Hate being subjected to conductor snobbery myself but if our projects occupied by sleeping people 5, 10, 15 years in the future:
Romex by itself installed by a beer-engineer will probably do famously, it's where the wire has a slight flaw, been knotted, kinked or nicked, spindled over an unseen barrier pulling it or flexed back and forth on itself a dozen or more times when routing or rerouting, and/or where Joe Somewhen CHANGES what we've installed after replacing X, Y or Z for the third time that risks ramp up exponentially in these vehicles that may be in motion 40+ hours a week and then occupied 24/7 by children and drunks... err, senior citizens.
My 1972 & '73 Airstream trailers have/had a hundred-plus pounds of romex installed by six-pack-before-lunch production line drunks without burning but the trailers have a Cadillac ride and usually are in motion only a few days of the year. I also pulled every bit and got $2+ a pound scrap value out of it.
https://www.amazon.com/dp/B000NUYBRK <--- One source of Marine grade cable, I did use the flat style myself. (
http://tinyurl.com/jhdp9qa) [[EDIT: changed single strand link to multi-strand] Wow - on test that link was not updated, still shows original, try it again
https://www.amazon.com/dp/B000NUYBRK]
http://www.powerstream.com/Wire_Size.htm <-- work the numbers. The only source of free power in the future would be heavying up conductors now. Example: 3+7+12+7+3 feet to go diagonally from corner to corner of my van - 32 feet, say a 10 amp @ 12 volt load... 5.48% drop with 10awg, 8.71% with 12awg, 13.83% with 14awg.
If the voltage drops then the current rises, the wattage draw spec of the load will be satisfied then add the cable losses so think 105.48% @ 10awg, 108.71% @ 12awg, 113.83% @ 14awg. How about 40 minutes free run time of the ceiling vent fan after 8 hours on if we go with 10awg versus 14awg? I ran 10awg from my 12V power center to the galley area, to the roof vent fan, to the possible black tank macerator pump, etc..
Solar feed lines are important - 300w solar at 12V nominal (parallel array) is 18~V to the charge controller so about 17A current. A basic rule is 3% or less drop from source to controls and then 5% to the individual loads from the distribution center. Remember the line size losses start at the solar panel itself when calculating, include the whips through the roof etc... Saving 5% is a free day of sunshine every month!