Copper Wire Resistance Table
Metric size (area per mm2) Resistance per 100metres for 2 wires (ohms)
1 4.2
2.5 1.5
4 0.92
6 0.62
10 0.37
16 0.23
25 0.135
35 0.105

Loss (watts)         =     amps2(squared) X Resistance (ohms)

or

w = I2  (current squared) X R (resistance in ohms)

Quoted allowable drop = no more than half an amp for a 12 volt system.