Precision Utility
UK Wire Size
Calculator
Standard
BS 7671
UK Supply
230V / 50Hz
Find the correct cable size for any UK electrical installation in seconds. Enter your design current, cable length and installation conditions — the calculator selects the smallest compliant conductor based on BS 7671 current capacity tables and voltage drop limits. Built for UK electricians, contractors and competent DIYers.
Cable Parameters
Recommended Cable Size
2.5 mm²
Cable Size
2.5 mm²
Current Capacity
27 A
Voltage Drop
5.40 V
Voltage Drop %
2.35%
How the wire size calculator works
Start by entering the design current your circuit needs to carry. This is the maximum load current in amps — for example, a 32A ring final circuit or a 20A radial for a kitchen.
Next, set the cable length. This is the one-way distance from the consumer unit to the furthest point of the circuit. Longer runs increase voltage drop, which may force you to use a larger conductor.
Choose your supply voltage (230V single phase or 400V three phase), the installation method and the insulation type. The calculator applies the correct derating factor for your installation method and adjusts current ratings if you select XLPE insulation.
The result shows the smallest cable size that satisfies both the current capacity requirement and the BS 7671 voltage drop limit of 5%. If no standard cable size can meet both criteria, you will see a warning.
What you need to know about BS 7671 cable sizing
BS 7671 (the IET Wiring Regulations) is the national standard for electrical installations in the UK. Appendix 4 contains current-carrying capacity tables and voltage drop data for every common conductor and installation method.
Cable sizing involves two checks that must both pass:
- Current capacity — the cable must safely carry the design current without overheating, accounting for derating factors like installation method, grouping and ambient temperature
- Voltage drop — the drop across the cable must not exceed 5% of the nominal supply voltage (11.5V on 230V, 20V on 400V)
- Cables in conduit or trunking cannot shed heat as easily, so their effective capacity is reduced by a derating factor
- XLPE insulation tolerates higher temperatures than PVC, giving roughly 20% more current capacity for the same conductor size
This calculator covers the most common domestic and commercial scenarios. For complex installations with multiple grouping factors, high ambient temperatures or harmonic currents, refer to BS 7671 Appendix 4 directly or consult a qualified electrician. All electrical work in England and Wales must comply with Building Regulations Part P.
Frequently asked questions
What wire size do I need for a 20A circuit in the UK?
For a 20A circuit using PVC-insulated cable clipped direct, you would typically need 2.5 mm² cable, which has a current capacity of 27A. However, if the cable run is long, you may need to upsize to 4.0 mm² to keep voltage drop within the BS 7671 limit of 5%.
What is the maximum voltage drop allowed under BS 7671?
BS 7671 limits voltage drop to 5% of the supply voltage for most installations. On a 230V single-phase supply that means a maximum drop of 11.5V. On a 400V three-phase supply the limit is 20V.
How do I calculate voltage drop in a cable?
Voltage drop is calculated by multiplying the cable's mV/A/m rating by the design current in amps and the cable length in metres, then dividing by 1000 to convert from millivolts to volts. The result must not exceed 5% of the supply voltage.
Does installation method affect cable sizing?
Yes. Cable enclosed in conduit or trunking cannot dissipate heat as effectively as cable clipped directly to a surface or run in free air. BS 7671 applies derating factors — typically 0.725 for conduit and 0.9 for trunking — which reduce the cable's usable current capacity.
What is the difference between PVC and XLPE insulated cable?
PVC cable is rated to 70°C and is the most common type in domestic installations. XLPE (cross-linked polyethylene) cable is rated to 90°C, so it can carry more current for the same conductor size. XLPE is often used in commercial and industrial settings.
How do I size a cable for a three-phase supply?
The process is the same — select the smallest cable whose derated current capacity meets your design current and whose voltage drop stays within 5%. The difference is the supply voltage is 400V instead of 230V, which gives you a higher maximum voltage drop of 20V.