What is the maximum voltage drop allowed in the UK?

BS 7671 18th Edition limits voltage drop to 3% for lighting circuits (6.9V on a 230V supply) and 5% for all other circuits (11.5V on a 230V supply). These limits apply from the origin of the installation (typically the meter) to the furthest point of the circuit.

How do I reduce voltage drop on a long cable run?

There are two main ways: increase the cable size (a larger cross-sectional area has lower resistance) or reduce the cable length. For very long runs, such as to an outbuilding, you may need to go up one or two cable sizes beyond what the current capacity requires. SWA cable may also help as it can handle longer runs. Use our Cable Sizing Calculator to find the right cable for your current and installation method.

Does voltage drop matter for short cable runs?

For typical domestic cable runs under 10-15 metres, voltage drop is rarely an issue. It becomes critical for longer runs such as garden buildings, EV chargers at the end of a long drive, or commercial installations. Always check both current capacity and voltage drop.

What is mV/A/m and how is it used?

mV/A/m stands for millivolts per amp per metre. It is the voltage drop rate for a given cable size listed in BS 7671 tables (e.g., Table 4D1B). The formula is: Voltage drop (V) = (mV/A/m x current x length) / 1000. This gives the total voltage drop in volts for the cable run.

Is the UK supply voltage 230V or 240V?

The UK nominal supply voltage is 230V +10% / -6% as harmonised with European standards. In practice, most UK supplies still deliver around 240V at the meter. BS 7671 calculations use 230V as the reference. A measured supply of 240V gives slightly more headroom on voltage drop limits.

Voltage Drop Calculator — Check BS 7671 Cable Voltage Drop

How We Calculate This

This calculator uses voltage drop data from BS 7671 18th Edition tables to determine whether your cable run complies with UK wiring regulations. Voltage drop occurs because all conductors have resistance, and the longer the cable run, the more voltage is lost.

The formula

Voltage Drop (V) = (mV/A/m × Current × Length) ÷ 1000

For example, a 2.5mm² twin and earth cable has a voltage drop rate of 18 mV/A/m. If you draw 20A over a 15m run: (18 × 20 × 15) ÷ 1000 = 5.4V drop, which is 2.3% of 230V — well within the 5% limit for power circuits.

UK limits per BS 7671

Lighting circuits: Maximum 3% voltage drop (6.9V on 230V supply)
Other circuits: Maximum 5% voltage drop (11.5V on 230V supply)

These limits apply from the origin of the installation to the furthest point of the circuit. If the voltage drop exceeds these limits, you need to increase the cable size or reduce the circuit length. The values used are from BS 7671 Table 4D1B for twin and earth and Table 4D2B for SWA cables.

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Voltage Drop Calculator — UK Wiring Regulations