How the Cable Size Calculator Works

Enter the circuit load, cable run length and how cable will be installed. The calculator works out design current, applies relevant correction factors and recommends the smallest standard cable size that carries load safely while keeping voltage drop within limits.

Under BS 7671, the IET Wiring Regulations, every cable must satisfy three checks. It must carry design current continuously without overheating. Voltage drop along run must stay within 3 percent for lighting and 5 percent for other circuits. And the earth fault loop impedance must le protective device disconnect fast enough under a fault. This tool covers first two, third depends on supply impedance at your property and must be measured and verified on site by a qualified electrician.

Common UK Cable Sizes by Circuit

Domestic wiring in the UK uses flat twin and earth cable in a small set of standard sizes:

• Lighting circuits (6A): 1.0mm² or 1.5mm²
• Ring final socket circuits (32A): 2.5mm²
• Radial socket circuits (20A): 2.5mm²
• Cookers and hobs: 6mm² typically
• Electric showers 8kW and above: 10mm²
• EV chargers (7kW): commonly 6mm² or 10mm² depending on route and length
• Outdoor supplies to garages and garden offices: SWA armoured cable sized to load and run length

These are typical selections rather than rules, because the installation conditions can change the answer, which is what the next section is about.

Why Installation Method Changes Everything

A cable’s published rating assumes ideal conditions, 30 degree ambient temperature, clipped to a surface in free air, running alone. Real installations rarely look like that and BS 7671 applies correction factors for temperature, grouping with other cables and contact with thermal insulation.

Thermal insulation is one that catches people out. A cable surrounded by loft or wall insulation can lose half its rated capacity. This is exactly why the shower question generates so much debate, a 6mm² cable can pass the calculation for an 8.5kW shower when clipped direct, yet moment route passes through insulated voids derating makes it inadequate. That is why UK trade standard for showers of 8kW and above is 10mm² regardless, giving headroom for real world routing.

Voltage Drop on Long Runs

On longer cable runs voltage drop becomes deciding factor before current capacity does. The check is simple, voltage drop equals the cable’s millivolt per amp per metre figure times current times length, divided by 1,000. A 2.5mm² twin and earth cable drops 18mV/A/m, so a 20A load over a 15m run loses 5.4V, which is 2.3 percent of 230V and comfortably inside the 5 percent limit.

Runs to garden buildings and EV chargers are where this bites. A cable that easily carries current over 10 metres may need upsizing at 30 metres purely to stay inside the voltage drop limit.

The Legal Bit: Part P

In England and Wales, new circuits and significant alterations are notifiable under Part P of Building Regulations, meaning work must be done or certified by a registered competent person or notified to building control. This calculator is a design guide for understanding and planning. The final cable selection, installation and testing belong with a qualified electrician who can verify earth fault loop impedance and issue the certification your insurance and any future house sale will ask for.