Cable mV/A/m Rating

The millivolt-per-ampere-per-metre system converts the complex calculation of voltage drop (involving conductor resistance, reactance, power factor, and phase angle) into a single lookup value that can be directly multiplied by current and length. Standards and manufacturers publish mV/A/m values for two components: the resistive component (r) and the reactive component (x), or alternatively the combined value at a specific power factor. For DC circuits and unity power factor AC loads, only the resistive component matters. For lagging power factor loads, both components contribute: Vd = (mVr × cos φ + mVx × sin φ) × Ib × L / 1000. The mV/A/m value decreases with increasing conductor size (lower resistance per metre) and is higher for aluminium than copper (higher resistivity). XLPE cables have slightly lower mV/A/m values than PVC cables of the same size because their lower operating reactance marginally reduces the combined value. For three-phase circuits, the tabulated three-phase mV/A/m values already account for the √3 factor. This simple system makes voltage drop checks fast and routine, avoiding errors that might occur with more fundamental impedance calculations.