4x1,5mm2 or 8x0,8mm2 (4*2) i'll be safe for trigger the 3 relays at the same time for 600 to 800m long? What do you think?Yes, you need to use the total wire length. Technically, half of the voltage drop will be on the positive wire, and half on the negative. But the total drop is all we're really concerned with. Also, if you trigger more than one relay at a time, the voltage drop will be higher, because the current in the common return wire will be higher.