120V system
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- What is the 120V complaint? Start at the pedestal with a meter or a plug-in tester before you blame the coach. Expect: Pedestal: 120 V hot to neutral, 120 V hot to ground, about 0 V neutral to ground. 50 A: both legs 120 V to neutral, 240 V between legs.
- Is there power at the pedestal and at the end of the cord? Meter the pedestal receptacle, then the coach end of the shore cord with it plugged in. Expect: 120 V at every step
- Power at the main breaker? Meter the line side and load side of the main breaker in the panel. If the coach has a transfer switch, meter its shore input and its output. Expect: 120 V on both sides of the main with it on
- Dead outlets: GFCI or branch? Most RV outlets daisy-chain off one GFCI in the bathroom or kitchen. Find it and press reset. Expect: One GFCI feeds the bathroom, kitchen counter, outside and often the bedroom outlets
- What trips the GFCI? Unplug everything on the circuit. Reset. Expect: A GFCI trips at about 5 mA of leakage to ground
- Breaker trip: instant or after a while? Reset it with the loads off, then add loads. Expect: A 20 A breaker holds 20 A continuously; a 15 A breaker holds 15
Causes and repairs
Pedestal breaker off or pedestal fault
Reset the pedestal breaker. If it trips again with the coach unplugged, it is the campground's problem.
Tools: multimeter
Have the park fix it. Don't jumper around a dead pedestal.
Shore cord or inlet failed
Burned blades on the plug, a melted inlet on the coach, or a broken conductor. 30 A plugs that ran hot are the usual find.
Tools: multimeter, flashlight
Inspect both ends for heat. Replace a burned plug or inlet. Check the inlet's wiring on the back for a loose terminal.
Transfer switch failed
The relay didn't pull in for shore, or the contacts burned. Common in coaches with a generator. Check the relay coil gets power from the shore side and listen for the click.
Tools: multimeter
Clean and tighten the terminals (they loosen and burn). Replace the switch if the coil is dead or the contacts are pitted.
Main breaker failed or loose
A main that reads line voltage in and nothing out while on is bad. Also look for a loose or heat-damaged wire on it.
Tools: multimeter, screwdriver
Replace the breaker. Torque the lugs. Check the neutral bar while the cover is off.
GFCI tripped
Something downstream leaked current or the outside outlet got wet. If it stays reset, you are done; if it trips again, see the GFCI branch.
Tools: none
Reset. Dry the outside outlet and check its cover.
Open in the outlet chain
Outlets are wired through with push-in or screw clamps. One loose backstab kills everything after it. Find the last working outlet and open the next one.
Tools: multimeter, screwdriver
Fix the loose connection. Replace backstabbed outlets with screw terminals.
Leakage in the wiring
A downstream neutral grounded, water in an outside box, or a hardwired device (water heater element, fridge heater, converter) leaking to ground.
Tools: multimeter, insulation tester
Disconnect downstream outlets one at a time from the GFCI's load side. When it holds, the fault is in what you just disconnected. Ohm each element to ground.
Appliance leaking to ground
A water heater element with a cracked sheath, a fridge AC heater, a space heater, or a coffee maker with a wet base.
Tools: multimeter
Ohm the heating element to its case: must be O.L. Replace the leaking element or appliance.
Outside outlet getting wet
Water in the box or a cover that doesn't seal.
Tools: hand tools
Dry it, replace the outlet and the in-use cover. Seal the box penetration.
Short on the branch
A shorted appliance cord, a nail through a wire, or a burned outlet. Unplug everything and try; if it still trips, it is in the wiring.
Tools: multimeter
Isolate by unplugging. Open the outlets on the circuit and look for scorching. Megger the run if you have one.
Overload or a motor drawing too much
Add up what's on the breaker. Then clamp the appliance: an AC compressor that pulls locked-rotor amps and stays there is shorted or has a bad capacitor.
Tools: clamp meter
Clamp running amps and compare to the data plate. Move loads to another circuit. See the AC guide for compressor tests.
Weak breaker or a hot connection
Breakers age and trip low, especially in a hot panel. A loose wire on the breaker heats it and trips it too.
Tools: clamp meter, infrared
Tighten the wire. Clamp the load: under rating and still trips, replace the breaker.
Stop: open neutral
Lights bright in one room and dim in another, electronics dying, is an open neutral on 50 A service. Without the neutral, the two 120 V legs float and one side sees up to 240 V. Unplug now.
Tools: multimeter
Check the pedestal neutral, the plug and inlet neutral blade (often burned), the transfer switch, and the panel neutral bar. Fix the open before you plug back in. Replace what burned.
Stop: hot skin
Any AC voltage from the coach skin to earth over a couple of volts is a fault: a missing ground, reversed polarity, or a leaking appliance. Unplug the coach.
Tools: multimeter, non-contact tester
Test with a non-contact tester or meter from the skin to a ground rod. Fix the ground path (pedestal, cord, inlet, panel ground bar, frame bond). Find the leak with every breaker off then on one at a time.
EMS or surge protector refusing power
The protector saw low or high voltage, open ground, reversed polarity, or open neutral and is doing its job. Read its code.
Tools: multimeter
Fix the pedestal problem or move sites. Low voltage under load (under about 104 V) means the park's wiring, a long cord, or too much load. Don't bypass the protector to 'make it work'.
Keep going on 120V and shore power
Learn it
- Shore power: 30A vs 50A
- Breakers, GFCIs and the EMS
- The lost neutral (50A danger)
- Transfer switches and generator power
Prove it
- Half the coach is dead
- Bathroom outlets are dead
- Lights surge and the microwave died
- Shocked at the door handle