Agree with @sirepair. If you need to run a wire for some really heavy duty farkle (like a Stebel horn or high intensity NON-LED driving lights) you can bump it up to 16 ga or even 14 ga. I usually only use 14 or 12 ga for a ground from the battery to my remote located ground bar and smaller stuff after that. Here is a wire size chart designed for stranded automotive (and mc) wiring showing amps and length. Length is round trip from power source (battery pos) through fuse block to farkle and back to ground (battery neg or ground bar like I use if connected direct to battery with the heavy wire). I know about other wire charts like this that recommend heavier wire but those charts are mainly for house wiring where solid core wire is used and wires are buried in walls causing less opportunity for heat to dissipate. Distances are not that great on a bike but can be sometimes more than you think also. And wire used in mc farkle installs is stranded core so can carry just a bit more amps. I usually just go to the local O'Reilly's discount auto parts store for those spools of wire in almost any color you want. I usually try to keep close to the wire color standard of the bike on which I'm working.
As other have said, 18 gauge is fine for just about everything except heavy power consumers such as front-end lighting, heated clothing, and aftermarket horns. I would go up one gauge (to thicker wire) if you are going to install a Powerlet connector, because you can't be sure what will eventually be plugged into it.
Direct Bury Irrigation Wire would be perfect. Contact an irrigation contractor (shouldn't be too hard to find one in CA), and tell them that you would like to buy 15 feet of multi-strand direct bury cable from them. The stuff comes in all sizes from 2-strand to 14 strand. It is 18 gauge single-strand (in other words, one solid conductor per wire), not multi-strand, but if you want sheathed cable, I think that is the closest you are going to get to it.
An easy answer is Ancor. They make many types and size gauge of multiple stranded conductor wires in a common sheath. You want stranded conductor wire on a vehicle as it is less affected by vibration over the life span of the vehicle. and forms into complex curves or bends as necessary to snake through tight spaces. Saving money here is short-sighted. Tinned copper is even better as it resists the corrosion that turns copper to green dust in short order. Solid conductor wire is stiff and fatigues (breaks) from road and motor vibration and is a poor choice to use here. I buy it (Ancor) in primary color rolls and by the foot for various projects. West Marine sells it in a store relatively near your house in CA or find it online form other sources. Color coding wire helps trace out circuits later - make a detailed diagram of your project and save it in the owner's manual for later or for the next owner.
www.ancorproducts.com
www.cableorganizer.com
Here is my Eastern Beaver fuse box strategically place at rear, except i need advise on a switched power source for the relay, i was eyeing the white connector in photo,
