Cable Sizing Guide - How to Calculate the Correct Cable Size UK
Selecting the correct electrical cable size is a legal requirement under BS7671 wiring regulations and a critical factor for the safety, performance, and longevity of any electrical installation. An undersized cable will overheat, trip protective devices, and present a fire risk. An oversized cable costs more than it needs to.
This guide explains how cable size is calculated in the UK, the four factors that determine the correct size (current, voltage drop, installation method, and ambient conditions), common cable sizes for domestic and commercial applications, and how to use our free cable size calculator for a quick answer.
Need a quick answer? Use our free cable size calculator to work out the correct cable size for your installation in seconds. Just enter the load in amps, the cable run length, and the installation method.
How to Calculate Cable Size in 4 Steps
Cable size calculation in the UK follows a standard process defined by BS7671 (the IET Wiring Regulations, 18th Edition). The cross-sectional area of the conductor, measured in square millimetres (mm²), must be sufficient to carry the design current without excessive voltage drop or overheating. Four factors determine the correct cable size.
Calculate the Design Current (amps)
Start with the load in watts or kilowatts. Divide watts by voltage to get amps. For a 7kW EV charger on a 230V supply: 7000 / 230 = 30.4 amps. For 3 phase loads (400V), use P / (1.732 × 400). This is the current the cable must carry continuously.
Apply Correction Factors for Installation Method
Cable current carrying capacity depends on how it is installed. Cables in insulation, grouped with other cables, or in high ambient temperatures carry less current than cables in free air. BS7671 Table 4D gives the reference current rating, and correction factors (Ca, Cg, Ci) reduce this to the actual rating for your installation.
Check Voltage Drop Over the Cable Length
BS7671 limits voltage drop to 3% for lighting and 5% for other circuits. Long cable runs need larger conductors to keep voltage drop within limits. A 30 amp circuit running 30 metres in 2.5mm² cable may be fine for current rating but fail on voltage drop - you would need 4mm² or larger.
Verify the Circuit Protective Device
The MCB, RCD, or fuse protecting the circuit must operate before the cable is damaged. The protective device rating must be equal to or less than the cable's corrected current carrying capacity, and greater than or equal to the design current.
Our cable calculator does all four steps automatically. For detailed current ratings and correction factors, see our cable selection chart.
Common Cable Sizes and Applications UK
The most common cable sizes used in UK domestic and commercial installations range from 1mm² to 25mm². The table below shows typical applications for each size - always verify with a full calculation for your specific installation.
| Cable Size | Typical Current Rating | Common Applications |
|---|---|---|
| 1.0mm² | 11-14 amps | Lighting circuits, small lighting loads |
| 1.5mm² | 14-18 amps | Lighting circuits, radial socket circuits under 16A |
| 2.5mm² | 20-27 amps | Ring main sockets, 20A radial circuits |
| 4mm² | 26-37 amps | Cooker circuits, 32A radial, longer 20A runs |
| 6mm² | 34-47 amps | Electric showers (up to 9.5kW), cookers, EV chargers (7kW) |
| 10mm² | 46-65 amps | Large showers, sub-main cables, EV chargers on long runs |
| 16mm² | 61-87 amps | Sub-mains, meter tails, consumer unit feeds |
| 25mm² | 80-115 amps | Main tails, sub-mains to detached buildings |
Current ratings shown are approximate and vary by cable type (T&E, SWA, flexible), installation method (clipped direct, in conduit, buried), and ambient temperature. Always refer to BS7671 Appendix 4 or use a cable calculator for compliant sizing.
Voltage Drop - Why Cable Length Matters
Voltage drop is the reduction in voltage along the cable caused by the cable's own resistance. The longer the cable and the higher the current, the greater the voltage drop. BS7671 sets maximum permitted voltage drop at 3% for lighting circuits and 5% for other circuits, measured from the origin of the installation (the consumer unit) to the equipment.
For a 230V single phase supply, 3% equals 6.9 volts and 5% equals 11.5 volts. If voltage drop exceeds these limits, you must use a larger cable size. This is why long outdoor runs to garages, outbuildings, or EV charging points often need a larger cable than the current rating alone would suggest.
Rule of thumb: For every doubling of cable length, voltage drop doubles. A 20 metre run has twice the voltage drop of a 10 metre run at the same current. For long runs, always go up a cable size.
How Installation Method Affects Cable Size
The way a cable is installed directly affects how much current it can safely carry. A cable buried in thermal insulation cannot dissipate heat as effectively as one clipped to a wall in free air. BS7671 defines seven reference installation methods (A to G) with different current ratings for the same cable size.
Common Installation Methods
- Method A: Cable in conduit in a thermally insulated wall - reduced current rating
- Method B: Cable in conduit on a wall - standard rating
- Method C: Cable clipped direct to a wall or surface - standard rating
- Method D: Cable buried directly in the ground or in a duct - reduced rating
- Method E: Multicore cable in free air - full rating
- Method F: Single core cables in free air - full rating
- Method G: Single core cables spaced in free air - highest rating
For underground installations using SWA armoured cable, method D applies. For surface-clipped T&E in domestic wiring, method C is typical. Grouping (multiple cables touching) and ambient temperature above 30°C further reduce the current rating.
Cable Type and Conductor Material
Conductor material and cable type both affect the current carrying capacity and the correct size for an application. Copper is the standard conductor in UK installations, offering high conductivity and flexibility. Aluminium is used for larger sub-mains and underground supplies where weight and cost matter - aluminium requires approximately 1.6 times the cross-sectional area of copper to carry the same current.
Common Cable Types for Different Applications
- Twin and Earth (T&E): Domestic fixed wiring - lighting, socket, cooker and shower circuits
- SWA armoured cable: Outdoor, underground, industrial mains power - see our SWA cable range
- Single core 6491X/6491B: Panel wiring, control panel installations, meter tails
- Flexible H07RN-F: Trailing supply cable, construction sites, temporary power
- MV 11kV cable: Utility distribution, DNO and IDNO networks - see our MV cable range
Cable Sizing FAQ
How do you calculate cable sizing?
Calculate cable size by determining the design current (watts / voltage = amps), applying correction factors for installation method and ambient temperature, checking the voltage drop over the cable length does not exceed BS7671 limits (3% lighting, 5% other), and verifying the circuit protective device matches the corrected current rating. Use our cable calculator to automate all four steps.
Do I need 1.5mm or 2.5mm cable for sockets?
UK ring main socket circuits require 2.5mm² twin and earth cable protected by a 32A MCB. 1.5mm² is not suitable for socket ring mains - it is used for lighting circuits (typically protected by 6A or 10A MCB). A 20A radial socket circuit can use 2.5mm² up to around 30 metres, with 4mm² needed for longer runs.
Can a 6mm cable take 40 amps?
6mm² twin and earth cable has a current rating of approximately 34-47 amps depending on installation method. Clipped direct (method C) it is rated around 47A, but in insulation (method A) it drops to around 34A. For a 40 amp circuit, 6mm² is suitable if the installation method allows - otherwise 10mm² may be required.
What size cable do I need for a 7kW EV charger?
A 7kW EV charger draws approximately 30 amps at 230V. For typical installations, 6mm² twin and earth or 6mm² SWA is commonly used for cable runs up to around 20 metres. Longer runs from the consumer unit to the charge point may need 10mm² or larger to keep voltage drop within BS7671 limits. Always confirm with a calculation based on the actual run length and installation method.
What size cable do I need for a 9.5kW electric shower?
A 9.5kW electric shower draws approximately 41 amps at 230V. 10mm² twin and earth cable protected by a 45A or 50A MCB is typically required, although 6mm² can be used for very short runs where installation method allows. Always verify with a full calculation.
How is cable size measured in the UK?
UK cable size is measured by the cross-sectional area (CSA) of the conductor in square millimetres (mm²). Common sizes are 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95mm². The CSA refers to each individual conductor - a "2.5mm²" twin and earth cable has 2.5mm² of copper in each of the live and neutral conductors, plus a smaller earth conductor.
What size cable is needed for lighting?
UK domestic lighting circuits typically use 1.0mm² or 1.5mm² twin and earth cable protected by a 6A or 10A MCB. 1.5mm² is the more common modern choice, offering better voltage drop performance for longer runs and additional capacity for LED driver inrush currents.
Buy Cable in the Correct Size
Once you have calculated the correct cable size, we stock the full range in every standard specification. Cable can be cut to length per metre from our cable cutting machine at Wrexham, or supplied in full drums with fast delivery from six UK branches.
- SWA armoured cable - 1.5mm² to 95mm², 3 core and 4 core, BS5467 and BS6724
- Low voltage cables - T&E, single core, panel wiring, flexible
- Medium voltage cables - 3.3kV to 33kV utility and industrial
- High voltage cables - specialist supply for transmission and grid
- Cable glands and cable lugs - termination accessories for every cable size
Free cable cutting service. Order the exact length you need and we cut it from the drum at Wrexham, reducing waste and project cost. Learn more about our cable cutting service.
Cable Sizing Support From Our Technical Team
Not sure which cable size is right for your installation? Our technical team has over 55 years of combined experience supplying cable for domestic, commercial, industrial, and utility projects across the UK. We can help you specify the correct cable type, size, and accessories - and get the best trade price.