PVC Conduit and Oval Conduit — Surface Cable Wiring, Junction Boxes, and Fittings for UK Electricians

PVC Conduit and Oval Conduit — Surface Cable Wiring, Junction Boxes, and Fittings for UK Electricians

PVC conduit is one of the most versatile materials in an electrician's toolkit. Whether you're chasing cable into a plasterboard partition, running exposed wiring along a garage wall, or protecting T&E behind tiles, the right conduit keeps the installation clean, compliant, and future-proof. This guide covers the main types, sizing rules, fitting techniques, and when each type is the correct choice under BS 7671.

Oval Conduit vs Round Conduit — When to Use Each

These two types are often confused, but they serve quite different purposes.

Oval conduit (also called flat conduit or capping) is designed to sit in a chase cut into brick, block, or plasterboard. It's slim — typically 16mm or 20mm wide and around 8–10mm deep — so it minimises the depth of the chase needed. Once the cable is laid in, the conduit is fixed with saddles and skimmed over with plaster or board finish. The oval profile keeps the conduit close to the wall and distributes pressure evenly if someone drives a fixing near it.

Round conduit is a full circular-section tube, available in 16mm, 20mm, 25mm, and 32mm diameters. It's used for surface-mounted exposed runs — along garage walls, in plant rooms, outbuildings, and commercial installations — or buried in concrete floor slabs. Round conduit is far more mechanically robust than oval capping and is suitable for installations where the wiring needs physical protection rather than just containment.

Under BS 7671 Regulation 522.8, mechanical protection must be appropriate to the environment. Oval conduit in a plastered chase on a domestic wall is fine; oval conduit on a garage wall where it could be hit by a car door is not — that wants round conduit or trunking.

Sizing: How Many Cables Fit?

BS 7671 Appendix 5 defines cable grouping factors, and the same logic applies to conduit fill. The general rule is that you should be able to draw cables through without excessive friction — the conduit should be no more than 45% full by cross-sectional area when you're pulling multiple cables.

In practice for domestic work:

• 20mm oval conduit: comfortable for one or two 1.5mm² or 2.5mm² T&E cables side by side, lying flat
• 25mm oval conduit: suits larger cables or three-core CY / SY screened cables
• 20mm round conduit: two or three 1.5mm² singles, or one 2.5mm² T&E with a draw cord alongside
• 25mm round conduit: more generous fill for ring main cable or multiple singles

Always plan draw points. For conduit runs over 5–6 metres, or with more than two 90° bends, fit an inspection elbow or conduit box at each bend. Trying to pull cable through a long run with tight bends without an access point will damage the cable sheath and defeat the purpose of the conduit.

Fixing and Spacing

Saddles or space-bar saddles (for round conduit) should be spaced at:

• Horizontal runs: 750mm–1000mm centres for 20mm conduit
• Vertical runs: 1000mm–1500mm centres
• Always fix within 150mm of each junction box, bend, or entry into equipment

For oval conduit in chases, use oval conduit clips fixed every 500–600mm. Don't rely on the plaster alone to support the conduit — it must be mechanically fixed first.

Conduit Boxes and Junction Points

Every conduit installation needs conduit boxes at junction points, bends, and entry points to luminaires or socket plates. The main types are:

• Through box (straight coupling): inline access for pulling cables
• Elbow (90° bend): changes direction at a right angle
• Tee box: splits a run in two directions
• Intersection / 4-way box: cross junction for four conduit runs converging
• Angle box: a square or round surface-mounted box for joining cables at a corner

Conduit boxes used as junction boxes must comply with BS EN 60670. The box itself is only the enclosure — if you're terminating cables in the box rather than just passing through, you must use appropriate connectors (WAGO levers or similar) and the box must be accessible for inspection or maintenance. Sealed-in junctions are not permitted under BS 7671 Regulation 526.3 unless specifically rated as maintenance-free.

Conduit on a Lighting Circuit

Surface-mounted round conduit is the method of choice in many commercial and industrial installations where you need to add circuits to an existing building without chasing. The sequence is:

1. Mark the route and fix conduit saddles at correct centres
2. Cut conduit to length — use a fine-tooth saw and deburr the cut end with a file or deburring tool so the cable sheath isn't damaged on draw-in
3. Fit the conduit into saddles and push into conduit boxes
4. Thread a draw cord through the run before adding more bends
5. Pull cables through — use cable lubricant on longer runs
6. Terminate at the fixture or accessory, leaving adequate tail length (150mm minimum from the face of the box)

For domestic lighting in a bathroom or shower room (Zones 1–2), surface-mounted conduit must have an IP rating appropriate to the zone. Standard PVC conduit is not rated for Zone 1 spray — if you're running a supply to a fan or lamp in Zone 1, use IP65-rated fittings and check that the conduit entry points are sealed.

Oval Conduit Behind Tiles

When running cable behind ceramic or porcelain tiles — behind a shower enclosure or kitchen splashback — oval conduit is essential. Even if the tiles sit on cement board or direct adhesive, the cable must be mechanically protected; tile alone is not adequate protection against accidental penetration (BS 7671 Reg 522.6.202).

Use 20mm oval conduit in a shallow chase, or clip it to the surface before tiling over. Ensure the conduit terminates in a conduit box at each end so the cable can be drawn out without disturbing the tiles if it ever needs replacing.

Earthing Conduit Correctly

Steel conduit can be used as a protective conductor (CPC) where it meets the requirements of BS 7671 Table 54.2 — the conduit must be continuous, mechanically sound, and all joints must be confirmed as electrically connected. PVC conduit cannot serve as a CPC and must always contain a separate earth conductor.

On domestic installations, PVC oval or round conduit will always run T&E cable (which has its own CPC) or singles with a separate green/yellow earth. There's no ambiguity — PVC doesn't conduct, so a separate earth is always needed.

Inspection and Testing After Installation

After completing a conduit installation, the cables inside should be tested in the normal way as part of the periodic inspection or initial verification:

• Insulation resistance (typically ≥1 MΩ at 500V DC for each circuit)
• Continuity of CPC (R1+R2 loop)
• Correct polarity throughout
• PSCC / EFLI for each circuit at the furthest point

The conduit itself doesn't change these tests, but ensure all conduit box lids are fitted before the test — open enclosures can introduce measurement anomalies on IR tests if the damp atmosphere affects surface leakage.

Choosing the Right Conduit for the Job

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