How to Wire a 12 Volt Electrical System in an RV

by John Cagney Nash
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A well-thought-out 12-volt electrical system in a recreational vehicle, or RV, allows for extended periods of remote camping and the reliable starting of the engine. The system comprises two separate 12-volt sub-systems; one serves the chassis system and runs the automotive part of the vehicle, and the other serves the coach and runs the "house" part of the vehicle. Successfully wiring the systems requires considerable planning and preparation.

Step 1

Use a battery isolator to ensure the chassis system cannot be depleted by use of the coach system's equipment. A battery isolator prioritizes charge from the running engine to the chassis battery, then switches charge to the coach battery, but prevents the "house" systems from drawing on the chassis battery.

Step 2

Design the coach system to safely answer all your predicted amperage and voltage needs for power outlets, lighting and appliance usage. Use the manufacturer's literature to calculate values to include the water pump, motors for slide-outs and the furnace fan even if the furnace runs on propane. Purchase a deep cycle battery or bank of batteries adequate to the predicted usage. It is possible to wire a bank of batteries in two ways: connecting the batteries in parallel will increase amp hour capacity, and connecting the batteries in series will increase the voltage capacity. Always install a battery disconnect master switch close to the battery or bank of batteries so the fuse board can be isolated.

Step 3

Make a diagram of all the wiring circuits and appliances necessary to answer all your predicted needs. Because direct current (DC) passes in one direction, from the battery and through an appliance to ground, each circuit must start at a fused connection to the coach battery and end at ground. When translating the diagram to installation, consider the easiest and most practicable ways to route the wires to keep them unobtrusive while also minimizing disruption to the fabric of the RV.

Step 4

Determine whether a monitor panel which centralizes gauges and dials and monitors battery charge will be useful. If so, plan to install it where access is easy and convenient but where it will not be subject to excessive moisture or accidental contact.

Step 5

Choose an inverter based on the requirements of your RV's 120-volt appliances. Microwaves, most televisions and some RV water heaters require 120 volts to operate, as do accessories brought from the home, such as hair dryers and coffee makers. An inverter produces 120-volt power from the 12-volt battery, but does so at a considerable penalty to the battery's charge.

Step 6

Plan a location from which to run an umbilical cord that will connect the RV to shore power. The cord should have a 30- or 50-amp plug at one end, and a panel of circuit breakers at the other. Locate a converter near the breaker panel, to transform 120-volt current from the shore connection to 12-volt for your systems. The converter should be considered part of the 12-volt system because it automatically charges the coach battery when the shore power cord is plugged in.

Step 7

Design a fuse board for the 12-volt system, and locate it close to where the shore power cord enters the RV. Beyond protecting your RV from the dangers of unregulated power, the fuse board will also act as a distribution panel, splitting the power drawn from the coach battery down into smaller wires which serve separate entities, such as individual appliances or circuits for lighting or outlets. The entire run of wire to the fuse board should be of an equal gauge to that between the battery positive terminal and the battery disconnect master switch; usually four-gauge.

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