Being an electrician,I have to chime in on this subject.I have had this discussion before many times.This is what I tell everyone:
1)Solid core wire is a no-no.Electricity has a natural tendancy to travel on the surface of the copper,not the middle.Stranded wire actually has more surface area,ergo better conductivity.
Just as Kisinana said,the wire will be under continuous vibration which is hard on solid wire-ever tried to find a wire broken inside the insulation?Minimum 17 strand conductor is best
2)Tinned wire is the best</u> wire for corrosive environments,but not the only wire you can use.I personally use what is called "cabtire"-The cable designation is called SJOW.Many strands,thick rubber jacket,very flexible and cheaper than marine grade cable.
3)Seal all your terminations!Even tinned wire will corrode over time and the only solution is to seal any area that will allow moisture to penetrate your insulation.Apply silicone to your terminal connections before you crimp,and use heat shrink tubing over the end of the insulation
4)House wire (NMD-90)is for dry locations only!The outer jacket and conductor insulation will not last.Water,sun and constant changes in temperature will turn the insulation brittle and crumble.Do not use!
Sorry for the rambling on,but I hope this will help you become better informed.
P.S.-12volt d/c requires larger diameter wire-dont be afraid to upsize
Good luck!
Hooch