Understanding your RV electrical system

The two systems, and why it matters

Your RV is wired like a small house and a large car at the same time. The 12V DC side is the car half: it comes from the house battery bank and runs everything that has to work when you are parked in a field with no hookups. The 120V AC side is the house half: it comes from a campground pedestal, a generator or an inverter, and runs the appliances that need real power.

Almost every confusing RV power symptom becomes simple once you assign it to one system. A furnace that will not light while you are plugged into shore power looks like a gas problem, but the furnace is a 12V appliance, so a dead battery explains it perfectly. A microwave that will not run on battery power looks like a fault, but the microwave is 120V and needs an inverter to work off the battery at all.

• 12V DC: interior lights, water pump, furnace blower and controls, range hood, fans, slide-outs, leveling jacks, awning motors, refrigerator and water heater control boards, monitor panel
• 120V AC: rooftop air conditioner, microwave, wall outlets, television, residential refrigerator, water heater electric element, converter

What the converter does

The converter takes 120V AC from shore power or the generator and converts it down to 12V DC. It does two jobs at once: it charges the house battery, and it powers the 12V system directly so the battery is not being drained while you are hooked up.

This is why a failed converter is so easy to miss. Everything keeps working normally for a day or two, because the battery is quietly carrying the whole 12V load on its own. Then the battery flattens and it looks like the battery has failed, when actually it was never being charged.

The converter usually lives inside the power distribution center, the panel that also holds the 12V blade fuses and the 120V breakers. It has its own AC breaker and, on most units, two DC output fuses mounted on the converter itself. Those output fuses are a very common failure point and are frequently overlooked because they are not in the main fuse block.

What an inverter does, and why not every RV has one

An inverter does the opposite of a converter: it takes 12V DC from the battery and produces 120V AC, so you can run household devices without hookups or a generator. Many RVs ship without one, or with a small inverter wired to only a couple of outlets.

Inverters are limited by battery capacity, not just by their own rating. A 2000W inverter can technically run a microwave, but doing so pulls close to 200 amps from the battery bank, which a single lead-acid battery cannot sustain for long. This is the main practical reason most RVs cannot run a rooftop air conditioner off battery power without a substantial lithium bank.

Shore power, 30 amp and 50 amp

A 30 amp RV service is a single 120V leg providing roughly 3600 watts in total. A 50 amp service is two 120V legs providing roughly 12,000 watts. That is not a 60 percent increase, it is more than three times the capacity, which is why 50 amp coaches can run two air conditioners and a 30 amp coach cannot.

Adapters let you plug a 50 amp coach into a 30 amp pedestal, but they do not create capacity. You still have 3600 watts to share across everything you switch on.

Voltage quality at the pedestal matters as much as amperage. Long extension cords, undersized adapters and busy campgrounds all sag the voltage, and low voltage is genuinely destructive to air conditioner compressors and other motors. A surge protector with voltage monitoring is a worthwhile investment, because it disconnects the coach when voltage drops out of a safe range.

A diagnostic order that actually works

When something electrical stops working, resist the temptation to start at the device. Work from the source outwards, because a fault upstream produces symptoms that look exactly like a failed appliance.

1. Decide which system the device belongs to: 12V or 120V.
2. If 12V: check the battery disconnect switch, then measure battery voltage at the posts, then check the main fuse, then the branch fuse for that circuit.
3. If 120V: check the pedestal breaker, then the RV main breaker, then the branch breaker, then any GFCI outlet in the circuit. A tripped GFCI in a bathroom commonly kills outlets elsewhere in the coach.
4. If everything on one system is dead, the fault is at the source. If only one device is dead, the fault is at that branch.
5. Only after the supply path is proven good should you suspect the appliance itself.

Battery basics worth knowing

House batteries are deep-cycle batteries, designed to be discharged and recharged repeatedly. That is a different job from a starting battery, which delivers a huge burst and is then immediately recharged. Using the wrong type in either role shortens its life considerably.

A flooded lead-acid battery should not routinely be discharged below about 50 percent of its capacity, which in practice means a 100 amp-hour battery gives you about 50 usable amp-hours. Lithium batteries can safely use far more of their rated capacity and last considerably longer, but they usually need a converter with a compatible charge profile, which is the step people miss when upgrading.

Resting voltage is a rough but useful state-of-charge gauge for lead-acid: about 12.7V is full, 12.4V is around three-quarters, 12.0V is about half, and below 11.8V is deeply discharged. Take the reading with no load and no charging for a couple of hours, or it will read high.