Cable Voltage Drop Calculator

How cable voltage drop works

When to use this: sizing a sub-main to an outbuilding, running power to a machine at the far end of a workshop, wiring a 12 V solar or automotive system, or working out why the motor at the end of a long run keeps tripping on undervoltage.

Every conductor has resistance. Push current through it and some of the supply voltage is consumed by the cable itself rather than delivered to the load. The formula is just Ohm’s law applied to the cable:

ΔV = 2 × L × I × ρ / A  (DC and single-phase)
ΔV = √3 × L × I × ρ / A  (balanced three-phase)

L is the one-way run in metres, I the current in amps, ρ the resistivity in Ω·mm²/m, and A the cross-section in mm². The factor of 2 is there because the current has to come back — the loop is twice the cable length.

Three-phase uses √3 because in a balanced system the three phase currents are 120° apart and largely cancel in the return path. The practical result: the same cable carries the same current with about 13% less drop on three-phase than single-phase.

The limits

Neither figure is a hard legal limit in most jurisdictions — both appear as recommendations or informational notes. In practice they are what an inspector expects and what equipment manufacturers assume, so treat them as binding unless you have a specific reason not to.

Lighting gets the tighter limit because incandescent output falls off sharply with voltage, and because flicker from voltage sag on other circuits is most visible in lighting.

Why low-voltage systems suffer most

The limit is a percentage, and that is what makes low-voltage DC so unforgiving. Consider the same 1 V of drop:

It compounds, too. For the same power, a 12 V system carries roughly 19 times the current of a 230 V one, and drop is proportional to current. This is why a 12 V solar installation uses conductors that look absurd for the wattage involved, and why the industry keeps moving to 48 V wherever it can.

Temperature matters more than people expect

Copper resistance rises about 0.393% per kelvin. The 20 °C figure in most tables is a reference condition, not an operating one:

A design that lands at exactly 5% at 20 °C is really at 6% once the cable warms up. If you are close to the limit, run the calculation at the cable’s rated temperature rather than the reference one.

Worked examples

Garage sub-main: 20 A over 30 m

ΔV = 2 × 30 × 20 × 0.017241 / 4 = 5.17 V
5.17 / 230 = 2.25%

Inside the 5% limit with room to spare. Note the cable is also dissipating 20 × 5.17 = 103 W as heat when fully loaded — over a year of continuous use that is 900 kWh, which is usually a stronger argument for the larger cable than the drop limit is.

The same run in 2.5 mm²

ΔV = 2 × 30 × 20 × 0.017241 / 2.5 = 8.28 V = 3.6%

2.5 mm² is rated for 20 A in free air, so it passes on current-carrying capacity. It passes a 5% limit too. But it fails a 3% lighting limit, and it wastes 60% more energy as heat. This is the case where current rating and voltage drop give different answers — and voltage drop is the one that should decide.

12 V DC over 10 m at 10 A

ΔV = 2 × 10 × 10 × 0.017241 / 2.5 = 1.38 V = 11.5%

Badly over limit despite a short run and a cable size that would be generous at 230 V. To hit 5% you would need about 5.8 mm² — call it 6 mm², which is more than twice the copper for a tenth of the power. That is the price of low voltage.

Copper or aluminium

Aluminium has about 61% the conductivity of copper, so it needs roughly 1.6× the cross-section for the same drop. Against that, it is far cheaper and much lighter, which is why service entrances and large feeders are routinely aluminium.

The real difficulties are mechanical rather than electrical. Aluminium creeps under sustained pressure, so terminations loosen over time unless they are torque-rated and listed for aluminium. It also forms an insulating oxide layer almost instantly on exposure to air, so joints normally need an anti-oxidant compound. Neither issue is a reason to avoid it — they are reasons to terminate it properly.

Reducing voltage drop

• Larger cross-section. Drop is inversely proportional to area, so doubling the cross-section halves the drop. Usually the first and simplest fix.
• Shorter run. Drop is directly proportional to length. Moving a distribution board closer to the load is often cheaper than upsizing a long cable.
• Higher voltage. This helps twice — the current falls for the same power, and the percentage is taken against a larger number. Going from 12 V to 24 V cuts the percentage drop by a factor of four for the same load.
• Split the load. Two circuits each carrying half the current have half the drop each, for the same total copper.

Size for both current and drop

Cable sizing has two independent constraints, and you take whichever demands more copper:

• Current-carrying capacity protects the cable. Exceed it and the insulation overheats and eventually fails. This is a safety limit.
• Voltage drop protects the load. Exceed it and equipment runs undervoltage — motors draw more current and run hot, electronics reset, lighting dims. This is a performance limit.

On short runs the current rating almost always decides. Past roughly 20–30 m, voltage drop takes over and frequently demands a cable one or two sizes larger than the current rating alone would suggest.