Voltage Drop FAQ

Multiply the cable’s mV/A/m value from standard tables by the design current in amperes and the cable length in metres, then divide by 1000 to get voltage drop in volts. For three-phase balanced loads, use the three-phase mV/A/m values directly. Convert to percentage by dividing by nominal voltage and multiplying by 100.

Yes, significantly. Cable impedance has resistive and reactive components. At low power factors (highly inductive loads), the reactive component increases the effective voltage drop. For a cable with separate r and x mV/A/m values, use: Vd = (mVr × cos φ + mVx × sin φ) × Ib × L / 1000. At power factor 0.8, voltage drop can be 10–15% higher than at unity.

Motors require adequate terminal voltage to develop rated torque. During starting, when current is 5–8 times normal, voltage drop across the supply cable increases proportionally. If terminal voltage drops below approximately 80% of rated, the motor may fail to accelerate to full speed or may stall. This can damage the motor and trip upstream protection.

Consider these alternatives: reduce the cable route length by relocating the distribution board closer to the load; increase supply voltage (e.g., 400V instead of 230V reduces current by 42%); use cables with lower mV/A/m values (XLPE instead of PVC); improve load power factor with capacitor correction; or split the load across multiple circuits from different distribution points.

Yes. DC circuits have only resistive voltage drop (Vd = I × R × 2L for two-wire circuits). AC circuits have both resistive and reactive components, making the voltage drop dependent on power factor and cable reactance. For small cables below 16mm², reactance is negligible and AC voltage drop approximates DC values. For larger cables, reactance becomes significant.

LED drivers typically accept a wide input voltage range (185–265V for 230V nominal), but voltage drop still affects dimming performance and driver efficiency. Most standards limit lighting circuit voltage drop to 3% (6.9V on a 230V circuit). For dimmed LED circuits, keeping voltage drop below 2% ensures consistent light output and prevents flickering at low dimmer settings.

Conductor resistance increases with temperature — copper resistance rises approximately 0.4% per degree Celsius. Tabulated mV/A/m values are given at the conductor’s maximum operating temperature (70°C for PVC, 90°C for XLPE). At lower actual operating temperatures, real voltage drop will be less than calculated — the tabulated values represent worst case.

Calculate at both. Check steady-state voltage drop at design current against the standard limit (3–5%). Separately calculate transient voltage drop during motor starting or load switching to ensure equipment can start successfully. Starting voltage drop is temporary and allowed to exceed normal limits, but terminal voltage must remain above the motor’s minimum starting voltage.

In a ring final circuit, current splits between two paths from the distribution board to the load point. The voltage drop is calculated based on the current in the more heavily loaded leg, which depends on the load position around the ring. For a uniformly loaded ring, maximum voltage drop occurs at the midpoint and is one-quarter of the voltage drop for the equivalent radial circuit.