Voltage Drop on a Long Wire Run: Why Distance Forces a Thicker Gauge

Every wire is a small resistor. Push current through it and some of your supply voltage gets spent heating the copper instead of reaching the load. On a short jumper between two terminals that loss is too small to notice. Stretch the same wire 50 metres out to a workshop, a pump, or a string of lights, and the loss can grow large enough to dim a lamp, trip a contactor, or cook a motor over time. That spent voltage is what electricians call voltage drop, and it is the single biggest reason a long run needs fatter cable than a short one.

This post walks through how voltage drop behaves, the rule of thumb that keeps installations safe, and a full worked example you can reproduce in the voltage drop calculator in about ten seconds.

What actually drives the drop

The drop across a two-wire run follows one compact formula:

Vdrop = 2 × L × I × ρ / A

L is the one-way length, I is the current, ρ is the resistivity of the metal, and A is the cross-sectional area of the conductor. The factor of two is the part people forget: current flows out to the load on one conductor and returns on the other, so it crosses the full run length twice. A 50 m run presents 100 m of copper to the current.

Three variables in that formula are levers you control, and they pull in clear directions:

More current means more drop. Double the amps and you double the volts lost. A wire carrying 40 A drops twice what the same wire drops at 20 A.
More length means more drop. Drop is directly proportional to run length. Move the load twice as far and the drop doubles again.
More cross-section means less drop. Resistance is inversely proportional to area, so a thicker conductor drops less. Step from 4 mm² up to 8 mm² and you halve the drop on the same circuit.

Put plainly: drop rises with current and length, and falls with a thicker wire. Distance is usually the variable you can't change, the load defines your current, so when a run comes up short on voltage your most direct fix is to go up a wire size.

The 3% rule

Numbers in volts are hard to judge in isolation. Is 8 volts of drop fine or a problem? It depends entirely on the supply voltage, which is why the practical limit is expressed as a percentage.

The widely used guidance: keep voltage drop on a branch circuit at or under 3% of the supply voltage, and keep the feeder plus branch combined at or under 5%. On a 230 V system, 3% is about 6.9 V. On 120 V it is about 3.6 V. On low-voltage DC the budget shrinks fast: 3% of 12 V is only 0.36 V, which is why a short run of thin wire can starve a solar charge controller.

Staying inside that window is not pedantry. Equipment is rated to run on voltage within a band, and a chronic 5 to 8% sag leaves incandescent and halogen lights visibly dimmer, makes LED drivers work harder, and forces motors to draw extra current to deliver the same torque, which means more heat and a shorter life. The 3% target keeps a comfortable margin before any of that starts.

A worked example

Suppose you are running a 20 A circuit 50 metres out to a detached workshop on a 230 V single-phase supply, and your first instinct is 4 mm² copper. Plug it in:

Current I = 20 A
One-way length L = 50 m
Cross-section A = 4 mm² = 4 × 10⁻⁶ m²
Copper resistivity ρ ≈ 1.68 × 10⁻⁸ Ω·m

Vdrop = 2 × 50 × 20 × 1.68 × 10⁻⁸ / (4 × 10⁻⁶) ≈ 8.4 V

As a percentage that is 8.4 / 230 ≈ 3.65% — over the 3% line. The load at the far end sees about 221.6 V instead of 230 V.

Now apply the lever. Bump the conductor from 4 mm² to 6 mm²: area goes up by 1.5×, so the drop falls by the same factor to about 5.6 V, or roughly 2.4%. That clears the limit comfortably. Step up once more to 8 mm² and the drop halves from the original to about 4.2 V (1.8%). The fix for a long run was never a clever trick — it was simply more copper.

That same arithmetic is what the calculator does instantly, and it flags the moment your percentage crosses 3% so you never have to eyeball it.

Copper, aluminium, and three-phase

Material matters too. Aluminium is cheaper per metre but carries about 58% more resistance than copper of the same size, so an aluminium run drops roughly 1.58 times what copper would. That is why aluminium feeders are usually specified one or two gauges larger to land at the same percentage. If you reuse the copper resistivity for an aluminium run, you underestimate the drop by about a third and can leave the load quietly under-volted.

Circuit type shifts the result as well. A balanced three-phase line has no separate neutral return, so its coefficient is √3 instead of 2. For the same current, length, and conductor, the three-phase drop comes out at √3 / 2 ≈ 0.866 of the single-phase value — one reason long-distance distribution favours three-phase.

When you are choosing between candidate sizes, it helps to pair this with a wire gauge calculator so you can cross-check the gauge against its ampacity, not just its voltage drop. The two constraints are separate: a wire can be thick enough to carry the current safely and still drop too much voltage over a long haul.

How I use this on a real job

The first time voltage drop bit me, I had wired a run of LED floodlights about 40 metres down a driveway on thin two-core cable, sized only by what would safely carry the current. Everything tested fine on the bench. Installed, the far lights came up noticeably dimmer than the near ones, and I spent an evening convinced I had a bad fixture. The fixtures were fine. The cable was eating nearly 6% of the supply by the time it reached the end of the run. Now I check the percentage before I cut a single length of cable, and I treat anything over 3% as a prompt to go up a size rather than a number to argue with. It has saved me more re-pulls than any other habit.

The short version

Voltage drop is the price you pay for distance and current, and the only structural way to buy it back is conductor area. Keep branch circuits at or under 3% of the supply, double-check the round-trip factor of two, use the right resistivity for your metal, and let the percentage — not the raw volts — tell you whether a run passes. Run your real numbers through the voltage drop calculator, and if the figure comes in red, the answer is almost always one more wire size.

Made by Toolora · Updated 2026-06-13