Voltage drop:
why a long, thin cable loses volts on the way

A cable is a resistor in series with your load

Copper is a good conductor, not a perfect one. Every metre of cable has a small resistance, and when current flows through that resistance a little voltage is lost along the way. The lost voltage appears as heat in the cable and as a lower supply at the far end.

For a 230 V mains socket a loss of 2 or 3 volts goes unnoticed. For a 12 V pump, a camera or a string of LED lights, the same loss is a large fraction of the supply and can show as dim lamps, slow motors or equipment that resets itself. That is why the topic comes up most often in solar, caravan, CCTV and low-voltage lighting installations.

Three things control the size of the drop: how much current the load draws, how long the cable is and how thick it is. The material enters through its resistivity, a constant that is lowest for silver and copper and noticeably higher for aluminium.

It is useful to separate the two jobs a cable has. As a safety component, it must not overheat, which depends on current and installation. As a performance component, it must deliver enough voltage at the load, which depends on length. A short heavy-duty feed and a long thin extension can both be perfectly safe yet behave entirely differently at the far end.

The formula for a two-wire circuit

Current has to go out to the load and come back, so a circuit of one-way length L uses 2 × L of conductor.

The figure by itself has no meaning until you compare it with the supply. On 230 V, 2.75 V is 1.2 per cent. On a 12 V system it is 22.9 per cent, which leaves the load with only about 9.2 V.

Turning volts into a percentage and a limit

Wiring codes and equipment makers set limits on the allowed drop, commonly a few per cent of the nominal voltage, often tighter for lighting than for other loads. The numbers differ between countries and installation types, so take the limit from the wiring code or the product's data sheet that applies to your job.

A load that draws a fixed power takes less current at a higher voltage, so a real 24 V system carrying the same power would draw half the current, and the drop would fall to about 1.4 V, or 5.7 per cent. Raising the voltage is one of the most effective cures for a long low-voltage run.

Ways to reduce the drop

1. 1Use a larger cross-section. Doubling the area halves the drop. For the 12 V example, holding the loss to 3 per cent, or 0.36 V, needs 2 × 10 × 1.72 × 10⁻⁸ × 20 ÷ 0.36, about 19.1 mm², which in practice means a 25 mm² cable.
2. 2Shorten the run by moving the supply, battery or inverter closer to the load.
3. 3Raise the system voltage. A 24 V or 48 V design needs less current for the same power.
4. 4Share the load across parallel cables, but only if the installation rules allow paralleling and the cables are matched.
5. 5Use good terminations; loose or oxidised joints add resistance that the formula does not capture.

Copper is not the only choice. Aluminium is lighter and cheaper but has about 1.6 times the resistivity, so the same 20 m, 10 A run in 2.5 mm² aluminium would drop roughly 4.5 V instead of 2.75 V. It is typically used in larger sizes on mains feeders where the extra area is easy to accommodate.

Do the arithmetic in a consistent order every time. Fix the supply voltage first, then the load current from the power rating, then the run length, and only then choose a cross-section. Decisions made in that order seldom need undoing, whereas picking a cable size first and testing it afterwards leads to repeated rework.

A second case: a workshop socket

Consider a socket circuit feeding a power tool in a shed at the end of the garden. The tool draws 16 A at 230 V, the cable runs 30 m from the house and the installer used 4 mm² copper.

That figure looks modest, and for a resistive load it is fine. Motors are more demanding because starting current can be several times the running current, and the momentary drop at start-up is correspondingly higher. A tool that stalls when switched on but runs well afterwards may be telling you about the cable and not the tool.

When you size cable for a motor, work out the drop at the starting current as well as the running current, using the starting figure from the motor's data plate or manual, and compare it with the limit the manufacturer gives for start-up.

Temperature, AC and three-phase circuits

Resistance rises with temperature. Copper increases by about 0.39 per cent for every degree Celsius, so a cable running at 70 °C has roughly 20 per cent more resistance than at 20 °C, and the 2.75 V drop becomes about 3.3 V. Cables bundled in conduit or buried in thermal insulation run warmer than the open-air figure.

For AC circuits the same formula is a good first approximation with small conductors and resistive loads. Larger cables add reactance, and inductive loads such as motors have a power factor below 1, so installers use the tables in the standards for accuracy. In balanced three-phase circuits the line-to-line drop is √3 × I × ρ × L ÷ A, because the currents are phased and do not simply double the length.