Cable Sizing

BS 7671 Voltage Drop Calculations

How to calculate cable voltage drop using BS 7671 Appendix 4 mV/A/m tables, correction factors and impedance conversion.

Updated May 27, 2026

BS 7671 Appendix 4 gives a practical method for calculating voltage drop in low voltage cables. The method uses tabulated resistive and reactive voltage drop values, expressed in mV/A/m, and applies them to the design current, route length and load power factor.

This note sits alongside the broader Voltage Drop Calculations for Cable Sizing article and focuses on the BS 7671 table method.

Allowable voltage drop

BS 7671 gives allowable voltage drop limits depending on the type of final circuit and the supply arrangement.

Supply arrangementLightingOther uses
Low voltage installation supplied directly from a public LV distribution system3%5%
Low voltage installation supplied from a private LV supply6%8%
Allowable voltage drop limits from BS 7671 Appendix 4
Allowable voltage drop limits from BS 7671 Appendix 4.

BS 7671 voltage drop calculation

The BS 7671 tables give voltage drop values for different cable types and installation arrangements. The values are related to the cable maximum operating temperature and the line-to-neutral or line-to-line voltage, depending on whether the circuit is single-phase or three-phase.

Where the conductor is expected to operate below its maximum temperature, BS 7671 includes a temperature correction factor:

Ct=230+tp–(Ca2Cg2Cs2Cd2–Ib2It2)(tp–30)230+tp

The voltage drop can then be calculated using the tabulated resistive and reactive components:

vd=Ib⁢L⁢[Ctcos(φ)(mV/A/m)r+sin(φ)(mV/A/m)x]

In this expression, Ib is the design current, L is the cable length, Ct is the temperature correction factor and φ is the load power factor angle.

Using table values as impedance

The mV/A/m values can also be converted to resistance and reactance for use in impedance-based voltage drop calculations.

DC circuits

R=(mV/A/m)r2×1000 X=0

Single-phase AC circuits

R=(mV/A/m)r2×1000 X=(mV/A/m)x2×1000

Three-phase AC circuits

R=(mV/A/m)r3×1000 X=(mV/A/m)x3×1000

For three-phase circuits the BS 7671 mV/A/m values are divided by √3 when converting them to the per-line impedance values used by CENELEC TR 50480 style calculations.

Power factor

The cosine term in the voltage drop equation is the load power factor:

cos(φ)

Where load power factor is not known, the selected design assumption should be stated because it affects the balance between the resistive and reactive voltage drop components.

Related Articles