Pick your panel and the route length — get the right wire cross-section with voltage drop, power losses and the safety margin checked.
Every meter of cable has resistance, and the current flowing through it turns part of your solar power into heat. The voltage drop follows one formula:
ΔU = 2 × L × I × ρ(T) / SL is the one-way route length (the 2 accounts for the return path), I is the operating current, S is the cross-section in mm², and ρ is the conductor resistivity — which grows with temperature. A rooftop cable at 70 °C conducts about 20% worse than the 20 °C textbook value. That is why this calculator asks about temperature while most others silently assume a cold wire.
Take a string of 12 × 580 W panels (Vmpp 42.5 V, Impp 13.8 A) and a 25 m route to the inverter. String voltage is 510 V. On 2.5 mm² copper at 85 °C — the calculator's default hot-roof conditions — the drop is 1.19% — above the 1% target. On 4 mm² it falls to 0.74%, about 52 W at full power.
Over a year that difference is roughly 20 kWh — and the safety check matters more than the losses: the same 4 mm² cable must also carry 1.25 × Isc on the hottest day. When both checks pass, the cable is right.
For a typical residential string (8–14 panels, one string per cable) 4 or 6 mm² solar cable covers most installations. The exact answer depends on the route length and string voltage — a long run at low voltage needs a bigger cross-section. Use the calculator above with your real panel and length.
It is the widely recommended design target, not a legal limit. Every percent of drop is energy you paid panels for and never receive. Going to 2–3% is safe and sometimes reasonable on a tight budget — the table shows exactly how many kWh per year each option costs.
Yes, but match by actual cross-section, not by the closest-looking number: 10 AWG is 5.26 mm² — noticeably thinner than 6 mm². The table shows the nearest AWG for every metric size; when in doubt, round up.
A cable's safe current rating assumes it can shed heat. Bundled in a conduit on a hot roof, the same wire safely carries 30–40% less current than in free air. IEC 60364-5-52 defines correction factors — the calculator applies them so a cable that passes the loss check cannot fail on safety.
No. Once the drop is well under 1%, extra copper buys almost nothing — the per-year column in the table makes that visible. The money is usually better spent on raising the string voltage (more panels in series), which cuts losses quadratically for free.
Not outdoors. PV wire (H1Z2Z2-K per EN 50618) is double-insulated and rated for UV, weather and up to 120 °C on the conductor — ordinary PVC cable cracks on a roof within a few years. After the roof entry, regular cable of the same cross-section is acceptable indoors where local rules allow it.
Power alone doesn't decide it — the run length and string voltage do. A typical 5 kW array is one or two strings of 10–14 A, so 4 mm² usually covers one-way runs up to roughly 20–35 m at a 1% drop, and 6 mm² up to roughly 35–50 m depending on string voltage. Enter your exact panel and length above for the precise answer.
Typical copper PV-wire sizes for a residential string (13 A at 335 V). Find your one-way run length, read the cross-section — then confirm with the calculator above using your exact panel and solar panel wiring layout.
| Cable | Max at 1% | Max at 2% | Safe current |
|---|---|---|---|
| 2.5 mm² | 15 m | 29 m | 33 A |
| 4 mm² | 23 m | 47 m | 45 A |
| 6 mm² | 35 m | 70 m | 57 A |
| 10 mm² | 59 m | 117 m | 80 A |
| 16 mm² | 94 m | 188 m | 108 A |
Lengths assume a hot-roof conductor at 85 °C; safe current is derated per IEC 60364-5-52 (on-surface pair, 70 °C ambient). Longer run or lower voltage — move one size up.
