Estimate cable size and voltage drop for domestic circuits
Cable sizing must be confirmed by a qualified electrician (Part P competent person). This calculator provides guidance only based on BS 7671 (IET Wiring Regulations 18th Edition). Actual installation requires full design including protective device coordination, earth fault loop impedance, adiabatic equation checks, and thermal constraints. All domestic electrical work must comply with Part P of the Building Regulations.
Selecting the correct cable size for an electrical circuit is one of the most important safety decisions in any domestic electrical installation. An undersized cable can overheat under load, creating a serious fire risk. An oversized cable wastes money and can be difficult to terminate in standard accessories. In the UK, cable sizing is governed by BS 7671, the IET Wiring Regulations (currently the 18th Edition, Amendment 2), which is the national standard for electrical installations. This calculator provides simplified guidance for common domestic circuits such as ring mains, radial socket circuits, cooker circuits, electric shower circuits, lighting, and immersion heaters. It estimates the recommended cable size based on the circuit type, cable route length, and installation method, and performs a basic voltage drop check against the BS 7671 limit of 5 percent of the supply voltage (11.5V on a 230V supply). IMPORTANT: This calculator is a simplified estimator only. It does not replace a full electrical design by a qualified electrician. Actual cable sizing requires consideration of many additional factors including protective device characteristics, earth fault loop impedance, adiabatic equation compliance for earth conductors, grouping factors for cables run together, ambient temperature correction, and thermal insulation effects. All domestic electrical work in England and Wales must comply with Part P of the Building Regulations, which requires most work to be carried out or certified by a Part P registered competent person.
To estimate cable size for a domestic circuit: 1. Select the circuit type from the dropdown. Common circuits include ring main (the standard socket circuit serving a floor of a house), radial sockets (an alternative to a ring, often used for garages or extensions), cooker circuit, electric shower (available in 7kW, 9kW, and 10.5kW ratings), lighting, and immersion heater. 2. Enter the cable route length in metres. This is the total length of cable from the consumer unit (fuse board) to the furthest point of the circuit, measured along the actual cable route including runs through walls, floors, and roof spaces. For a ring main, enter the total length of the ring (both legs combined). 3. Select the installation method. Cables clipped directly to a surface have the best heat dissipation and highest current-carrying capacity. Cables in conduit or trunking have slightly reduced capacity. Cables enclosed in thermal insulation (such as running through loft insulation) have significantly reduced capacity and may need to be upsized. 4. Review the results. The calculator shows the recommended cable size (e.g., 2.5mm squared twin and earth), the MCB (miniature circuit breaker) rating, the estimated voltage drop in volts and as a percentage, and whether the voltage drop is within the BS 7671 5 percent limit. 5. ALWAYS have your cable sizing confirmed by a qualified electrician before purchasing materials or starting work. This calculator does not account for all the factors required by BS 7671, and incorrect cable sizing is a serious safety hazard.
The cable sizing process follows a simplified version of the BS 7671 method: Step 1 -- Design current: Each circuit type has a design current based on the load it serves. A ring main is protected by a 32A MCB, a cooker by a 32A MCB, a 9kW shower draws approximately 40A (9000W divided by 230V), and a lighting circuit typically uses a 6A MCB. Step 2 -- Cable selection: Based on the design current and the installation method, a cable is selected from the current-carrying capacity tables in BS 7671 Appendix 4. The calculator uses a simplified lookup table for the most common domestic circuits. For example, a ring main uses 2.5mm squared twin and earth cable, which has a current-carrying capacity of 27A per conductor when clipped direct -- but because a ring has two paths, the effective capacity is higher. Step 3 -- Voltage drop check: BS 7671 requires that the voltage drop from the origin of the installation (the consumer unit) to the furthest point of the circuit does not exceed 5 percent of the nominal supply voltage. For a 230V supply, this is 11.5V maximum. Voltage drop is calculated as: design current (A) multiplied by cable length (m) multiplied by the mV/A/m figure for the cable (from BS 7671 tables), divided by 1000. For ring mains, the effective length is half the total ring length because current splits both ways. Step 4 -- Derating for insulation: When cables are enclosed in thermal insulation, a correction factor (typically 0.5 for cables fully surrounded by insulation) reduces the current-carrying capacity. This often means a larger cable size is needed. The calculator applies a 1.2x increase to the mV/A/m value and adjusts the maximum length threshold to account for this. If the voltage drop exceeds 5 percent, the calculator flags this and recommends increasing the cable size. In practice, an electrician would also check earth fault loop impedance, adiabatic compliance, and coordination with the protective device characteristics.
BS 7671 (IET Wiring Regulations, 18th Edition) is the definitive UK standard for electrical installations. Part P of the Building Regulations (England and Wales) makes most domestic electrical work notifiable -- meaning it must be carried out by a Part P registered competent person or inspected and tested by Building Control. The Electrical Safety Standards in the Private Rented Sector (England) Regulations 2020 require all rental properties to have an EICR (Electrical Installation Condition Report) every 5 years. Cable sizes shown here are for standard PVC-insulated twin and earth (T&E) cables to BS 7671. For special locations (bathrooms, swimming pools, saunas), additional regulations apply including RCD protection and supplementary bonding requirements. Always ensure cables are routed within safe zones as defined in BS 7671 and are protected from mechanical damage where required.