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4 mm² on the DC strings, 4 mm² on the AC side to a 20 A MCB, 6 mm² for the earth.
A 3 kW inverter puts out about 13.6 A on Pakistan's 220 V single-phase supply — 3000 ÷ 220 — so the AC run sits on a 20 A MCB with copper rated 36 A. On the roof, a single string of six panels works at roughly 17.5 A of design current, and 4 mm² carries that out to a 34 m route before voltage drop passes 3%. Longer than 34 m, step up one size.
Most quotes name one size and call it "solar cable". A rooftop system has three separate electrical jobs and each one takes a different cable. The panels feed the inverter on direct current, through single-core cable run outdoors. The inverter feeds the house on 220 V alternating current, through ordinary multi-core copper. And the aluminium frames the panels bolt to have to be bonded to earth.
A 3 kW rooftop solar system in Pakistan needs 4 mm² single-core DC cable for the panel strings and the run down to the inverter, 4 mm² three-core copper for the AC run from the inverter to the distribution board on a 20 A MCB, and 6 mm² for earthing the array frames and the inverter body. Those sizes hold for a 17.5 A DC design current and a route of 34 m or less, with the cable clipped direct at 30°C ambient. A longer roof-to-inverter route, a run pulled through conduit on a hot roof, or a panel plate showing a higher short-circuit current pushes the DC side to 6 mm².
| Run | Current | Cable | Capacity | Protection |
|---|---|---|---|---|
| Panel string, DC | 17.5 A design | 4 mm² single core | 36 A | DC isolator |
| Inverter to AC board | 13.6 A | 4 mm² 3 core | 36 A | 20 A MCB |
| Array frames and inverter earth | — | 6 mm² | — | — |
Everything below runs off two numbers printed on the back of your own panels: 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, six panels in one series string, and an 18 m route from the array down to the inverter. Put your own figures in the same places.
The millivolt figure already covers the trip out and the trip back, so the 18 m in that sum is the route length, measured one way. The cable you buy is twice it.
Heat is rarely what limits a 3 kW string — even 2.5 mm² carries the current. Distance is what decides the size. This is the longest one-way route each conductor covers before the string loses 3% of its voltage in the cable.
| Size | Current capacity | Voltage drop | Longest route at 3% |
|---|---|---|---|
| 2.5 mm² | 27 A | 18 mV/A/m | 21 m |
| 4 mm² | 36 A | 11 mV/A/m | 34 m |
| 6 mm² | 46 A | 7.3 mV/A/m | 52 m |
| 10 mm² | 63 A | 4.4 mV/A/m | 86 m |
Read it as a decision, not a spec sheet. Inverter in the roof room, array on the same slab: 4 mm². Inverter in a ground-floor store room three storeys down and across the house: measure the route, and if it beats 34 m take 6 mm². Full spec and today's rate sit on the 4mm DC cable and 6mm solar cable pages.
Past the inverter it is an ordinary 220 V circuit, and it is sized the ordinary way. A 3 kW inverter at full output draws 3000 ÷ 220 = 13.6 A. Apply the same 1.25 continuous-duty margin — 13.6 × 1.25 = 17 A — and the next breaker up is 20 A. The cable then has to clear the breaker, not just the load: 4 mm² three-core copper carries 36 A on two loaded conductors, 16 A clear of the MCB.
Voltage drop on that run is trivial at these lengths. Over 8 m at 13.6 A: 11 × 13.6 × 8 ÷ 1000 = 1.2 V, about half a percent of 220 V. The three cores are live, neutral and earth — 4mm three-core, off the standard cable range, is the usual pick. Bond the array frames and the inverter body back to the house earth with 6 mm²; the solar earthing cable guide covers the terminations.
This is the question nobody answers, and it is the one that decides what you pay. Sizes are cheap to look up; metres are what the quote is made of. Here is the count for one stated layout, so you can adjust it against your own roof.
| Run | Route | Cable to buy | Size |
|---|---|---|---|
| Row to row, DC | 4 m | 8 m (4 red, 4 black) | 4 mm² |
| Array to inverter, DC | 18 m | 36 m (18 red, 18 black) | 4 mm² |
| Inverter to AC board | 8 m | 8 m of 3 core | 4 mm² |
| Frames, inverter, earth pit | 20 m | 20 m single core | 6 mm² |
Totals for that layout: 44 m of 4 mm² DC, evenly split red and black, plus 8 m of 4 mm² three-core and 20 m of 6 mm² earth. With 15% for slack, order 25 m of each colour. One warning on ordering: coil lengths in this market are not standard. Ask for the metres, not "one coil", and check whether a quoted price is per metre or per coil before you compare it with anyone else's. Send us the route lengths on WhatsApp and we will quote the metres.
Single-core stranded copper for the strings and the home run, drawn on the Pilone line in Lahore. Plain annealed class 5 copper — not tinned. A 100 m length holds about 3.6 kg of conductor at 35.6 kg/km, which is the quickest way to tell full copper from a mixed draw: 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 AC side of the same system takes 4mm three-core, off the standard cable range, and the earth takes 6mm earth cable. All DC and solar cable sizes.
We draw the copper in Lahore and sell it at the day's rate — no dealer margin. Send the three route lengths and your city and we will price the metres: Punjab in 1–2 days, rest of Pakistan in 2–4.