RV, Van & Marine Power

RV, Van & Marine Power calculators — each with the formula, a worked example and a reference table. Results are estimates for planning and education; verify all sizing with a licensed electrician and your AHJ.

Low voltage means high current — and heavy cable

RV, van and marine electrical systems mostly run at 12 or 24 volts DC, and that low voltage changes the rules. Delivering the same power at 12 V draws roughly ten times the current of a 120 V circuit, so DC runs need surprisingly heavy cable and voltage drop becomes the dominant constraint — many builders hold 12 V circuits to a tighter drop than the 3% used on household wiring, because a volt lost on a 12 V system is a large share of the supply. Size those runs with the 12V DC Wire Size calculator, measuring the real routed length out and back.

Start any build by tallying the loads. The Appliance Amp Draw calculator converts each appliance's wattage into the DC amps it pulls from the battery, and the Battery Runtime calculator tells you how long the bank lasts from its amp-hours and usable depth of discharge. To keep the battery charged off the vehicle alternator while driving, the DC-DC / Alternator Charger calculator sizes the charger and its cabling; to charge from the sun, the RV Solar calculator matches panel wattage to your daily use and available roof space.

The underlying cable method is the same one covered in 12V wiring and DC wire size for RVs and vans. Marine installations carry extra requirements (ignition protection, tinned wire, ABYC practice) beyond these general estimates. Size fuses and breakers to protect the wire, and verify a full build with a qualified RV or marine electrician.