Electrical Cable Size Calculator
Calculate the minimum cable cross-sectional area (CSA) for any circuit based on current load, cable run length, and how the cable is installed. Based on BS 7671 (18th Edition).
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Electrical Cable Sizing Guide
What's the key thing to understand about UK Standard Cable Sizes?
Standard T&E cable sizes in the UK: 1.0mm² (6A) — lighting circuits. 1.5mm² (16A) — lighting, small appliances. 2.5mm² (25–27A) — ring main (power sockets), standard 20A circuits. 4mm² (32A) — cooker circuits under 10kW, showers under 8kW. 6mm² (40A) — showers, large cookers. 10mm² (50A) — large cookers, EV chargers. 16mm² (63A) — high-load EV chargers, large commercial equipment. 25mm² (80A) — very high load applications. These current ratings are for clipped surface installation — buried or enclosed cables in insulation need a lower rating applied, since they lose heat less easily than a cable clipped directly to a surface.
What should I know about Voltage Drop?
Voltage drop occurs over any cable due to resistance: V_drop = I × R × L × 2, where R is resistance per metre and L is cable length (×2 for return path). BS 7671 permits a maximum of 3% voltage drop for lighting circuits and 5% for other circuits. At 230V supply: 3% = 6.9V, 5% = 11.5V. Example: 2.5mm² T&E has a resistance of approximately 7.41 mΩ/m at 70°C. For a 16A circuit over 20m: V_drop = 16 × 0.00741 × 20 × 2 = 4.74V = 2.06% — within the 3% limit.
What's the key thing to understand about Correction Factors?
Cable current ratings must be reduced (derated) for higher ambient temperatures, grouping with other cables, and enclosed/buried installation. If cables are grouped together in a bundle or trunking, the derating factor can be 0.7 or lower for groups of more than 4 cables. This means a 2.5mm² cable carrying 25A when clipped in air can only carry 17.5A when grouped with 4 other circuits. Always calculate with the installation-specific rating, not the maximum rating. These calculations are covered in detail in BS 7671, the UK wiring regulations, which is why qualified electricians use reference tables and software rather than working from first principles on every job.
When to Use a Qualified Electrician?
In England and Wales, most electrical work in dwellings is notifiable under Part P of the Building Regulations and must be either carried out by a registered competent person (Part P scheme member), or notified to and inspected by a local Building Control authority. Notifiable work includes: new circuits, consumer unit replacement, work in kitchens and bathrooms, and work in any zone around a bath/shower. Non-notifiable work, which householders can do themselves, includes replacing like-for-like fittings such as a socket or light switch, provided it's outside a special location like a bathroom zone.