RV Electrical, 12 Volt and 120 Volt

An RV runs two electrical systems that touch in a few specific places. The 12 volt direct current system feeds lights, water pump, furnace controls, slide and jack motors, the propane detector and most appliance logic boards, and it is supplied by the house batteries. The 120 volt alternating current system feeds outlets, air conditioning, microwave and the electric element in the water heater, and it comes from shore power, the house generator, or an inverter. Confusing the two is the root of most self diagnosed electrical problems.

The converter and the inverter are the bridges. A converter, often a Magnum, Xantrex or Progressive Dynamics unit, takes 120 volt alternating current and produces the direct current that charges batteries and runs the DC loads. An inverter runs the other direction, taking battery current and producing 120 volts for selected circuits when no shore power is present. Many coaches use a combined inverter charger. Knowing which box you have, and which subpanel it feeds, determines whether a dead outlet is a shore power fault or a battery fault.

A 30 amp service is a single 120 volt leg on a three prong TT-30 plug, which is 3,600 watts total for the entire coach. A 50 amp service is two 120 volt legs on a four prong 14-50 plug, which is 12,000 watts, not 6,000, and that distinction is the whole reason larger coaches use it. A 50 amp coach on a 30 amp pedestal through an adapter is not getting half power. It is getting one leg's worth of everything, and the load management decisions become yours.

Practical math prevents nuisance trips. A 15,000 BTU Dometic or Coleman rooftop unit draws roughly 12 to 16 amps running, with a much higher startup surge. A residential microwave takes 12 to 14. An electric water heater element takes 12. On 30 amp service you can run one air conditioner and one large appliance, and that is the honest budget. Coaches with an energy management system shed loads automatically, while coaches without one trip the pedestal breaker, and repeated tripping is what cooks pedestal contacts and plug ends.

The automatic transfer switch decides whether the coach draws from shore power or the house generator, and it is a frequent failure point that gets misdiagnosed as everything else. Contacts arc, terminals loosen, and one leg drops out, so half the 120 volt circuits go dead while the other half work fine and the generator seems fine too. That symptom pattern, half the outlets dead on shore power, is the transfer switch or a lost neutral, and it deserves a meter before any part gets ordered.

Onan and Cummins house generator faults follow their own tree: fuel supply, start battery, control board fault codes, and the generator's own transfer relay. The house generator is in scope for us. The chassis engine is not, and we do not perform engine or drivetrain work of any kind. Diagnostic work on any of this bills at $285 per hour with a one hour minimum, because it requires scan tools, documented fault codes and load testing rather than parts substitution. That hour is credited against the authorized repair.

Lithium iron phosphate batteries are a real improvement in usable capacity and cycle life, and a drop in swap is rarely just a swap. The existing converter's charge profile is written for lead acid and often has no lithium setting, which means undercharging or a chemistry mismatch. Alternator charging on a chassis with no DC to DC converter can push a lithium bank harder than the alternator tolerates. Solar controllers need reprogramming. Cable sizing that was adequate for a lead acid bank may not be adequate for lithium discharge rates.

A correct conversion is a system project: battery bank, compatible charger or converter settings, DC to DC charging where the chassis charges the house bank, a battery monitor with a shunt such as a Victron unit, appropriate fusing at the bank, and a low temperature charging strategy, since LiFePO4 must not be charged below freezing without internal heaters. Expect a Victron, Xantrex or Magnum ecosystem decision rather than a battery purchase. This work sits in the systems tier at 3 business days to 2 weeks and bills at $260 per hour.