Solar panel to inverter cable: what size, and how far can it run?

The route decides the size, not the kilowatts

Ask what cable a solar system needs and you will be told a size for a kW rating. That is the wrong axis. Panels in series all pass the same current, so a string of ten panels carries no more amps than a string of five — what changes is the voltage. Two systems of the same size can need different cable, and two systems of very different sizes can need the same cable. What actually sets the size is the string's design current, the string's voltage, and the distance from the array down to the inverter.

The panel-to-inverter run on a Pakistani rooftop is sized by route length at the string's design current, held to a 3% voltage drop. At 17.5 A of design current — a 14 A panel short-circuit current with the standard 1.25 margin — 4 mm² single-core DC cable reaches a 31 m route on a 200 V string and 6 mm² reaches 46 m, with both conductors clipped direct at 30°C ambient. Heat is not the constraint on this run: 4 mm² is rated 36 A and 6 mm² is rated 46 A, against 17.5 A flowing. Distance is the constraint, and paralleling two strings into one run halves every distance in the table below.

How far each size runs

Add up the Vmp printed on each panel in one series string — five panels at 37 V is 185 V, ten is 370 V — then take the column closest to that figure and read down.

Two adjustments to make it yours. If your panel plate reads a short-circuit current other than 14 A, scale every figure by 17.5 divided by your own design current. If two strings are combined into one run before it reaches the inverter, the current doubles and every distance in the table halves — that is the single change that catches installers out, and it is why a combined run often needs the next size up. Rates and full specs sit on the 4mm, 6mm and 16mm DC cable pages.

Working out your own run

Take a real case. Eight panels in one series string, plate reading Isc 14 A and Vmp 37 V. The array sits on a second-floor roof; the route down to the inverter, along the parapet, through the shaft and across to the wall, measures 26 m.

• Design current. 14 × 1.25 = 17.5 A. Panels beat their plate figure on a bright, cold morning and hold it for hours, so the string is sized on the higher number.
• String voltage. 8 × 37 = 296 V.
• Voltage budget. 296 × 0.03 = 8.88 V allowed in the cable.
• 4 mm². 11 × 17.5 × 26 ÷ 1000 = 5.01 V, which is 1.7%. It passes, and the limit for this string works out at 46 m.
• 6 mm². 7.3 × 17.5 × 26 ÷ 1000 = 3.32 V, or 1.1%, with the limit at 69 m.
• Cable to order. 26 × 2 = 52 m, plus about 15% for slack, bends and re-terminations: 30 m of red and 30 m of black.

Both sizes pass at 26 m, so why take 6 mm²? Because measured routes grow. The tape says 26 m; the installer routes around a water tank and it comes out at 33 m. 4 mm² still passes at 33 m on this string, but it is spending most of its margin, and there is nothing left if a panel is added later. On a run that is buried in trunking and never touched again, the size with headroom is the cheaper decision.

Route length is not cable length

Every voltage-drop figure on this page counts the route once. That is not an oversight: the millivolt-per-amp-per-metre value already includes the trip out and the trip back, which on a DC string is the positive conductor and the negative conductor. So the distance you measure with a tape is the number you put in the calculation, and the metres you order are double it.

This is where quotes stop matching each other. One supplier prices the route, another prices the cable, and the two numbers differ by a factor of two before anyone has argued about the rate. Ask for the metres of cable, ask whether the number quoted is per metre or per coil, and check that the length being priced is twice your measured route. Coil lengths in this market are not standard either, so "one coil" is not a quantity — ask how many metres are on it. Send us your route length and we will do the doubling and quote the metres.

One last thing this run is not: it is single-core, positive and negative in separate cables, each with its own sheath so a single nick cannot bridge them. There is no 3 core DC cable for a solar string — multi-core belongs on the AC side of the inverter, and the DC cable vs AC cable guide sets out which goes where. Nor is house wire a substitute: 7/29 is built for indoor AC at 220 V, not for a roof at a few hundred volts DC in June.

The two cables this run is usually made of

Both are drawn in Lahore in plain annealed copper — class 5 stranded, not tinned. Tinning matters in humid coastal service, where bare strands corrode at cut ends; inland, with the strands sealed inside properly crimped MC4 connectors, plain copper is what most Pakistani rooftops run. Weigh a coil against the conductor figures above if you want to know whether you were sold full copper. Longer runs and combined strings go up to 10mm DC cable; the whole ladder is on the DC and solar cable range.

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