Electrical Cable Size Calculator

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.

What should I know about Why Cable Size Depends on More Than Just Current?

Selecting cable size isn't simply matching a cable's rated current capacity to the circuit's expected load — BS 7671 (UK Wiring Regulations) requires derating for installation method (cable clipped to a surface in free air can carry more current than the same cable buried in thermal insulation or bunched with other cables, since heat dissipation differs significantly), ambient temperature, and grouping with other circuits. A cable that's technically rated for 20A in free air might only be safely rated for 13-15A once correction factors for insulation and grouping are applied — using the uncorrected rating is a common and potentially dangerous shortcut that can lead to cable overheating under sustained full load, even though the circuit's protective device (fuse or MCB) might not trip, since the device is rated to protect against a short circuit or overload current, not necessarily against a cable that's simply undersized for its actual installation conditions.

What do I need to know about Earth Fault Loop Impedance and Cable Size?

Beyond current-carrying capacity and voltage drop, cable size (specifically the circuit protective conductor/earth size) must also be adequate to ensure a fault current is high enough to trip the circuit breaker quickly in the event of a fault — this is governed by earth fault loop impedance (Zs) calculations in BS 7671, which depend on cable length, cross-sectional area, and the supply's own characteristics. A cable that's adequately sized for normal operating current might still fail this disconnection-time requirement on a long circuit run, which is why qualified electricians calculate Zs as a separate check rather than relying on current-carrying capacity tables alone — this is a genuinely specialist calculation (not covered by a simple online sizing calculator) and is exactly the kind of check that distinguishes DIY cable sizing estimates from a certified electrical installation, reinforcing why any new circuit, particularly one on a long run, should be verified or installed by a qualified electrician.

What do I need to know about Part P Notifiable Electrical Work?

Part P of the Building Regulations covers electrical safety in dwellings in England (Wales has similar but separate rules), and it determines which electrical jobs must be notified to a local authority Building Control body or carried out/certified by a registered competent person electrician. Notifiable work generally includes any new circuit (such as a new socket circuit, a cooker circuit, or a circuit for an outbuilding), consumer unit replacement, and any work in a 'special location' such as a bathroom or outdoors, even if it's just extending an existing circuit in those zones. Non-notifiable work — such as adding a socket or light fitting to an existing circuit in a normal room, replacing a damaged accessory, or like-for-like repairs — can usually be done without notification, though it should still meet the wiring regulations (BS 7671) and ideally be checked by a competent person. Choosing cable sizes correctly is only half the job: for notifiable work, a registered electrician (or Building Control-approved self-certification) is required by law, and selling a property with uncertified notifiable electrical work can cause real problems at conveyancing. If in doubt about whether a job is notifiable, treat it as if it is and get it checked rather than risk an invalid installation.

Common UK Circuit Cable Sizes at a Glance

While the calculator above works out the correct size for your specific run, it helps to know what a "normal" result looks like for common domestic circuits, so you can sanity-check the output and spot an input error (such as a run length typed in the wrong units).

Worked Example: Checking Volt-Drop on a 25m Shower Cable Run A 9.5kW electric shower draws roughly 9,500W / 230V ≈ 41.3A. Using 10mm² twin-and-earth cable, a typical volt-drop figure from manufacturer tables is around 4.4 millivolts per amp per metre (mV/A/m). Over a 25m run, the volt drop is approximately 41.3A x 25m x 4.4 mV/A/m ÷ 1000 ≈ 4.5V. Expressed as a percentage of the 230V supply, that is about 2.0% — comfortably within the 3% limit BS 7671 generally allows for final circuits in a domestic installation (and well under the 5% overall limit from the origin of the installation). This is an indicative check only; always have the final design verified by a qualified, registered electrician, since earth fault loop impedance, diversity, and the specific protective device also affect what cable size is actually compliant.

Ambient Temperature and Grouping Correction Factors in Practice

A cable's tabulated current-carrying capacity assumes specific reference conditions — typically an ambient temperature around 30°C and a cable installed on its own, not bundled with others. Real installations rarely match this exactly, which is why correction factors exist and why two cables of the identical size can have very different safe current ratings depending on how and where they're installed.

The insulation-depth factor is the one most commonly missed by non-professionals: a cable buried within loft insulation, even briefly where it crosses joists, can need stepping up a full cable size compared with the same run clipped to an open surface, because insulation traps heat around the cable far more effectively than open air does. This is exactly the kind of detail a qualified electrician checks as standard and is a good reason not to rely solely on an online calculator for the final design of a circuit buried in or passing through insulated areas.