Question 1

What size conduit do I need?

Accepted Answer

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.

Question 2

When should I use steel conduit instead of PVC?

Accepted Answer

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.

Question 3

What is the minimum bend radius for conduit?

Accepted Answer

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.

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 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

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

UK Conduit Sizes & Cable Fill

Standard Conduit Sizes

Cable Fill — Number of Singles per Conduit

Conduit Material Types

Minimum Bend Radius

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