By Chris Dougherty
Chris Dougherty is a certified RV technician. Here is a letter he received from a reader while he was serving as RVtravel.com’s technical editor.
A couple weekends ago, we were using close to the rated 30 amp draw on our trailer with fridge, water heater, periodic microwave, HVAC, etc., when my son suddenly told me that the power plug outside was smoking slightly. When I looked, sure enough it was melting in the socket!
I actually had three ammeters running at the moment (genny output, AC input, and an experimental graphing smart monitor) and they all agreed we were at about 27 amps for the last few minutes before meltdown. Yes, close to maximum but not quite over. Just to check for a defective main breaker, after replacing the plug, I intentionally put 32A draw on for a few seconds, and the breaker popped immediately. How normal is it for slightly less than 30A to still dangerously overheat the shore cord?
I would have expected a margin of safety that apparently doesn’t exist here. Is there anything else I should check? If this IS normal for borderline draws, perhaps the newsletter should warn folks to stay further below the max draw? —Wolfe
This is a great point, and it’s not just because of the amp draw. This is likely the result of resistive heating. Resistive heating occurs in an electrical connection where power can’t flow freely, usually because of a poor or dirty connection.
Let’s look at a hose. Water is measured in pressure (psi) and flow (gpm or gallons per minute.) Firefighters use different size hoses for different water flows. Would you see a firefighter go into a fire with a garden hose? No, because it can’t flow enough water. On the same note, a firefighter is in a fire and he loses pressure on his line, only to find that it got caught under a door which restricted the flow.
The same thing happens with electrical conductors. If the contact in a plug doesn’t connect firmly due to age, heating, corrosion and/or being bent, then as it tries to pass the electrical energy it starts to have arcing at the contact, which heats the metal, it expands, and the problem worsens. Over time, with repeated heating, this will get worse.
Campground plugs/receptacles are continuously connected and disconnected, and abused in other ways. So, if the terminals on either side were bent and/or corroded, this would lead to exactly what you experienced. The plug and receptacle assembly normally can pass this amperage without an issue. Unfortunately the contacts do wear on both sides which can lead to this condition.
You will have to replace the plug on your shore power cord. Unfortunately, because of maintenance and wear issues on the campground side, you probably can’t prevent this from happening. Many RVers never experience it, but those that do, including me, just have to make the repairs.