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Cable Sizing Calculator — BS 7671 18th Edition
What size cable do I need? Calculate the correct cable cross-section for your circuit based on BS 7671 18th Edition wiring regulations. Uses UK standard cable sizes and installation methods.
Design current Ib — the maximum current the circuit will carry
One-way run length from the consumer unit to the load
UK nominal supply voltage (default 230V single-phase)
BS 7671 limit: 3% lighting, 5% other circuits (default 3%)
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
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How We Calculate This
This calculator uses current carrying capacity data from BS 7671 18th Edition (IET Wiring Regulations). Cable sizing depends on four key factors: the design current of the circuit, the installation method, the cable type, and the conductor material.
How cable sizing works
Step 1: Determine the design current (Ib) — the maximum current the circuit will carry under normal conditions.
Step 2:Select a protective device rating (In) that is greater than or equal to Ib but does not exceed the cable’s capacity (Iz), so that Ib ≤ In ≤ Iz. For example, if Ib = 28A you would typically select a 32A MCB.
Step 3: Using the appropriate BS 7671 table for your installation method and cable type, find the smallest cable whose current carrying capacity (Iz) meets the requirement.
Note: this tool sizes the cable so that its tabulated capacity (Iz) is at least your design current (Ib). It does not pick the protective device for you — once you have chosen a standard device rating (In), confirm that Iz ≥ In for your cable and method before relying on the result.
BS 7671 reference tables
- Table 4D5: Flat twin and earth (6242Y), copper, 70°C thermoplastic — Reference Method C (clipped) and Method 100 (in insulation). Table 4D5 tabulates no conduit or trunking column, so those ratings are taken from Table 4D2A Reference Method B, one two-core cable single-phase, which is standard practice for flat cable and errs on the safe side
- Table 4D4A: 2-core PVC steel-wire-armoured (SWA), copper — Method C (clipped direct) and Method D (direct in ground)
- Table 4F3A: Flexible cords (3183Y / 3093Y), copper, single-phase
- Tables 4D5 / 4D4B: Voltage-drop (mV/A/m) columns used for the volt-drop check
The calculator compares your design current against the tabulated current carrying capacities for each standard UK cable size and recommends the smallest cable that meets the requirement. Tabulated ratings assume a 30°C ambient with no grouping or thermal-insulation derating — apply BS 7671 Appendix 4 correction factors (Ca, Cg, Ci, Cc) for your conditions. Always verify with a qualified electrician and check voltage drop for longer cable runs. For the full Table 4D5 figures at a glance, see our BS 7671 cable size reference table.
Working figures for this are collected in our Cable Ampacity Table reference table.
Frequently Asked Questions
A standard UK ring final circuit uses 2.5mm² twin and earth cable with a 32A MCB. This is the standard configuration for socket outlets in domestic installations per BS 7671. The ring configuration means the cable is looped from the consumer unit and back, giving it an effective capacity suitable for 32A protection.
Installation methods affect how well a cable can dissipate heat. Method C (clipped direct to a surface) allows the highest current because heat escapes easily. Method B (in conduit or trunking) is more restrictive. Cables enclosed in thermal insulation (Method 100) have the lowest ratings because heat cannot escape, so you typically need to upsize the cable.
Steel Wire Armoured (SWA) cable is chosen for outdoor runs, underground installations, and anywhere the cable needs mechanical protection — garden buildings, outbuildings, EV chargers with long runs and commercial installations. SWA is selected for that protection and for buried/outdoor use, not for higher current capacity: clipped-direct ratings are similar to twin and earth (for example 2.5mm²: SWA 28A per BS 7671 Table 4D4A versus twin and earth 27A per Table 4D5 Method C). For EV charger installations specifically, see our EV Charger Cable Sizing Calculator.
Yes. BS 7671 limits voltage drop to 3% for lighting circuits (6.9V on a 230V supply) and 5% for other circuits (11.5V). A cable may have sufficient current capacity but still fail on voltage drop over long runs. Always check both current capacity and voltage drop — use our Voltage Drop Calculator alongside this tool.
Aluminium conductors are rare in UK domestic work and are used mainly in larger commercial and industrial supplies, and in some distribution tails. Aluminium has lower conductivity than copper, so a larger cross-section is needed for the same current, and aluminium terminations need special care to avoid loosening. Flat twin and earth and ordinary flexible cords are not manufactured in aluminium, and BS 7671's aluminium tables only begin at larger sizes (16mm² and above), so this calculator covers copper conductors only. For most domestic work, copper twin and earth is the standard choice.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.