Voltage Drop: Why Long Cable Runs Need Bigger Conductors
Last reviewed 2026-11-10
Every conductor has resistance, so every amp that flows along a cable costs a little voltage. On a short run the loss is negligible. On a long run, such as a pump at the far end of a farm, a car park lighting circuit or a PV array a hundred metres from its inverter, the loss can grow until the equipment at the end no longer gets the voltage it was designed for. This guide explains where voltage drop comes from, how it is calculated, and why the usual cure is a larger conductor.
What causes voltage drop
Ohm's law does most of the work. A conductor of resistance R carrying current I loses a voltage I × R along its length. Resistance in turn depends on three things:
- Material (ρ, resistivity). Annealed copper at 20 °C has a resistivity of 0.017241 Ω·mm²/m, the international annealed copper standard of 1/58. Aluminium is about 64 % higher at 0.028264 Ω·mm²/m. For the same voltage drop, an aluminium conductor needs roughly 1.6 times the cross-section of a copper one.
- Length (L). Resistance is proportional to length. Double the run and you double the drop.
- Cross-sectional area (A). Resistance is inversely proportional to area. Double the area and you halve the drop.
That last relationship is why long runs need bigger conductors. Length cannot be changed, and current is set by the load, so the cross-section is the only lever left. A cable that is perfectly adequate for current-carrying capacity can still be too small for voltage drop once the run gets long enough.
Temperature matters more than most people expect
Resistivity rises with temperature. For copper the coefficient is about 0.00393 per °C at 20 °C, and for aluminium about 0.00403:
A PVC-insulated cable running at its full rated current may have a conductor temperature of 70 °C, and an XLPE cable can reach 90 °C. At 70 °C, copper resistance is about 1 + 0.00393 × 50 = 1.197 times its 20 °C value, so voltage drop is nearly 20 % higher than a cold calculation suggests. If the cable is heavily loaded, use the operating temperature. If it is lightly loaded and runs cool, using 70 °C anyway gives a conservative answer.
AC circuits add reactance and power factor
In AC circuits the conductor also has inductive reactance X, from the magnetic field around it. Its effect depends on how the load current lines up with the voltage, so the practical formula combines resistance and reactance through the load power factor:
Here R and X are per metre of one conductor, and L is the one-way route length. The factor 2 covers the outgoing and return conductors of a single-phase or DC circuit. In a balanced three-phase circuit there is no return current in the neutral. The √3 factor converts the drop in each phase into the line-to-line drop, which is compared with the line-to-line voltage.
For small cables the resistance term dominates and reactance hardly matters. Reactance for typical multicore low-voltage cables is in the region of 0.08 mΩ/m and changes little with size, while resistance falls steadily as the conductor grows. For conductors of about 120 mm² and above at low power factor, the reactance term becomes significant. Upsizing then gives diminishing returns, and parallel cables or a higher voltage become more attractive.
A worked example
A 32 A three-phase load at PF 0.85 sits 50 m from a 400 V distribution board, fed by a 16 mm² copper cable running at 70 °C:
Comfortably within any common limit. Now stretch the same circuit to 250 m. Voltage drop is proportional to length, so it rises fivefold to about 3.9 %. That passes a 5 % limit but fails a 3 % one. A 25 mm² cable brings it back to about 2.6 %. Try the numbers in the voltage drop calculator, which also searches the standard size range for the smallest conductor that meets your limit.
Why the limits exist
Equipment is designed to work within a voltage band around its rating. Outside that band, problems start to appear:
- Motors develop torque roughly in proportion to the square of voltage. A 10 % voltage deficit means about 19 % less starting torque. A motor driving a constant load draws more current at low voltage and runs hotter.
- Lighting output falls and some drivers flicker or shut down at low voltage.
- Electronics and contactors may drop out, especially during motor starting, when the transient drop is several times the running value.
- Energy is wasted. The voltage drop multiplied by the current is power turned into heat in the cable, and it is paid for in every hour of operation.
Codes and design guides therefore recommend or require maximum drops. The figures vary. In the US, informational notes in the NEC suggest 3 % for a branch circuit and 5 % for feeder plus branch circuit combined. These are recommendations rather than mandatory rules. In the UK, BS 7671 Appendix 4 gives 3 % for lighting and 5 % for other uses for installations supplied from a public low-voltage network. IEC 60364-5-52 contains comparable guidance in an informative annex. Check the rules that actually apply to your project, including any utility or client specifications, which may be stricter.
Voltage drop is only one check
A conductor that meets the voltage-drop limit still has to satisfy several other requirements:
- Current-carrying capacity for the installation method, grouping, ambient temperature and insulation, from IEC 60364-5-52, NEC Table 310.16 and its correction factors, or the national equivalent.
- Protection. The overcurrent device must protect the conductor, and in earthed systems the circuit impedance must be low enough for the device to disconnect a fault within the required time. Long runs can fail this check before they fail voltage drop.
- Short-circuit withstand during the time the fault current flows.
The final size is the largest of the sizes these checks call for. On short runs ampacity usually decides. On long runs voltage drop or fault disconnection usually does.
Practical ways to reduce voltage drop
- Upsize the conductor. The simplest and most common fix. Moving up one standard size typically cuts resistance by 30–40 %.
- Run conductors in parallel. Two identical conductors per phase halve both R and X. This is useful above about 240 mm², where single cables become hard to handle.
- Raise the voltage. For the same power, current falls in proportion to voltage and percentage drop falls with its square. This is why PV strings, long feeders and transmission lines run at higher voltages.
- Move the distribution point. A sub-distribution board closer to a cluster of loads shortens the many final circuits.
- Improve power factor. Lower current means a smaller drop, but this helps only where reactive current is a large share of the total.
A note on accuracy
Calculating resistance from resistivity and nominal area, as most quick calculators do, gives the DC resistance of a theoretical solid conductor. Real stranded conductors are specified by a maximum resistance in IEC 60228, or tabulated in NEC Chapter 9 Table 8, which is slightly higher. For large conductors, AC resistance is higher again because of skin and proximity effects. For final design, use tabulated or manufacturer resistance and reactance values. The Engineerly calculator lets you enter a tabulated resistance for exactly this reason.
This guide is general education. Always design to the code and standards in force where the installation is built.