The formula behind it
A (mm²) = 2 × ρ × L × I ÷ ΔV — where ρ is copper's resistivity (0.0175 Ω·mm²/m), L the one-way length, I the current, and ΔV the drop you allow. The factor 2 covers the round trip through both conductors. We then round up to the next standard cable size.
Line loss is money
Every volt dropped becomes heat: loss = drop × current. On a 40 A battery line, a lazy 5% drop wastes ~100 W continuously — a panel's worth of power lost inside the wall.
What voltage drop is acceptable in a solar system?
Keep it within 2–3%. Beyond that you are paying for energy the cable turns into heat — and thin cables running hot are a fire risk.
Why does higher voltage need thinner cable?
For the same power, doubling the voltage halves the current — and cable size depends on current. This is why panel strings (300 V+) use thin wire while 48 V battery lines need thick copper.
Copper or aluminium wire for solar?
Copper. It conducts ~60% better, terminates more reliably, and is the standard for rooftop DC wiring.