Voltage Drop Calculator — UK Wiring Regulations

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)

Three-phase circuits

The tabulated mV/A/m figure is the single-phase (two-conductor) drop. On a balanced three-phase circuit the line-to-line drop is √3÷2 (about 0.866) of that value, and the result is checked against the 400V line-to-line supply rather than 230V. This calculator applies the 0.866 factor automatically when you select three-phase.

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 mV/A/m values used are from BS 7671 Table 4D5 for flat twin and earth (6242Y, tabulated up to 16mm²) and Table 4D4B for multicore steel wire armoured (SWA) cables.