What cable size do I need for a 32A ring final circuit?

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.

What is the difference between installation methods in BS 7671?

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.

When should I use SWA cable instead of twin and earth?

Steel Wire Armoured (SWA) cable should be used for outdoor runs, underground installations, and anywhere the cable needs mechanical protection. It is commonly used for garden buildings, outbuildings, EV chargers with long runs, and commercial installations. SWA generally has higher current ratings than twin and earth due to better heat dissipation. For EV charger installations specifically, see our EV Charger Cable Sizing Calculator.

Do I need to consider voltage drop when sizing cables?

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.

Can I use aluminium cables in domestic installations?

Aluminium cables are less common in UK domestic installations but are used in larger commercial and industrial settings. Aluminium has lower conductivity than copper, so you need a larger cable size for the same current. Aluminium cables also require special terminations. For most domestic work, copper twin and earth is standard.

Cable Sizing Calculator — What Size Cable Do I Need? BS 7671

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 equal to or greater than Ib. For example, if Ib = 28A, you would 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) is equal to or greater than In.

BS 7671 reference tables

Table 4D1A: Single-phase copper conductors, twin and earth cables
Table 4D2A: Single-phase copper conductors, SWA cables
Table 4D4A: Single-phase copper conductors, flexible cables
Table 4H1A-4H4A: Aluminium conductor equivalents

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. Always verify with a qualified electrician and check voltage drop for longer cable runs.

Frequently Asked Questions

Related Calculators

Voltage Drop Calculator

Check voltage drop complies with UK wiring regulations.

Maximum Demand Calculator

Calculate maximum electrical demand for domestic installations.

Circuit Breaker Sizing Calculator

Determine the correct MCB rating for your circuit.

Socket Spacing Calculator

Plan socket layouts with Part P guidance on spacing and positioning.

Lighting Layout Calculator

Calculate lighting requirements based on lux levels for each room type.

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Cable Sizing Calculator — BS 7671 18th Edition