being chased into masonry walls. If your project involves electrical work in solid walls (and almost all UK extensions do), conduit will be on the materials list.

Common mistakes

Ordering 20mm when you need 25mm. The price difference between 20mm and 25mm oval conduit is negligible (67p per 3m length at Toolstation). But trying to pull 2.5mm twin-and-earth through 20mm conduit with even one gentle bend will have your electrician cursing your name. Buy 25mm unless the run is dead straight and under a metre.

Cutting chases too shallow. The conduit must sit fully below the wall surface with room for fill on top. A 20mm oval conduit needs a chase at least 12-15mm deep. If you're using plastic clips instead of nail fixings, add another 3-4mm. A conduit that sits proud of the wall creates a visible ridge under the plaster that no amount of skimming will hide.

Pre-fixing conduit before the electrician arrives. Your electrician needs to position conduit based on the actual cable routing plan. If you've already fixed conduit in positions that don't match the routes, it all has to come out and be redone. Cut the chases. Leave the conduit unfixed. Let the electrician do the positioning.

Ignoring insulation derating. This catches people planning full conduit systems in insulated stud walls (timber frame with quilt insulation). When conduit containing sheathed cable is surrounded by insulation, the cable's current-carrying capacity drops significantly. A standard 2.5mm twin-and-earth cable that normally handles a 20A ring final may need to be upsized to 4mm or even 6mm when enclosed in conduit surrounded by insulation. This is a specialist calculation. If your extension has insulated stud walls rather than masonry, your electrician needs to account for derating factors in the cable sizing. Don't assume standard cable sizes are adequate without checking.

Using round conduit in wall chases. Round conduit needs a deeper chase than oval. On a standard 100mm block wall, the horizontal chase depth limit is 16mm. A 20mm round conduit won't fit within that limit. Oval conduit, at roughly 10mm deep, fits comfortably. Use oval for chases, round for surface-mounted runs.