When people plan a solar installation, the panels and the inverter get all the attention. The cable connecting them is usually an afterthought — ordered last, chosen by habit, and almost never checked against the actual run length of the system. That is a quiet mistake, because an undersized DC cable does not fail loudly. It simply wastes a slice of the energy your array generates, every daylight hour, for twenty-five years.
The physics is straightforward. Every metre of cable has resistance, and pushing current through resistance converts some of your solar power into heat. The longer the run and the higher the current, the more you lose. Installers describe this as voltage drop, usually expressed as a percentage of system voltage. Good practice for the DC side of a solar system is to keep it under 2–3%. On a short roof run of ten metres, almost any compliant cable will pass. On a forty-metre run to a garden-mounted array or a detached garage, the same 4mm² cable that was fine on the roof can push losses well beyond acceptable limits.
The decision most homeowners and small installers face is between 4mm² and 6mm² solar cable, occasionally stepping up to 10mm² for long runs. The temptation is to save a few pounds per metre by staying small. But work the numbers over a system’s lifetime: a persistent 4% voltage drop on a 5kW array can cost more in lost generation over a decade than the price difference of the heavier cable several times over. Copper is cheap compared with sunlight you have already paid to capture.
There is a second, less discussed reason to size cable properly: heat. A cable running near its ampacity limit in a hot loft or conduit ages faster, and its insulation degrades sooner. Modern solar cable to the EN 50618 standard (the H1Z2Z2-K type that has replaced the older PV1-F designation) is rated for 120°C and long service, but the rating assumes the conductor is not being pushed to its limit around the clock.
So how do you actually check? The calculation itself is not complicated — current, length, conductor cross-section and a resistivity constant — but it is easy to get a decimal place wrong, and the tables in wiring regulations are not written for quick comparisons. The practical route is to run your figures through a free PV cable sizing calculator, which compares 4mm², 6mm² and 10mm² side by side for your run length, string current and system voltage, and shows the drop percentage against common project targets. Two minutes with real numbers beats guesswork, and it gives you a defensible spec to hand to whoever supplies the cable.
A few practical rules of thumb are worth keeping. Always calculate with the actual routed cable length, not the straight-line distance — cable follows walls, rails and conduit, and the real run is often 20–30% longer than it looks. Use the string’s maximum current, not its typical operating current. And remember that voltage drop applies to the return path too: a ten-metre distance between array and inverter means twenty metres of conductor in the loop.
None of this is exotic engineering. It is the kind of small, unglamorous decision that separates an installation that performs to its datasheet from one that mysteriously underdelivers. The panels convert the sunlight; the cable’s only job is not to waste it. Spend the two minutes, size it once, and it will repay you silently for decades.
