Electrical Systems (12 VDC and 120 VAC)
An RV carries two electrical systems that share a chassis: 12 VDC house and chassis circuits, and 120 VAC shore, generator, and inverter circuits. The exam tests Ohm's law and voltage drop, battery chemistry and charging, converter and inverter behaviour, shore power configurations, bonding and grounding, overcurrent protection, and disciplined diagnostic method.
12 VDC
- Ohm's law and power: V = IR and P = VI, so a 60 W load at 12 V draws 5 A.
- Ampacity and voltage drop are separate checks. A conductor can be thermally fine and still drop excessive voltage over distance.
- Batteries: flooded, AGM, gel, and lithium iron phosphate. Charging profiles differ. A converter with a fixed 13.6 V output will not fully charge lithium and will overcharge flooded cells.
- Flooded state of charge at rest: 12.7 V is 100 percent, 12.2 V is about 50 percent, specific gravity 1.265 is full.
- Parallel adds capacity, series adds voltage. Mixing ages or chemistries shortens life.
120 VAC
- 30 A shore service is 120 V on three wires (hot, neutral, ground) for about 3,600 W. 50 A service is 120/240 V on four wires for about 12,000 W.
- Neutral and ground are bonded only at the source, at the pedestal or generator. A second bond in the RV panel creates parallel return paths, nuisance GFCI trips, and shock hazard.
- GFCI protection is required in wet locations; test with the test button and a plug tester.
- Transfer switches must switch hot and neutral together where required by design; inverters need correct neutral bonding.
Worked example
A 15 A pump sits 20 ft from the battery on 10 AWG copper, 10,380 circular mils. Voltage drop = 2 x K x I x L / CM = 2 x 12.9 x 15 x 20 / 10,380 = 0.75 V, which is 6.2 percent of 12 V and over the 3 percent target. Moving to 8 AWG at 16,510 CM gives 0.47 V, or 3.9 percent. Going to 6 AWG gives 0.30 V, or 2.5 percent, and passes.
Traps
- Leaving the meter on DC and reporting no voltage on an AC circuit, or the reverse.
- Assuming a converter is charging because it reads 13.2 V; that may be float only.
- Replacing a blown fuse with a larger one instead of finding the overload.
- Believing a 50-to-30 A adapter adds capacity. It does not; load management is the technician's job.
- Chasing a 12 V fault without checking chassis grounds, which corrode first.