Conduit Size Guide: How to Choose the Right Diameter for Your Cables
Don't get stuck halfway through a pull. Learn how to calculate exactly what size conduit you need for your cables, every time.
1️⃣ Why Size Matters (It's Not Just About Fitting In)
Novices think: "If the cable physically fits inside the tube, it's fine." This is wrong.
Overfilling conduit causes three major problems:
- ? Heat Build-up: Tightly packed cables cannot dissipate heat. This can de-rate your cable capacity or even melt insulation.
- ?♂️ Impossible Pulls: Friction increases exponentially. If you pack it tight, you will likely snap your draw tape or strip the cable sheath when pulling.
- ? Compliance Fail: The IET Wiring Regulations (BS 7671) have strict "Space Factor" rules to ensure safety.
2️⃣ The Golden Rule: Limit Fill to 45%
A good rule of thumb for safe installation is the 45% Rule.
Never fill the internal cross-sectional area of the conduit by more than 45%.
This leaves 55% "air space" inside the tube. This air space is vital for cooling the cables and allows enough room to navigate bends without jamming.
3️⃣ Capacity Cheat Sheet (Standard Singles)
Assuming a straight run up to 3 meters (adding bends reduces capacity!), here is a safe guide for standard stranded (6491X) single core cables:
| Cable Size (Singles) | 20mm Conduit Max | 25mm Conduit Max | 32mm Conduit Max |
|---|---|---|---|
| 1.5mm² | 7 cables | 12 cables | 20+ cables |
| 2.5mm² | 5 cables | 9 cables | 15 cables |
| 4.0mm² | 3 cables | 6 cables | 10 cables |
| 6.0mm² | 2 cables (Tight!) | 5 cables | 8 cables |
| 10.0mm² | Max 1 | 3 cables | 5 cables |
*Estimates based on typical UK stranded singles. Always check specific cable manufacturer OD (overall diameter).
4️⃣ Solid vs. Stranded: What’s the Difference?
Solid Core (Twin & Earth): Stiff and flat. Harder to pull around bends. You usually need larger conduit just to accommodate the flat profile.
Stranded (Singles): Flexible and round. Much easier to pull and packs more efficiently into round conduit.
Tip: If running Twin & Earth in conduit, treat the flat width as your "diameter" and assume it takes up closer to 60% effective space due to its awkward shape.
5️⃣ The "Plus One" Rule (Future Proofing)
Electrical needs rarely shrink; they only grow. Adding data lines, extra circuits, or smart home wiring is common.
The Strategy: Calculate what you need today, then go one size up.
- Need 20mm? → Install 25mm.
- Need 25mm? → Install 32mm.
Leaving a generic "draw rope" in your spare space allows you to pull a new cable through in minutes, years down the line, without chasing out walls again.
Get the Right Size First Time
Browse our full range of conduit sizes, from neat 20mm to heavy-duty 32mm.