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UK Conduit Sizes & Cable Fill
20 mm, 25 mm and 32 mm conduit in PVC and galvanised steel with cable fill calculations and bend radius requirements.
Standard Conduit Sizes
| Nominal Size (mm) | External OD (mm) | Internal Bore (mm) | Internal Area (mm²) | 45% Fill (mm²) |
|---|---|---|---|---|
| 16 | 16 | ~12.6 | ~125 | ~56 |
| 20 | 20 | ~16.1 | ~204 | ~92 |
| 25 | 25 | ~21.1 | ~350 | ~158 |
| 32 | 32 | ~27.3 | ~585 | ~263 |
Cable Fill — Number of Singles per Conduit
Based on single-core PVC-insulated cables (6491X) at 45% fill:
| Cable Size (mm²) | Cable OD (mm²) | 20 mm conduit | 25 mm conduit | 32 mm conduit |
|---|---|---|---|---|
| 1.0 | ~5.8 | 3 | 5 | 9 |
| 1.5 | ~6.5 | 2 | 4 | 7 |
| 2.5 | ~7.8 | 1 | 3 | 5 |
| 4.0 | ~9.0 | 1 | 2 | 4 |
| 6.0 | ~10.5 | 1 | 1 | 3 |
Values are approximate — always verify with manufacturer data and BS 7671 tables.
Conduit Material Types
| Type | Material | Application | Notes |
|---|---|---|---|
| Round PVC | Rigid uPVC | Concealed domestic, surface light commercial | Light, cheap, easy to work |
| Oval PVC | Rigid uPVC (oval section) | Concealed in plaster (shallower chase) | Reduces chase depth in walls |
| Flexible PVC | Corrugated PVC | Awkward routes, ceiling voids | Easy routing, not for surface mounting |
| Galvanised steel | Hot-dip galvanised steel | Industrial, commercial, fire-rated areas | Can serve as CPC, high mechanical protection |
| Stainless steel | 304 / 316 stainless | Corrosive environments, food processing | Expensive, specialist use |
Minimum Bend Radius
| Conduit Size (mm) | Min. Internal Bend Radius (mm) |
|---|---|
| 16 | 40 |
| 20 | 50 |
| 25 | 63 |
| 32 | 80 |
Related Calculators
See the Cable Trunking Sizes for trunking options, or the UK Cable Sizes for cable specifications. See also the Conduit & Trunking Fill Calculator.
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Frequently Asked Questions
The conduit size depends on the number and size of cables it must carry. The 45% fill rule applies — cables should not occupy more than 45% of the conduit internal cross-sectional area. For a single 2.5 mm² T&E cable, 20 mm conduit is sufficient. For 2-3 cables, 25 mm is usually needed. For 4+ cables or larger cable sizes, 32 mm is required. Always calculate the actual fill.
Steel (galvanised) conduit provides mechanical protection and can also serve as the circuit protective conductor (CPC/earth), reducing the need for a separate earth wire. It is required in some commercial and industrial settings for fire resistance and mechanical protection. PVC conduit is lighter, cheaper, corrosion-resistant and easier to work with — standard for most domestic and light commercial concealed wiring.
The internal radius of any bend must be at least 2.5 times the external diameter of the conduit (BS 7671). For 20 mm conduit, minimum bend radius is 50 mm. For 25 mm, it is 62.5 mm. For 32 mm, it is 80 mm. Tighter bends damage cables during pulling and restrict future cable withdrawal. Use factory-made bends or a proper conduit bender — never heat PVC conduit with a naked flame.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.