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

A standard domestic ring final circuit at 32A uses 2.5mm² twin and earth cable (6242Y) clipped direct or in conduit at standard ambient temperature. This gives a current-carrying capacity of 27A per conductor, with both conductors in parallel on a ring providing 54A effective capacity — sufficient for the 32A protective device.

What cable size for a 9.5kW shower?

A 9.5kW shower at 230V draws approximately 41.3A, requiring a 45A MCB. In typical domestic installations where the cable runs through ceiling voids (touching insulation, Ci=0.75), the required derated capacity is 45÷0.75=60A, which requires 10mm² cable (65A tabulated). Clipped direct throughout with no insulation contact, 6mm² (47A) would technically pass the current check — but in practice, shower cables almost always run through insulated ceiling voids, making 10mm² the correct and safe choice. Always verify with a qualified electrician.

What is voltage drop in electrical circuits?

Voltage drop is the reduction in voltage along the length of a cable due to its resistance. BS 7671 limits voltage drop to 3% of nominal voltage for lighting circuits (6.9V on a 230V supply) and 5% for power circuits (11.5V). Longer runs or higher currents may require a larger cable CSA to keep voltage drop within limits even if current capacity is satisfied.

What is a BS 7671 correction factor?

Correction factors reduce the tabulated current-carrying capacity of a cable to account for real-world installation conditions. Ca adjusts for ambient temperature above 30°C. Cg adjusts for cables grouped together (mutual heating). Ci applies when cable runs through thermal insulation. The cable must be sized so its derated capacity still exceeds the protective device rating.

Energy

Cable Size Calculator

Q: Do I need an electrician to size cables?

In England, Wales and Scotland, new circuits and significant alterations must be designed and signed off by a competent person registered under Part P of the Building Regulations (or notified to local authority building control). This calculator provides guidance only — all electrical installation work must be verified and certified by a qualified electrician.

BS 7671 cable sizing guidance for domestic circuits — lighting, ring finals, radials, cookers, showers and EV chargers. Applies correction factors and checks voltage drop.

Last reviewed 15 May 2026

BS 7671:2018+A4:2026 (18th Edition)

Voltage drop check

6 circuit types

Guidance only

Guidance only — not a substitute for professional design. New circuits and significant alterations to existing wiring must be designed and certified by a competent person registered under Part P of the Building Regulations (or notified to local authority building control). This tool provides indicative cable sizes for reference purposes. Always have electrical installation work verified and signed off by a qualified, registered electrician.

Rule of thumb: Standard UK lighting circuits (6A) typically use 1.0mm² or 1.5mm² cable, while ring final socket circuits (32A) use 2.5mm². High-power appliances like 9.5kW showers usually require 10mm² to account for insulation derating.

Circuit Details

Select circuit type and enter load and installation details

Circuit Type

Load

Design current (Ib) will be calculated as W ÷ 230V.

Protective Device Rating (MCB / fuse)

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Installation Method — select the worst point on the cable route

Number of grouped circuits

Ambient temperature (°C)

Circuit length (m, one way)

Measure the cable route from consumer unit to furthest point. Used for voltage drop calculation.

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Recommended

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Design current

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Ib (A)

Derated capacity

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It (A) required

Voltage drop

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V (limit —)

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Correction factors applied (BS 7671)

Cable options for this installation

6242Y flat twin & earth (PVC/PVC) — current ratings from BS 7671 Table 4D5

Earth fault loop impedance (Zs) — on-site verification required

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Common Domestic Cable Sizes — 6242Y Flat Twin & Earth

Shower values assume ceiling void routing (Ci=0.75). Cooker applies BS 7671 diversity. All others: clipped direct, no grouping, ≤30°C, 15m run. Use the calculator for your actual conditions.

Circuit	MCB	Cable CSA	Notes
Lighting	6A	1.0mm²	15.5A tabulated · includes 1.0mm² CPC
Ring final (sockets)	32A	2.5mm²	27A per leg · includes 1.5mm² CPC
Cooker (up to 12kW)	32A	6mm²	BS 7671 diversity reduces Ib to ~28A · includes 2.5mm² CPC
Shower (8.5kW)	40A	10mm²	Ceiling void routing (Ci=0.75) · includes 4mm² CPC
Shower (10.5kW)	50A	16mm²	Ceiling void routing (Ci=0.75) · Ib=45.7A > 45A MCB · 6mm² CPC
EV charger (7.4kW / 32A)	32A	6mm²	47A tabulated · includes 2.5mm² CPC

Table for guidance only. Correction factors for grouping, temperature and insulation may require a larger cable. Voltage drop must also be checked for longer runs. Always verify with a qualified electrician.

BS 7671 Guide

How Cable Sizing Works

The three cable sizing criteria

Every circuit cable must satisfy three criteria under BS 7671. First, current-carrying capacity: the cable must carry the design current (Ib) continuously without overheating. Second, voltage drop: the voltage reduction along the cable length must not exceed 3% for lighting or 5% for power circuits. Third, earth fault protection: the earth fault loop impedance (Zs) must be low enough to ensure the protective device disconnects fast enough under a fault.

This tool checks the first two criteria. The third — loop impedance — must be calculated and tested by a qualified electrician on-site as it depends on the supply impedance (Ze) unique to each property.

Correction factors explained

Ca (ambient temperature): Tabulated cable ratings assume 30°C ambient. Above 30°C the cable cannot dissipate heat as effectively — Ca reduces the rated capacity. At 40°C for PVC insulation, Ca = 0.87.

Cg (grouping): Cables touching each other generate mutual heating. Two grouped circuits: Cg = 0.80. Three: 0.70. Six or more: 0.57. Each cable in the group must be sized for its derated capacity.

Ci (thermal insulation): Cable running through thermal insulation cannot dissipate heat. Touching insulation on one side: Ci = 0.75. Fully enclosed in insulation: Ci = 0.50. This halves the effective current rating — avoid burying cables in insulation where possible, or use a significantly larger CSA.

Voltage drop calculation

Voltage drop (Vd) is calculated as: Vd = (mV/A/m) × Ib × L / 1000 where mV/A/m is the millivolt drop per amp per metre for the chosen cable CSA (from BS 7671 Appendix 4), Ib is design current in amps, and L is the one-way cable length in metres.

The result must not exceed 6.9V (3% of 230V) for lighting circuits, or 11.5V (5%) for other circuits. Long runs may require a larger cable CSA than current capacity alone demands — the calculator flags when this is the governing criterion.

FAQ

Common Questions

What cable for a 32A ring final?

A standard ring final uses 2.5mm² 6242Y twin and earth. The ring arrangement means both legs are in parallel, giving an effective capacity of 2×27A = 54A — well above the 32A protective device. Voltage drop must be checked for the full ring length, which for a typical 30–40m ring is usually within limits with 2.5mm².

What cable for a 9.5kW shower?

A 9.5kW shower draws approximately 41.3A. With a 45A protective device and typical ceiling void installation (Ci=0.75), 10mm² cable is required. The derated capacity at Ci=0.75 is 65×0.75=48.8A, which exceeds the 45A MCB. Always verify with a qualified electrician for your specific installation conditions.

Does cable size affect voltage drop?

Yes — larger cables have lower resistance and therefore lower voltage drop per metre. On long runs, voltage drop may require a larger cable than current capacity alone demands. For example, a lighting circuit on a 30m run at 6A current may technically fit 1.0mm², but the voltage drop may require 1.5mm² to stay within the 3% limit.

What is Part P of the Building Regulations?

Part P requires that electrical installation work in dwellings is carried out to BS 7671 standard and either certified by a registered competent person (NICEIC, NAPIT, ELECSA etc.) or notified to local authority building control. Adding new circuits, extending circuits in kitchens or bathrooms, and installing consumer units all fall within Part P scope. Minor additions in dry areas (like adding a socket to an existing circuit) may be exempt.