Cable Size / Voltage Drop Calculator

Calculate the minimum cable size based on voltage drop, current load, cable run length and installation method per BS 7671 / IET Wiring Regulations.

kW
V
metres

How to Use This Calculator

How to Use This Calculator

  1. Cable Type – Choose the cable you plan to install. Twin & Earth is standard domestic wiring; SWA (Steel Wire Armoured) is used for outdoor runs, outbuildings, and higher-current circuits.
  2. Installation Method – How the cable will be routed. Clipped direct to a surface gives the best current rating; enclosed in insulated walls requires derating.
  3. Circuit Type – Non-lighting circuits allow up to 5% voltage drop; lighting circuits are limited to 3% (BS 7671 regulation 525.1).
  4. Ambient Temperature – The expected maximum surrounding air temperature. Higher temperatures reduce the cable's current-carrying capacity.
  5. Number of Circuits & Layout – How many cables share the same route and whether they are bunched or laid side by side. More cables grouped together means more derating.
  6. Power – The load in kilowatts (kW) that the circuit will carry.
  7. Voltage – Supply voltage, normally 230 V in the UK (single phase).
  8. Cable Length – The one-way distance from the consumer unit to the final point of the circuit, in metres.

The calculator determines the design current, applies temperature and grouping correction factors, and checks every standard cable size against both the current-carrying capacity and the maximum allowable voltage drop to find the minimum compliant cable.

Common Questions

What is voltage drop and why does it matter?

Voltage drop is the reduction in voltage as electricity travels along a cable. Excessive voltage drop means appliances receive less than their rated voltage, which can cause poor performance, dimming lights, or overheating. BS 7671 limits it to 3% for lighting and 5% for other circuits.

What cable do I need for a cooker or shower?

A typical 9–10 kW electric shower or cooker on a 230 V supply draws around 40–45 A, usually requiring 10 mm² Twin & Earth cable clipped direct, though the actual size depends on cable run length, installation method, and grouping.

Do I need to apply correction factors?

Yes – BS 7671 requires correction for ambient temperature above 30 °C and for cables grouped together. This calculator applies both automatically based on your inputs.

Is this calculator suitable for three-phase circuits?

This version calculates for single-phase circuits only. Three-phase voltage drop calculations use different formulas and cable data tables.

Should I use this instead of a qualified electrician?

This tool gives a useful starting estimate but electrical work in the UK must comply with Part P of the Building Regulations. Always have cable sizing verified by a qualified electrician before installation.

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