30-Amp Coach with Voltage Collapse at the Pedestal
A 2019 30-amp travel trailer arrives with the complaint that the interior lights dim and the microwave clock resets every time the roof air conditioner compressor starts. The customer is plugged into a campground pedestal through a 50-to-30-amp adapter and a 25 ft, 10 AWG cord. With all loads off, the technician measures 121 V at the pedestal 30-amp receptacle. With the 13.5 kBtu air conditioner, the 55-amp converter and a few small loads running (about 25 A total), the pedestal voltage falls to 104 V, the distribution panel reads 101 V, and neutral-to-ground at the panel reads 2.8 V. The 30-amp main breaker is warm but has not tripped. The campground host says the pedestal was just rewired and suggests the technician put in a bigger breaker or use the 50-amp side.
1. Using the voltage readings, determine whether the fault is in the coach or the supply, and state what you would do before opening any panel.
Read the worked answer
Expected drop versus actual drop. 10 AWG copper is roughly 1.2 ohms per 1,000 ft. A 25 ft cord has about 50 ft of conductor round trip, so 0.06 ohm; at 25 A that is only about 1.5 V, plus a small adapter loss. Expected pedestal voltage under this load is about 119 V. The measured 17 V drop works out to about 0.68 ohm, roughly nine times the expected resistance.
Conclusion. The resistance is in the supply path, not in the coach branch circuits. Neutral-to-ground of 2.8 V at 25 A indicates about 0.11 ohm in the neutral path, which confirms a poor connection rather than a normal loaded neutral.
Likely culprits. Worn or heat-damaged 50-to-30 adapter blades, a pitted pedestal receptacle, loose breaker terminations in the pedestal, a damaged cord cap, or an unnoticed long 16 AWG extension.
Immediate action. Shed loads, shut off the coach main, disconnect, and treat the pedestal as unsafe until tested by the campground electrician. Do not open a pedestal you do not own, do not raise the breaker size, and do not defeat the ground. Test the pedestal with a known 12–15 A load such as a heat gun; if it still collapses, request another site.
2. Outline the test sequence that isolates the faulty segment, and describe what repairs are acceptable and unacceptable.
Read the worked answer
Sequence.
- Kill all coach loads and confirm no-load voltage at the pedestal.
- Apply a known resistive load at the pedestal and record the voltage; a collapse here condemns the pedestal or its breaker terminations.
- Move the same load to the female end of the cord through the adapter and compare. The difference between readings is the loss in the adapter and cord.
- Thermally inspect and flex-test the cord cap, adapter and receptacle; look for discoloration, green corrosion and heat marks.
- Repeat at the coach panel main lugs to confirm the coach inlet and transfer switch are not contributing.
Acceptable repairs. Replace a damaged cord, adapter or inlet; have the campground electrician torque or replace pedestal terminations; relocate to a healthy site.
Unacceptable. Installing a 50-amp breaker on 30-amp conductors, filing or spreading adapter blades, removing the ground pin, or adding a second parallel cord. The 30-amp breaker protects 10 AWG conductors and the TT-30 receptacle; it is not a voltage regulator. After repair, load-test again and expect roughly 114 V or better at the coach panel with normal loads, and less than about 1 V neutral-to-ground.
3. Explain to the customer, in plain numbers, why 104 V damages equipment and why the 50-amp side of the pedestal is not the fix.
Read the worked answer
At 104 V the coach is 13% below nominal. Motors are constant-power loads: when voltage falls, current rises. The air conditioner compressor draws more current on start and run, its windings and start capacitor run hotter, and nuisance trips or premature failure follow. The converter must draw more input current to hold 12 V output, so it also heats up. Electronic controls see brief dips at compressor start, which is why the microwave clock resets.
Plugging into the 50-amp receptacle through the adapter does not change anything inside the coach: the coach still has a 30-amp main, 10 AWG wiring and a 30-amp inlet. It only changes which pedestal breaker feeds the same single hot leg. If the pedestal or cord is the resistance, the drop follows the current no matter which receptacle is used. A bigger breaker would allow more current through the same bad connection, adding heat and fire risk without raising voltage.
Tell the customer the repair is to find and correct the high-resistance connection, not to increase the breaker. Recommend an EMS with low-voltage cutoff set near 104–106 V so the compressor is protected when campground power sags.