I do agree with you Monkey Mark about the section. 1.5mm is a VERY minimum. I personally would put 2.5mm too.
Anyhow, with 1.5mm and 20m long. At 20 degree, resistance is 0.227 Ohms, and at 25 Amp, voltage drop is 5.6V (2,3%)
At 100 degree and all same, resistance is 0.316 Ohms, voltage drop is...