Electrical conduit provides mechanical protection and a neat, professional containment system for cables in domestic, commercial, and industrial installations. This guide covers PVC conduit, galvanised steel conduit, flexible conduit, sizing and fill calculations, bending techniques, and installation best practice to BS 7671.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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Electrical conduit is a tube or channel used to protect and route electrical cables within a building or structure. It serves three primary purposes: mechanical protection of the cables from physical damage, providing a defined cable route for neat and professional installation, and in some cases, acting as the circuit protective conductor (CPC) for the circuits it contains.
Conduit is classified as a wiring system under BS 7671 Chapter 52, which sets out the requirements for the selection and erection of wiring systems. The choice of conduit type, size, and installation method depends on the environment (indoor or outdoor, domestic or industrial), the number and type of cables to be contained, the required level of mechanical protection, and any specific environmental conditions such as temperature, moisture, or chemical exposure.
In the UK, the three main types of conduit used in electrical installations are PVC (polyvinyl chloride) conduit, galvanised steel (GI) conduit, and flexible conduit. Each has distinct characteristics, advantages, and applications. Understanding when to use each type — and how to size and install it correctly — is fundamental to professional electrical installation work.
See our trunking installation guide for the alternative containment system used where larger cable capacities or surface-mounted distribution is required.Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
PVC (polyvinyl chloride) conduit is the most widely used conduit type in domestic and light commercial electrical installations. It is lightweight, easy to cut and bend, corrosion-resistant, and significantly cheaper than steel conduit. PVC conduit does not conduct electricity, so it does not require earthing and cannot serve as a CPC.
PVC conduit fittings include bends (90-degree and 45-degree), tees, elbows, inspection bends, couplers, saddles, and terminal boxes. All joints are made using solvent cement (PVC adhesive), which creates a permanent, airtight bond. For some applications, push-fit fittings are available that allow easier disassembly but do not provide the same seal as solvent-welded joints.
A key limitation of PVC conduit is temperature sensitivity. Standard PVC conduit becomes brittle at temperatures below minus 5 degrees Celsius and softens above 60 degrees Celsius. It should not be used in direct sunlight for extended periods (UV degradation), near heat sources, or in cold-store environments. For these applications, steel conduit or special high-temperature PVC conduit should be used instead.
Galvanised steel (GI) conduit is the standard containment system for commercial, industrial, and hazardous area electrical installations. It provides superior mechanical protection, fire resistance, and electromagnetic screening compared to PVC conduit. The galvanised finish provides corrosion resistance in normal environments.
Steel conduit is available in 20 mm and 25 mm nominal sizes (the most common for general installation work) and 32 mm, 38 mm, and 50 mm for heavy-duty and multi-circuit applications. It is supplied in standard 3.75-metre lengths and is joined using threaded couplers, locknuts, and bushes.
When steel conduit is used as the CPC, all joints must maintain electrical continuity. Threaded joints with proper coupling and locknut installation provide this continuity. Where the conduit route includes accessories or junction boxes, an earth continuity strap or flyover lead should be used to bridge the joint and ensure unbroken continuity.
Flexible conduit provides mechanical protection for cables where a rigid conduit run is impractical — typically for the final connection to equipment that vibrates, moves, or needs to be repositioned for maintenance. It is used between a rigid conduit system and the equipment connection point.
Standard PVC flexible conduit (corrugated or smooth bore) provides light to moderate mechanical protection. It is commonly used for final connections to luminaires, small motors, and equipment mounted on vibrating structures. Available in 20 mm, 25 mm, and 32 mm sizes. PVC flexible conduit does not provide an earth continuity path and cannot serve as a CPC.
Metal flexible conduit (often called Copex, after a major brand) uses interlocking galvanised steel strip with a PVC outer covering. It provides much better mechanical protection than PVC flexible conduit and can carry an earth continuity path through the interlocking metal construction. Available in sizes from 16 mm to 50 mm. Metal flexible conduit is specified for connections to motors, transformers, and industrial equipment where vibration and movement are present.
Low Smoke Zero Halogen flexible conduit is specified where fire performance and low smoke emission are required — typically in commercial buildings, transport infrastructure, and public buildings. LSZH flexible conduit produces minimal smoke and no toxic halogen gases when exposed to fire.
Flexible conduit should only be used for the minimum length necessary — it should not be used as a substitute for rigid conduit over long runs. The typical maximum recommended length for a flexible section is 450 mm to 600 mm, though this depends on the application. Always use appropriate adaptors to connect flexible conduit to rigid conduit or equipment enclosures.
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Selecting the correct conduit size is essential for a successful installation. Too small, and cables cannot be drawn in without damage. Too large, and the installation is unnecessarily expensive and bulky. BS 7671 limits the proportion of conduit internal area that can be occupied by cables — this is the conduit fill calculation.
BS 7671 limits conduit fill to approximately 40% of the internal cross-sectional area of the conduit. This 40% maximum applies to the total cross-sectional area of all cables (measured over the insulation) as a proportion of the internal area of the conduit.
The 40% limit serves two purposes: it ensures cables can be drawn into the conduit without excessive force (which could damage the insulation), and it provides sufficient air space around the cables for heat dissipation. Cables packed too tightly in a conduit overheat because they cannot dissipate the heat generated by the current flowing through them.
For longer conduit runs or runs with multiple bends, a lower fill percentage (typically 35% or even 30%) is advisable to reduce drawing friction and prevent cable damage during installation.
| Conduit Size | Internal Diameter | Internal Area | 40% Fill Area |
|---|---|---|---|
| 20 mm PVC | 17.1 mm | 230 mm squared | 92 mm squared |
| 25 mm PVC | 21.2 mm | 353 mm squared | 141 mm squared |
| 32 mm PVC | 27.6 mm | 598 mm squared | 239 mm squared |
| 20 mm Steel | 16.1 mm | 204 mm squared | 81 mm squared |
| 25 mm Steel | 21.1 mm | 350 mm squared | 140 mm squared |
Elec-Mate's conduit fill calculator instantly determines the maximum number of cables for any conduit size and cable combination.
Try it free for 7 daysThe ability to produce neat, accurate bends in conduit is one of the key skills for an electrician. Poor bending results in kinks, flattened sections, and restrictive bends that prevent cables from being drawn through and look unprofessional.
PVC conduit is bent using heat — either a bending spring inserted into the conduit (for simple bends) or a PVC conduit bending machine that applies heat and pressure simultaneously (for precise, repeatable bends). When using a bending spring, heat the conduit evenly with a hot air gun along the section to be bent, then bend it smoothly around the spring. Allow the conduit to cool completely before removing the spring. Avoid overheating, which causes the PVC to blister and weaken.
Steel conduit is bent using a mechanical conduit bender (Hilmor or equivalent). The bender provides leverage and a former that guides the conduit into a smooth curve without kinking. Common bends include 90-degree sets, double sets (for negotiating obstructions), bridge bends (for crossing other conduit runs), and kick bends (for transitioning from horizontal to vertical). Accurate measurement, marking, and bender positioning are essential for producing bends that meet the required dimensions and angles.
Kinks occur when the bend is too tight or the bending force is applied at a point rather than distributed along the bend radius. Flattening occurs when the conduit is overbent or bent without proper support. Both defects restrict cable pulling space, increase friction, and can damage cable insulation during installation. If a conduit section is kinked or flattened, it must be replaced — do not attempt to straighten and re-use it.
The minimum bending radius for conduit depends on the type and size. For PVC conduit, the minimum internal bending radius is typically 4 times the conduit diameter. For steel conduit, the minimum is typically 2.5 times the diameter when using a proper bending machine. Always check the manufacturer's specifications for the specific conduit type being used.
A well-installed conduit system should be mechanically secure, provide a continuous and unobstructed path for cable drawing, and present a neat, professional appearance. Following installation best practice prevents problems during cable pulling and reduces the risk of future cable damage.
For commercial and industrial installations, the conduit system is typically installed during the first fix phase, before walls are lined and ceilings are closed. This allows the conduit to be concealed within the building fabric. For surface-mounted conduit in finished buildings (refurbishments, additions), ensure the conduit is routed neatly, level and plumb, with consistent fixing centres.
First fix electrical guide — covers conduit installation as part of the first fix sequence.Elec-Mate's AI installer agent provides conduit sizing guidance, bend calculations, and installation sequences.
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