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6 mm² per DC string, 10 mm² where two strings share a home run, 10 mm² four-core on the AC side to a 40 A MCB per phase, 10 mm² for the array earth.
A 20 kW inverter on a 400 V three-phase connection puts out 20000 ÷ (1.732 × 400) = 28.9 A per phase, which is a 40 A MCB. On the roof, forty panels usually run as four strings of ten, each at about 17.5 A of design current, and 6 mm² holds a 3% voltage drop out to an 86 m route on a 370 V string. The 25 mm² figure often quoted for a system this size is not the string size — it is what a single home run carrying all four strings at once needs, and most 20 kW roofs never build one.
Twenty kilowatts is the size at which the AC side gets easy and the DC side gets fiddly. On three phases the inverter output splits three ways and the cable to the board is modest. On the roof, forty panels are four strings, and the whole cable decision turns on how many of those four are asked to share a pair of conductors on the way down.
A 20 kW rooftop array in Pakistan needs 6 mm² single-core DC per string, 10 mm² where two strings are combined into one home run, 16 mm² or 25 mm² where all four are combined into a single pair, 10 mm² four-core copper on a 40 A MCB from a three-phase inverter to the AC board, and 10 mm² for bonding the array frames and the inverter body. Those sizes hold for forty panels wired as four series strings of ten, 17.5 A of design current per string, a 400 V three-phase supply, DC routes of 86 m or less per string, and cable clipped direct at 30°C ambient. Change the string layout and the DC answer changes with it; the AC answer does not.
| Run | Current | Cable | Capacity | Protection |
|---|---|---|---|---|
| Each panel string, DC | 17.5 A design | 6 mm² single core | 46 A | DC isolator |
| Two strings combined, DC | 35 A design | 6 mm² to a 43 m route, else 10 mm² | 46 A / 63 A | DC isolator, string fuses |
| All four strings combined, DC | 70 A design | 16 mm² to a 56 m route, else 25 mm² | 85 A / 112 A | DC isolator, string fuses |
| Inverter to AC board, 3-phase 400 V | 28.9 A/phase | 10 mm² 4 core | 57 A | 40 A MCB |
| Inverter to AC board, 1-phase 220 V | 90.9 A | 35 mm² 2 core | 138 A | 125 A |
| Array frames and inverter earth | — | 10 mm² | — | — |
Two numbers off the back of your own panels drive all of it: the short-circuit current, Isc, and the voltage at maximum power, Vmp. This worked example uses a plate reading Isc 14 A and Vmp 37 V, forty panels in four series strings of ten, and a 30 m route from the roof edge down to the inverter room. Put your own plate figures in the same places.
The millivolt figure already covers the trip out and the trip back, so the 30 m in those sums is the route, measured one way, and the cable you buy is twice it per pair. Forty panels is the count at 500 W each; thirty-four 585 W panels come to roughly the same 20 kW. What moves the cable is the string layout and the plate figures, not the panel count.
Read the left column for a string on its own MPPT input, the middle for the usual two-strings-per-input arrangement, and the right for a single combined home run carrying the whole array.
| Size | Capacity | Voltage drop | One string (17.5 A) | Two combined (35 A) | All four (70 A) |
|---|---|---|---|---|---|
| 4 mm² | 36 A | 11 mV/A/m | 57 m | 28 m, on a 1 A margin | not enough capacity |
| 6 mm² | 46 A | 7.3 mV/A/m | 86 m | 43 m | not enough capacity |
| 10 mm² | 63 A | 4.4 mV/A/m | 144 m | 72 m | not enough capacity |
| 16 mm² | 85 A | 2.8 mV/A/m | 226 m | 113 m | 56 m |
| 25 mm² | 112 A | 1.75 mV/A/m | 362 m | 181 m | 90 m |
| 35 mm² | 138 A | 1.25 mV/A/m | 507 m | 253 m | 126 m |
Three cells in the right-hand column say "not enough capacity" instead of a distance, and that is the whole argument about 25 mm² on a 20 kW system. Combine all four strings into one pair and heat, not length, settles the size: 70 A needs 16 mm² at minimum, and 25 mm² once that pair runs past 56 m. Split the array across two MPPT inputs instead — which is how most 20 kW inverters are built — and the same roof is wired in 10 mm², for a fraction of the copper. The 4 mm² row carries 35 A by a single amp before any derating at all, which is not a margin; treat that cell as a number, not a recommendation. Full spec and today's rate sit on the 6mm DC solar cable, 10mm DC cable, 16mm DC cable and 25mm DC cable pages.
A 20 kW inverter in Pakistan is normally a three-phase machine on a 400 V connection. Three phases split the output: 20000 ÷ (1.732 × 400) = 28.9 A per phase, times the 1.25 continuous-duty margin is 36.1 A, so a 40 A MCB. 10 mm² four-core carries 57 A with three conductors loaded, seventeen amps clear of that breaker, and a correction factor of 0.75 for conduit and a hot plant room still leaves 42.8 A — above the breaker. That margin is the reason the AC side of a 20 kW system is less trouble than the AC side of a 10 kW single-phase one.
The single-phase alternative is the row nobody prices before ordering: 20000 ÷ 220 = 90.9 A, which after the same margin is 113.6 A, a 125 A main and 35 mm² cable. Same inverter rating, three sizes of copper apart, decided by one line on the nameplate and by what your connection actually is. Check both before ordering anything on the AC side. Voltage drop on this run is rarely the constraint: 12 m of 10 mm² four-core at 28.9 A drops 4.4 × 28.9 × 12 ÷ 1000 = 1.53 V, well under 1% of 400 V. Breaker margin is. The four cores are three phases and neutral, with the earth as a separate conductor, off the standard cable range. If the system is going on net metering, the run on from that board to the utility meter is sized from your sanctioned load rather than from the inverter — the net metering cable size guide has that table.
Sizes are easy to look up; metres are what the quote is made of. On a 20 kW roof the number that surprises people is the home run: two pairs down the same 30 m route is 120 m of cable before a single panel is connected. Here is the count for one stated layout, to adjust against your own roof.
| Run | Route | Cable to buy | Size |
|---|---|---|---|
| String 1 to roof edge, DC | 8 m | 16 m (8 red, 8 black) | 6 mm² |
| String 2 to roof edge, DC | 6 m | 12 m | 6 mm² |
| String 3 to roof edge, DC | 6 m | 12 m | 6 mm² |
| String 4 to roof edge, DC | 8 m | 16 m | 6 mm² |
| Roof edge to inverter, DC, two pairs | 30 m | 120 m | 10 mm² |
| Inverter to AC board, 3-phase | 12 m | 12 m of 4 core | 10 mm² |
| Frames, inverter, earth pit | 45 m | 45 m single core | 10 mm² |
Totals for that layout: 56 m of 6 mm² and 120 m of 10 mm² on the DC side, both split evenly red and black, plus 12 m of 10 mm² four-core and 45 m of 10 mm² for the earth — 233 m of cable in all. With 15% for slack, order 32 m of each DC colour in 6 mm² and 69 m of each in 10 mm². One warning before you compare quotes on a job this size: coil lengths in this market are not standard, and price lists routinely print a coil price where a per-metre price belongs. Ask for the metres, ask which unit the number is in, and check both quotes against the same run.
The paired home runs on a 20 kW roof, and the bulk of its metres. Single-core stranded copper drawn on the Pilone line in Lahore — plain annealed class 5, 80 strands of 0.40 mm, not tinned. A 100 m length holds about 8.89 kg of conductor at 88.9 kg/km, which is the fastest check on whether you were sold full copper: put the coil on a scale.
Prices track the daily copper rate. Send the metres and your city on WhatsApp for the exact figure.
The strings themselves take 6mm solar cable, a single combined home run takes 16mm DC cable or 25mm DC cable, the AC run takes 10mm four-core off the standard cable range, and the earth takes 10mm earth cable. All DC and solar cable sizes. Installers buying several sizes at once should start at the solar installer supply page.
We draw the copper in Lahore and sell it at the day's rate — no dealer margin. Send the string layout, the roof-to-inverter route and whether your connection is single or three-phase: Punjab in 1–2 days, rest of Pakistan in 2–4.