If the battery is the heart of the van, then the electrical cables are its circulatory system. They take power from the battery to where it’s needed, either at 12 volts or via the inverter, which converts 12 volts DC to 120 volts AC to run regular household appliances.
120 volt cables can be as thin as 12-gauge diameter and still deal with all the power requirements of the van. However, 12 volt cables need to be sized very carefully to prevent them from overheating.
Because the voltage (“pressure” of electricity) is lower, the amperage (“volume” of electricity) that the cable carries has to be higher in order to provide the same total wattage (“energy”). Volts x Amps = Watts. And squeezing too many Watts into a thin cable will make it heat up, and in the worst case melt its housing or start a fire.
Luckily there are online calculators you can use to work out how thick your cables need to be. Most of the cables we ran in the van were fine at 10-gauge (and yes, lower gauge numbers means thicker cables). The lights and some of the lower-power devices could have used thinner cables, and I did use some 14-gauge I had lying around, but if you’re buying a 100-foot reel of cable, it makes sense to use it rather than buying additional thinner-gauge cable as well.
The solar charger has 2-gauge cables, and the run from the battery to the main bus bars and to the inverter is 4/0-gauge (“four-ought”, or four over zero gauge). The wires we used are welding cables. These have multiple strands, which means they are very flexible. It also means they crimp nicely when you want to add lugs to the end to bolt them on to battery terminals.
We also kept the distance between the high-current devices as small as possible. Greater distances also increase resistance, which drops the voltage you receive at the other end as well as wasting the energy as heat.
Once you start using these very large cables, you need tools that can cope with them. Even cutting that many copper strands is hard without a ratcheting cutter.
Crimping is possible with a $15 tool and a hammer, but I decided to buy a $45 hydraulic crimping press. It’s really fun to use and creates very sturdy crimps. Note the interesting translation of the warning sticker. It actually sums up my approach pretty well, I think.
Update: We’ve written more now about cable crimping.