No smoking – especially in shore power connections

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.

Dear Chris,
smokin-plugA 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

Dear 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.

##RVT830

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8 Thoughts to “No smoking – especially in shore power connections”

  1. DAVE TELENKO

    Right after we got our new M/H I decided to put in a 50 amp service in the garage. I had to buy a 50′ extension cord rated for 50 amps, that have 3 @ 6 gage & 1@ 8 gage ground. What my point was it came with a very cool heavy duty plastic cover for the male end. I now keep it on the male end in the M/H 50 amp plug as it has rough life when in storage & in use! I don’t know if they sell that separately or not! Actually I sent CAMCO a email asking them if they have them for 50 & 30 amp!

  2. Sherry Dawson

    I read and saved this comment from a reader in a previous issue. Mike Sokol agreed with it. I paraphrased it for my notes:
    “Regularly use a product that cleans and lubes the contacts, like De-Oxit. . .with the breaker off, of course. I use it every time I pull into an RV space before plugging anything into the pedestal. Along with a good quality EMS these should go a long way alleviating hot wire problems, assuming one uses the proper cord that is.” Sorry, I didn’t note the contributor’s name..

    Would this have helped in this case?

  3. Oldkartracer

    I always insert and remove my plug several times before I flip the breaker on. This helps to clean the oxides off the connector surfaces. Also notice how much force it requires. A new socket may take two hands to remove the plug. If the plug requires little force, the socket is worn and can allow a poor connection. I would check the plug and if is more than slightly warm, reduce current draw, ask the park to take a look at it or move to a different site.

  4. Scott Taylor

    Since the 30A circuit is likely the most commonly used connection, is it better, even with a 30A rig, to use a dogbone in the 50A outlet, if available, since that probably gets less use and might be in better condition?

  5. Rodney

    In industry Eason for a maximum of 80 percent load in a transformer and wiring . The 20 percent is reserved for startup in rush surge. With that 30 amps is only good for 24 amps continuous current. It was over done by 3amps over an extended period

  6. Bill Lampkin

    Yes you can keep this from happening. Simply replace the cord end with a twist-lock cord end like is used on boats and construction generators. Yes, you will have to keep a matching pigtail (dogbone) adapter so you can plug your twist lock cord into a blade type receptacle. But cord connection overheating can be eliminated. Try it, you’ll like it (and you will get more volts to your rig!)

  7. Traveling Man

    Without diving too deep with the Nat’l Electric Code (NEC), wire size will depend on the wire type used and the distance that it is run. Typically, a camper will not have this information. In some cases, campgrounds are built outside any authoritative jurisdiction that inspects the initial installation and they may not exactly use what is required.

    The receptacles are designed using IEEE and other Standards. So the plug (brand new) is usually not a problem.

    The real problems begin with wear and maintenance (or the lack thereof).

    Park Owners are looking to maximize budgets. They may or may not have a qualified maintenance person on site. In many cases, Parks subscribe to a Run-to-Fail maintenance program.

    On the other hand, many RV owners don’t maintain their equipment either. Check the prongs on your cord each time you plug it in or pull it out. Try not to expose the cord to direct sunlight (I know we are camping and that can be a challenge). Check the connections on your RV for arcing, burned spots, and loose fittings. Touch the fitting when you think about it checking for heat. Use a thermography gauge if you have one (for your tires for example). Use as short a cord as you can. Use the correct extension cord size for the amperage. Some people will try to use a 30 amp cord on a 50 amp circuit…It’s OK to use a 50 amp cord on a 30 amp circuit. Consider using a 90 degree fitting to reduce the stress on the cord and receptacle. Use a Power monitor that can help eliminate the risk to your RV while in a campground. I personally use the Progressive Industries 50A hard-wired unit (ems-hw50c). It has saved the rig on a couple of occasions now.

    Back to the electrical circuit design (assuming everything has been installed to code)….Circuits have a safety margin built in. On ANY circuit, they are designed to carry 80% of the rated load for long duration’s of time. But when running 100% for a long time, this can have devastating affects on the equipment. Go over 100%, you will trip the breaker. Breakers work on 2 principals (without the inclusion of GFCI or Arc-Blast rated breakers). They work on heat (due to high thermal load amperage) and instantaneous short circuits which cause a magnetic field to trip the breaker. So, how many RVer’s run their full 50 Amps? They are causing maintenance problems for RV parks. Breakers are designed to trip one time (Guaranteed). Most people will reset them without any future testing program or replacement program. The bottom line is that as Parks age, so does the equipment. The grease can harden in the breaker. The field of the magnetic coils can break down due to the excessive heat applied for long periods of time. The receptacles wear from plugging in cords and pulling out cords. Weather builds corrosion causing a higher resistance in breakers, receptacles, and splices. Higher resistance means more heat.

    There is not a lot can do about the Campground equipment. But checking your own equipment and understanding how heat affects the equipment will help you be safe.

    Happy Glamping!

  8. Ric

    Had the same problem and yes it turned out to be duty contacts. I now clean them regularly and especially before I go out on a trip. No problems sence.

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