Electrical - 12V DC

Most of the electrical in my van is 120V, except for three main things: the fridge, the water pump, and the toilet fan/vent.

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There are technically two separate electrical systems in any conversion or RV; We have the original DC electrical system that our engines use, that use the chassis of the vehicle, and we have our own separate electrical system we build into our living space. The first question for your electrical system is what type of system do I want to use?

Many people choose to do a DC setup for their living space, and connect their indoor batteries to their alternator in their vehicle electrical system to help charge them while they drive. This works well for a lot of people, and there are a lot of helpful people who can teach you how to do that. (But unfortunately, not me.) I chose to do a combination, AC and DC, for my living space - and I chose to separate it entirely from the electrical system of the engine (aside from grounding the inverter and AC breaker panel). I chose to invest in solar panels and an occasional generator instead.

The DC system for your vehicle uses the chassis of your vehicle as the "common ground", which means it uses your chassis as the conductive pathway from your equipment (a motor for example), for the negative wire, back to the battery to complete the circuit. DC only uses two wires, Red for positive and Black for negative, and one way or another, the circuit always connects back to the battery terminals. In a DC system, the negative/black line is called "Ground". [It is not the same thing as your ground wire in AC - The wire that brings the current back in AC is called a neutral.] Since there's so many parts to an engine that pulls current, it wasn't feasible to run two direct connections to each piece from the battery, so there is one direct connection for the positive, and the negative wire from the equipment is attached to the chassis, which is connected to your negative battery terminal, so you can essentially connect back to the battery anywhere on the vehicle. This is why it's called a "common" ground.

For the DC appliances in my home, I chose to do a direct line to each piece of equipment instead of using the chassis as a common ground. The reasoning for this was mainly because my refrigerator requires a direct isolated connection anyway, and the amp draw for the remaining water pump and toilet fan (normal computer fan) is very low, and the voltage drop through the chassis with that small of a current wasn't a problem I wanted to deal with.

So the DC inside the living space is separate from both the AC in the truck and the DC from the engine. I have simple connectors from the battery terminals that go to post terminals. The negative wires are wired from the post terminal directly to the appliances. The positive lines from the post terminals connect to DC thermal breakers, like what are used in vehicle DC systems, and then directly to the appliance.
 

DC TIPS:

 There are some misconceptions about DC that it's more dangerous than AC - an idea that influenced my choice to use primarily an AC system honestly. I've since learned that DC is quite safe in small enclosures, and can be done well. Here are some quick tips I've learned about DC that might help you:

Voltage drop: The voltage of your current will drop based on the length of your line to the appliance, and you compensate for this with your gauge of wire. For example, here is a picture from my pump's manual;

So I actually had 12g wire laying around, and that's what I used for my run (about 22 ft or so from where my battery was. Ran indoors inside conduit.) 12G wire can carry up to 20amp current, which was more than enough for my pump. Make sure you use a large enough wire gauge for your appliances!

Type of wire: You can use solid or stranded wire for DC - the main difference is practicality; It's easier to maneuver and utilize a stranded line than it is to use a solid. But you could use either one.