1mm, 1.5mm, 2.5mm or 4mm Cable: Which Twin & Earth Do You Need?

Specifying the wrong cable size is one of those mistakes that costs you on the return visit. This guide covers the most common domestic cable sizes — 1mm, 1.5mm, 2.5mm and 4mm Twin & Earth — and when each one is correct.

The Basics: What Does the Size Refer To?

Cable sizes refer to the cross-sectional area (CSA) of each conductor in square millimetres (mm²). A 2.5mm² cable has conductors with a cross-section of 2.5mm². Larger CSA = lower resistance = more current-carrying capacity.

Twin & Earth (T&E) cable — the standard for most UK domestic fixed wiring — has two insulated conductors (live and neutral) plus an uninsulated earth. It's specified for mains voltage (230V) circuits.

1mm² Twin & Earth

Current rating (clipped direct): 15A

Used for lighting circuits. A standard domestic lighting circuit typically runs at 6A with a 6A MCB — 1mm² is appropriate for this load. The most common use is the final ring or radial circuit for fixed lighting.

Also used for low-load signal circuits (doorbell transformers, some thermostat wiring) though purpose-made signal cable is sometimes more appropriate.

Don't use 1mm² for: Socket circuits, shower circuits, hob circuits, or any high-load application.

1.5mm² Twin & Earth

Current rating (clipped direct): 20A

Lighting circuits where runs are long and you want the extra headroom, or higher-load lighting (LED drivers, fluorescent luminaires). Some electricians default to 1.5mm² for all lighting as a belt-and-braces approach.

Also used for dedicated appliance spurs where the load is under 20A — fridge, freezer, some washing machines where a dedicated circuit is preferred over the ring.

2.5mm² Twin & Earth — The Workhorse

Current rating (clipped direct): 27A

The most-used domestic cable size. Protected by a 20A MCB on ring final circuits (the ring effectively halves the load) and used for:

• Ring final circuits (socket outlets) — the standard domestic ring
• Radial circuits up to 20A
• Dedicated appliance circuits: washing machine, dishwasher, tumble dryer, fridge-freezer
• Immersion heater circuits (with appropriate MCB)

If you're doing a rewire or extension and one cable size will go on the van for most jobs, 2.5mm² T&E is it. We cut it from the drum — any length, no minimum, no overbuying a 100m coil for a 12-metre circuit.

4mm² Twin & Earth

Current rating (clipped direct): 37A

Higher-load dedicated circuits:

• Electric showers (8.5kW and above)
• Electric hobs and ovens where load exceeds 2.5mm² rating
• EV charger supply cables (check manufacturer spec — some require 6mm² or 10mm²)
• Large instantaneous water heaters
• Outbuilding supplies where the final circuit is significant

For a 9.5kW shower: 9500W ÷ 230V = 41.3A. You need a 45A MCB and 6mm² cable — 4mm² isn't sufficient for this specific case. Always calculate actual load before specifying.

6mm² and Above

For high-load circuits — large showers, EV chargers, cookers, outbuilding feeds. See the wider electrical range for related accessories. Beyond the scope of this guide, but we stock it. Call us to confirm availability on specific sizes.

Derating: Why the Rated Current Isn't the Whole Story

The current ratings above assume cable clipped direct to a surface in open air (Installation Method C). If your cable is:

• Buried in thermal insulation — apply a derating factor (typically 0.5 for fully enclosed)
• Grouped with other cables — bunching reduces cooling; apply grouping factor
• In conduit — lower rating than clipped direct

Always refer to BS 7671 Appendix 4 for the correct current-carrying capacity for your installation method. The above figures are a starting guide, not a substitute for a proper calculation.

Quick Reference

Cut to Length from Our Acton Trade Counter

We cut Twin & Earth cable from the drum. Need 8 metres of 2.5mm² for a kitchen circuit extension? Pay for 8 metres. No 50m minimum, no leftover coil in the van.

This is an in-store service only — available at our Acton trade counter, Mon–Sat. Contact APM to confirm the size you need before visiting.

APM Plumbing & Electrical Supplies
24 Western Avenue, Acton, London W3 7TZ
📞 020 8702 8080 | 🌐 apmi.uk