Cable Size Calculator Uk

Estimate a suitable UK cable size using design current, voltage drop, installation method, conductor material, ambient temperature and grouping factors. This tool gives a practical first-pass recommendation for common low-voltage circuits in the UK.

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Expert guide to using a cable size calculator in the UK

A cable size calculator helps you estimate the minimum conductor size needed for a circuit so the cable can carry the design current safely and keep voltage drop within acceptable limits. In the UK, this sits within the wider framework of BS 7671, often called the IET Wiring Regulations, and practical design also considers installation method, ambient temperature, grouping, insulation type, protective devices and fault protection. The calculator above is designed as a high-quality planning tool for common low-voltage applications, such as radial circuits, EV charging points, workshop supplies, outbuilding feeds and small sub-mains.

The reason cable sizing matters is simple. If a cable is too small, it may overheat, nuisance trip, waste energy and create excessive voltage drop. If it is too large, the installation can become unnecessarily expensive, harder to terminate and less efficient from a budget perspective. Good design aims for the correct balance: a cable that is safe, compliant and practical to install. In UK practice, cable size is normally expressed in square millimetres, such as 2.5 mm², 6 mm², 10 mm² or 25 mm².

What the calculator is actually checking

At a simplified design stage, a cable size calculator usually checks two major things. First, the conductor must have adequate current-carrying capacity after any derating factors are applied. Second, the circuit should remain inside the chosen voltage drop limit, commonly 3% for lighting final circuits and 5% for many other circuits in a typical building design workflow. Professional design may also need to verify earth fault loop impedance, short-circuit withstand, disconnection times, harmonic content, starting current, neutral sizing, prospective fault current, correction for thermal insulation and manufacturer-specific data.

• Design current: the current expected under normal load conditions.
• Current-carrying capacity: how much current the selected cable can safely carry for the chosen installation method.
• Voltage drop: the reduction in voltage along the cable length due to conductor resistance.
• Correction factors: reductions in usable current capacity caused by heat, grouping, insulation and other site conditions.

Why UK installation method has such a big effect

Exactly the same 6 mm² cable can behave very differently depending on how it is installed. A cable clipped direct on a wall, in free air or on tray can lose heat more effectively than one run through conduit, trunking or thermal insulation. That means the free-air installation generally permits a higher current than the enclosed installation. This is why any useful cable size calculator asks for installation method rather than relying on conductor area alone.

For UK domestic and commercial work, this becomes especially important in lofts, risers, service voids and plant rooms. A cable passing through insulation for only a short section may be treated differently from one fully surrounded by insulation. Likewise, a bundle of circuits inside trunking may need significant grouping corrections. These details can move a design from 6 mm² to 10 mm² or larger very quickly.

Common copper size	Approx. resistance at 20C	Approx. clipped direct capacity	Approx. in conduit capacity	Typical UK use case
1.5 mm²	11.49 ohms/km	20 A	16 A	Lighting circuits and controls
2.5 mm²	6.90 ohms/km	27 A	22 A	Socket radials, short ring spurs, small loads
4 mm²	4.31 ohms/km	37 A	30 A	Cookers, heavier radials, small sub-mains
6 mm²	2.87 ohms/km	47 A	38 A	Showers, hobs, EV circuits over moderate distances
10 mm²	1.72 ohms/km	65 A	52 A	Sub-mains, larger appliances, EV chargers with long runs
16 mm²	1.08 ohms/km	87 A	70 A	Small commercial supplies and substantial sub-mains

How voltage drop changes the answer

Many people focus only on ampacity, but cable sizing in the UK is frequently dictated by voltage drop. A long run to a garage, garden office or EV charger may have plenty of current-carrying capacity on paper, yet still fail the voltage drop check if the cable is too small. This is common with 32 A and 40 A circuits over longer distances. In those situations, increasing the cable size reduces conductor resistance and helps maintain healthy terminal voltage at the load.

For single-phase circuits, the route length matters directly, and resistance produces a proportional voltage drop. Double the length and the drop roughly doubles. Increase the current and the drop rises accordingly. In practical terms, that is why a 32 A EV charger near the consumer unit may be fine on a smaller cable than the same charger at the far end of a driveway or detached outbuilding.

Copper size	Approx. max one-way length at 32 A, 230 V, 3% drop	Approx. max one-way length at 32 A, 230 V, 5% drop	Comment
2.5 mm²	10.4 m	17.4 m	Often limited by voltage drop on longer runs
4 mm²	16.7 m	27.9 m	Common step-up for moderate lengths
6 mm²	25.1 m	41.9 m	Popular for showers and EV charging
10 mm²	41.8 m	69.7 m	Useful where current and distance are both significant
16 mm²	66.8 m	111.4 m	Typical sub-main territory

Single-phase vs three-phase cable sizing

Single-phase installations dominate UK homes, while three-phase is more common in larger dwellings, farms, workshops, apartment buildings and commercial premises. A three-phase supply can deliver more power at a lower current per phase for the same total load, which can improve cable economy and reduce voltage drop impact. However, three-phase design also introduces phase balance, neutral current and equipment compatibility considerations. A proper cable size calculator should therefore ask whether the circuit is single phase or three phase before calculating design current from power.

As a simple example, a 12 kW load on 230 V single phase draws much more current than the same 12 kW on 400 V three phase. Lower current usually means a smaller cable may be possible, provided all other design checks pass. This is one of the reasons commercial designers often prefer three-phase distribution for larger loads.

Copper or aluminium?

Copper remains the standard choice for most domestic circuits in the UK because it has higher conductivity, smaller cross-sectional size for a given performance level and excellent termination reliability. Aluminium can be attractive on larger sub-mains and distribution runs because it is lighter and often cheaper per amp carried, but it needs more cross-sectional area to match copper performance. Termination practices, lugs, creep characteristics and jointing quality are all especially important when aluminium is used.

For this reason, many domestic calculators default to copper. The calculator above allows either material, but if you are working on a real design with aluminium conductors, always confirm the exact cable manufacturer data and termination method.

Common UK applications where people use a cable size calculator

• EV charger circuits, especially 7.4 kW single-phase chargers and longer driveway runs.
• Electric shower circuits where load current is high and protective device coordination matters.
• Cooker and hob supplies with diversity considered at the design stage.
• Garage, shed and garden office sub-mains where voltage drop often drives the final answer.
• Workshop and plant equipment feeds where motors and starting currents may matter.
• Commercial distribution boards and small three-phase feeders.

How to use the calculator properly

1. Choose whether you want to enter power in kilowatts or current in amperes.
2. Select single phase or three phase and confirm the system voltage.
3. Enter route length as one-way distance, not the return loop length.
4. Pick the maximum voltage drop target appropriate to the circuit.
5. Choose conductor material and insulation type.
6. Select the installation method that best matches the actual site arrangement.
7. Apply ambient temperature and grouping assumptions realistically.
8. Review the recommended cable size and check the chart to see why smaller sizes fail.

Important: A cable size calculator is a design aid, not a substitute for full verification. Final cable selection in the UK should be checked against BS 7671 tables, manufacturer data and all relevant protection requirements, including earth fault loop impedance, disconnection times and short-circuit withstand where applicable.

Authority sources worth checking

UK legislation: Electricity Safety, Quality and Continuity Regulations
Health and Safety Executive guidance on electricity
The IET hub for BS 7671 information

Frequent mistakes when sizing cable in the UK

One of the most common mistakes is using a standard current rating taken from a table without applying correction factors. Another is overlooking the impact of thermal insulation in loft spaces or walls. Designers also sometimes use the protective device rating as the actual design current, even when the load is substantially lower or higher. Long runs to detached buildings are another classic trap, because voltage drop can force a much larger cable than ampacity alone would suggest.

There is also confusion over route length. Most UK voltage drop data is applied using one-way route length, because the table factors already reflect the circuit arrangement. When using resistance-based calculations, the formula differs depending on whether the circuit is single phase or three phase. That is why a dedicated calculator is so useful: it keeps the arithmetic consistent and visible.

Final practical advice

If your calculation lands exactly on a marginal cable size, consider the real-world installation. Will the ambient temperature rise in summer? Could the load increase later? Is the route likely to share containment with additional circuits? Is there a preference for lower voltage drop to improve equipment performance? In many projects, stepping up one cable size delivers a better long-term outcome even if the smallest compliant size appears acceptable on paper.

For domestic work, examples such as EV chargers, showers and outbuilding sub-mains often benefit from conservative design because they combine meaningful current, real cable length and varying installation conditions. For commercial work, the need for documentation, discrimination, fault calculations and future spare capacity becomes even more important.

Use the calculator above as an informed starting point. It will help you compare design current, derated current-carrying capacity and estimated voltage drop across common cable sizes. Then confirm the final selection against the latest UK standards and the exact cable data for the product you intend to install.

Engineering note: the calculator provides a practical estimate for common UK low-voltage cable selection using resistance-based voltage drop and generic capacity tables. It does not replace project-specific design, certification or inspection and testing obligations.