Voltage Drop Calculator Guide: Keeping Voltage Where It Belongs

When current flows through a cable, the cable's resistance causes a small voltage to be lost along its length. The equipment at the far end therefore receives slightly less voltage than the supply provides. That loss is called voltage drop. In short runs at low currents it is negligible. In long runs, or at high currents, it can dim lights, slow motors, cause electronics to misbehave, and in extreme cases overheat the cable. Understanding voltage drop is therefore a basic part of sizing any cable run, from a garden lighting circuit to a workshop supply.

This guide explains the calculation, works through a complete example, and explains the limits that apply in practice. Electrical sizing is guidance only: local wiring codes and a qualified electrician govern any real installation, and this guide does not replace them.

Why cables drop voltage

Every conductor has resistance, which depends on its material, its cross-sectional area, and its length. Copper has low resistance, aluminium has higher resistance for the same size, a thicker cable has lower resistance than a thinner one, and a longer cable has higher resistance than a shorter one. When current flows, Ohm's law tells us that the voltage dropped across the cable equals current multiplied by resistance. That voltage is no longer available to the load.

The resistance of a cable is usually quoted per unit length, for example in ohms per kilometre, at a stated temperature. Resistance rises with temperature, so a cable carrying a heavy load, and therefore running warm, has slightly higher resistance than the same cable when cold. For most practical calculations, the resistance value at the cable's operating temperature is used, or a standard tabulated value that already allows for it.

Voltage drop is usually expressed both in volts and as a percentage of the supply voltage. The percentage is the more useful figure for judging whether the drop matters, because a drop of 9 volts is small on a 400 volt supply but large on a 230 volt supply, and very large on a 12 volt supply. Low-voltage systems are therefore far more sensitive to voltage drop than mains systems, and a 12 volt garden lighting run that would be trivial at 230 volts can lose a quarter of its voltage over a modest distance.

The calculation

For a single-phase circuit, current flows out along one conductor and back along the other, so the total cable length in the circuit is twice the route length. The voltage drop is therefore twice the route length multiplied by the current multiplied by the resistance per unit length. If resistance is given in ohms per kilometre and length in metres, divide by 1,000 to convert metres to kilometres.

For a three-phase circuit, the calculation is slightly different, because the currents in the three conductors partly cancel, and the standard formula includes a factor related to power factor. A calculator handles that case, but the principle is the same: longer runs, higher currents, and thinner cables all increase the drop.

Temperature, power factor, and the type of cable all affect the tabulated resistance values. Published tables give resistance for common cable sizes and types, and a calculator that uses those tables will be more accurate than one that uses a single generic resistivity figure. Always check which table or standard your calculator uses, and whether it matches the cable type you intend to install.

A worked example: a 30 metre run

Consider a single-phase 230 volt circuit supplying a load that draws 20 amps, over a route length of 30 metres, using copper cable with a resistance of 7.41 ohms per kilometre for the conductor size in question. This resistance figure is representative of a common small cable size; in a real design you would take the exact figure from the manufacturer's data or the applicable wiring tables for the cable you plan to use.

The voltage drop is calculated as follows. Twice the route length is 60 metres. Multiply by the current of 20 amps, giving 1,200. Multiply by the resistance of 7.41 ohms per kilometre, and divide by 1,000 to convert metres to kilometres: 1,200 multiplied by 7.41 equals 8,892, and dividing by 1,000 gives 8.892 volts. So the voltage drop is about 8.9 volts.

As a percentage of the 230 volt supply, that is 8.892 divided by 230, multiplied by 100, which equals about 3.87 percent. The voltage at the load is therefore about 230 minus 8.892, which equals 221.1 volts. Whether 3.87 percent is acceptable depends on the applicable wiring rules and the equipment concerned. Many wiring codes set a limit of around 3 percent or 5 percent for the total drop from the supply origin to the load, depending on the jurisdiction and the type of installation. At 3.87 percent, this run might already exceed a 3 percent limit, even though it is within a 5 percent limit, which shows why the applicable limit must be checked rather than assumed.

If the drop is too high, the usual remedy is to use a thicker cable, which has lower resistance per kilometre. Doubling the cross-sectional area roughly halves the resistance, and therefore roughly halves the voltage drop. Alternatively, the load can be moved closer to the supply, or the circuit can be split so that each run carries less current. Reducing the current is often the most effective change, because voltage drop is directly proportional to current.

A Voltage Drop Calculator performs this calculation for single-phase and three-phase circuits, for copper and aluminium, and for a range of cable sizes, and it expresses the result in volts and as a percentage. Enter the route length, not the total conductor length, because the calculator applies the doubling for the return conductor itself. Entering the total conductor length by mistake will double the result.

Why voltage drop matters for equipment

Most mains equipment is designed to operate over a range of voltages, often around plus or minus 10 percent of nominal, so a drop of a few percent is usually harmless. Motors are more sensitive: a motor starting on a long cable may draw a high starting current, causing a larger temporary drop, which reduces the starting torque and can cause the motor to stall or to take longer to start. Repeated slow starts overheat motors.

Lights are the most visible indicator. Incandescent and halogen lamps dim noticeably with a drop of a few percent, while LED lamps with regulated drivers are less sensitive, up to a point, beyond which they may flicker or switch off. Electronics with switch-mode power supplies generally tolerate a range of input voltages, but they may draw more current as the voltage falls, which increases the drop further in a feedback loop.

Low-voltage systems deserve special attention. A 12 volt lighting circuit with a drop of 1 volt has lost over 8 percent of its supply, which will visibly dim the lamps at the far end. A 12 volt circuit over 30 metres at moderate current can easily lose several volts unless the cable is generously sized. For low-voltage runs, calculate the drop before buying cable, not after the lights are installed and found to be dim.

Cable size, current capacity, and voltage drop together

Cable sizing involves two separate checks, and both must pass. The first is current-carrying capacity: the cable must be able to carry the load current without overheating, taking into account how it is installed, whether it is buried, in conduit, or in free air, and the ambient temperature. The second is voltage drop: even a cable that can safely carry the current may drop too much voltage over a long run.

In short runs, current capacity usually governs, and the cable size chosen for safety will also satisfy voltage drop. In long runs, voltage drop often governs, and a thicker cable is needed than current capacity alone would suggest. That is why a calculator that checks both, or a design that checks both separately, is important. Choosing a cable on current capacity alone for a long run is a common design error.

Protection also matters. The cable must be protected by a fuse or circuit breaker that disconnects quickly enough in a fault, and the fault-loop impedance, which depends on cable resistance, affects whether that protection operates correctly. That is a further reason why cable selection should be done as part of a complete design, not as an isolated voltage-drop calculation.

Practical tips

Measure the route length along the actual cable path, including vertical drops, bends, and the run inside the building, not the straight-line distance on a plan. A route that looks like 25 metres on a drawing is often 30 metres once the cable is actually run. Adding a small allowance for the measured length is prudent.

Check the supply voltage at the origin as well. If the supply itself is already low, for example 220 volts instead of 230 volts, a drop of 9 volts leaves only 211 volts at the load, which may be outside the equipment's tolerance even though the percentage drop looks modest. Voltage drop is additional to any variation in the supply itself.

For a new installation, record the calculation: route length, load current, cable size and type, calculated drop in volts and percent, and the limit applied. That record is useful for future changes, for inspection, and for diagnosing problems if equipment later misbehaves. If a future load is added to the same run, the drop must be recalculated for the new total current.

For neighbouring electrical work, the Voltage Divider Calculator covers the small-signal equivalent of this problem, and the Transformer kVA Calculator Guide explains how supply capacity is assessed. The Area Calculator is useful when estimating cable routes across a floor plan.

Disclaimer

This guide is educational guidance only. Electrical installations must comply with the wiring regulations and codes applicable in your country and region, and work on fixed electrical installations should be carried out or verified by a qualified electrician. Cable data, limits, and methods vary between standards, and the example resistance figure used here is representative, not a substitute for the manufacturer's data or the applicable tables for the cable you intend to use. If you are unsure about any part of a design, seek qualified advice before installing or energising a circuit. A voltage drop calculation is a necessary part of a safe design, but it is not the whole design, and it does not by itself make an installation safe.