PVC Conduit Guide: Round vs Oval, 20mm vs 25mm, and Fittings for Electricians

Conduit is one of those things that separates a tidy job from a messy one. Run your cables loose and stapled, and the next spark who touches it is swearing. Run them in conduit and the job is future-proof, inspectable, and looks professional. This guide covers everything you need to pick the right PVC conduit and fittings for the job.

PVC vs Steel Conduit: Choosing the Right Material

PVC (rigid thermoplastic) conduit is the default choice for most domestic and commercial surface wiring in the UK. It’s lightweight, easy to cut, resistant to corrosion, electrically non-conductive, and costs a fraction of metallic alternatives. It’s also self-extinguishing once the heat source is removed, which matters for fire safety in concealed routes.

Steel conduit (galvanised or stainless) is the choice for industrial environments where the conduit itself needs to withstand mechanical impact — factory floors, plant rooms, exposed external runs, anywhere cables could be physically damaged. Steel conduit also provides a continuous earth path when correctly installed and bonded, which is valuable in some installations. But it requires threading, specialist tools, and significantly more labour. For most domestic and commercial jobs, PVC is the right call.

Round vs Oval Conduit: Different Jobs

Round conduit (circular cross-section) is the standard for multiple-cable containment. It’s available in 20mm and 25mm diameters for general wiring, and up to 32mm and 50mm for larger cable bundles or industrial work. Fittings — couplers, bends, tees, boxes — are all designed around the round profile. This is what you’ll use for the vast majority of conduit work: chased surface wiring, sub-main runs, cable drops at distribution boards.

Oval conduit is for concealed single-cable chases. It sits in a narrow channel cut into brick or block, plastered over. The oval profile is shallower than round, so the chase depth is minimal. It’s not a multi-cable containment system — it’s protection for a single cable run in a wall, typically from a switch to a fitting or from a socket back to a ring. It doesn’t use the same fittings as round conduit.

Don’t confuse the two. Oval conduit in a surface-wiring application looks wrong, doesn’t fit standard fittings, and won’t pull multiple cables effectively.

20mm vs 25mm: Sizing Round Conduit Correctly

Conduit sizing is governed by cable fill. BS EN 61386 (the UK conduit standard) doesn’t specify a hard percentage, but the IET Guidance Note 1 and general practice uses a 45% fill ratio — the cross-sectional area of all cables in the conduit should not exceed 45% of the conduit’s internal CSA. Beyond this, drawing cables becomes difficult and heat dissipation is impaired.

In practice:

• 20mm conduit comfortably takes 3–4 x 2.5mm² twin and earth cables, or 6–8 x 1.5mm² singles, or equivalent combinations. This covers the majority of domestic socket and lighting circuits.
• 25mm conduit is needed when running larger cables (6mm² or above), higher cable counts, or when armoured tails need a larger entry into a distribution board.

Always size up slightly if in doubt. Drawing cables through a full conduit ruins the cable insulation and costs you time. Conduit is cheap. Cable replacement is not.

White vs Black: Surface vs Exposed

White is the standard colour for interior surface wiring — it blends with painted plaster walls and ceiling surfaces. Black is used for external runs, exposed areas, and anywhere white would look industrial or out of place (plant rooms, meter cupboards, external walls). The material and mechanical properties are identical; it’s purely aesthetic and application-driven.

Both colours are available across the full ProFix fittings range APM stocks.

Key Fittings: What You Need and When

Couplers join two lengths of conduit end-to-end. The ProFix 20mm Conduit Couplers (Pack of 10) are the go-to for straight runs. Use solvent cement on the joints for a permanent, sealed connection; push-fit is fine for temporary or dry-area runs.

Female adaptors connect conduit to a knockout in a metal back box, consumer unit, or distribution board. The ProFix 20mm Female Adaptors Black (Pack of 10) and White (Pack of 10) screw into the knock-out thread, providing a smooth cable entry without sharp edges that would damage insulation.

Normal bends provide a smooth 90° change of direction in a run. The ProFix 20mm Normal Bend is a fixed radius — cables must be pulled through rather than pushed. Use these on straight, accessible runs with few cables. The larger radius reduces pulling friction significantly compared to tight inspection bends.

Inspection bends have a removable cover plate, allowing access to the cable bundle at the bend. The ProFix 20mm Inspection Bend White is essential on longer runs with multiple direction changes — it allows you to pull in sections and inspect or re-pull without dismantling the conduit. Always use inspection bends when two or more 90° changes occur in a single draw.

Inspection tees provide a branch point with an access cover. The ProFix 20mm Inspection Tee White is used at junction points where cables split to different destinations, allowing future access to each branch.

Terminal boxes terminate a conduit run at a switch, socket, or fitting position. The ProFix 20mm Terminal Box White (1-way) provides a single conduit entry with a backplate for mounting accessories. Use where a single conduit drops to a switch or socket position.

H-boxes (4-way boxes) are used at junction points where conduits meet from multiple directions — ceiling roses, mid-run junctions, distribution points. The ProFix 20mm H-Box White accepts four conduit entries at 90° to each other, making it the standard fitting for conduit junction work.

Space bar saddles secure conduit to walls and ceilings. The ProFix 20mm Space Bar Saddles Black (Pack of 10) clip around the conduit and fix through with a single screw. Maximum spacing: 500mm on horizontal runs, 800mm on vertical runs per BS EN 61386 — but tighter spacing (every 300–400mm) gives a neater finish and reduces sag over time.

Draw Wire and Cable Pull Technique

Install all conduit, boxes, and fittings before pulling any cables. Threading draw wire through the completed conduit run before sealing boxes is the mark of an organised job — you can pull in any cable at any time rather than having to dismantle the installation.

For short, straight runs, cables can be pushed through by hand. For runs over 3–4m with any bends, use a fish tape or draw wire. Attach cables securely to the draw wire — a poorly attached joint that pulls off inside a conduit means cutting it out or fishing for hours. For tight pulls, cable pulling lubricant cuts friction significantly and protects the insulation.

Always pull cables smoothly at a consistent tension. Jerking or forcing cables around tight bends damages insulation at the bend points — damage that won’t show up on initial testing but can cause premature failure or insulation breakdown under load.

Flexible Conduit: Where Rigid Won’t Work

Rigid PVC conduit cannot accommodate vibration, slight misalignment, or connection to moving plant. Where a fixed conduit run terminates at a motor, transformer, or vibrating equipment, the final 300–600mm of connection should be in flexible conduit. Polypropylene flexible conduit handles the mechanical flex and prevents vibration from cracking rigid fittings or fatiguing the cable insulation.

It’s also used for short tail connections at consumer units and distribution boards where the cable needs to move slightly during installation and termination. Never run a full circuit in flexible conduit — it’s for short connection sections only.

Pick It Up in Acton Today

APM Electricals stocks the full ProFix conduit fittings range and conduit pipe in 20mm and 25mm, white and black, at the trade counter in Acton. 24 Western Avenue, W3 7TZ. Call 020 8702 8080 to check stock. Same-day collection, no minimum order.