Electrical Myths, Part 6: “Power Surge” Burned 30-Amp Plug

You’ve probably seen pictures or heard stories of charred 30-amp plugs plugged into a campground pedestal. You’ve also heard a variety of explanations, most of them incorrect or incomplete, but you know that the problem is more likely during the peak of air conditioning season, and related to the load from your RV. You hear talk of power surges, surge protectors and other gizmos, and aren’t sure what you need.

30-Amp Plug Basics

Here are a few things we should know about the 30-amp plug and receptacle commonly referred to as a 30-amp RV plug:

  • They are identified by their NEMA (National Electrical Manufacturer’s Association) designation: TT-30, with a “P” suffix to identify the plug, or end with male terminals, or “R” suffix to identify the receptacle, with female terminals.
  • It has 3 terminals: 2 flat-blade terminals, line and neutral, that are current carrying, and a round grounding terminal that only carries current in fault conditions.
  • Each of the two current carrying conductors connect via a fork-shaped female terminal inside the receptacle. When you insert the plug, the forks squeeze the male terminal, which spreads them slightly.
  • It must be wired for 120V AC service. It is not the same as a 3-wire “dryer” plug/receptacle, which is a 250V configuration/rating and has an “L”-shaped ground terminal.
  • For more discussion about power plugs, adapters, dog bones, etc., see our guide here.

What causes the plug to get hot?

The answer is largely the same as it was when we discussed space heaters a couple of years ago. Poor contact between the plug and receptacle combine with high current usage to generate heat.

But it isn’t “overloading” that’s the problem. If you overload the circuit, the circuit breaker trips. Both your RV and the pedestal should be equipped with a 30A circuit breaker. It only takes one of two working properly to interrupt power in a situation where the circuit is overloaded.

In cases like we see in the photos here, the problem is occurring at less than the 30A ratings for the circuit. As the female contact (inside the pedestal) is wears, the resistance at the interface increases. As you put a load on the circuit, the heat causes the female terminal to expand, reducing its contact pressure on your plug’s male terminals. That, in turn, means even more heat, and more expansion. It’s essentially a runaway scenario that would lead to a fire but for the fact that the expansion reduces the amount of current flowing through the circuit, usually at a rate that’s fast enough that you don’t get ignition of the plug and receptacle.

But that doesn’t mean no damage is done. The heat can damage the plug, receptacle, and the wiring leading to both. If you’re monitoring voltage inside your RV, and notice it starting to fall, check the plug. Also, check the plug at the side of the RV, if not permanently connected. Some warmth is normal, but it should never be too hot to touch, or hot enough to give off a noticeable smell.

Why does it seem like it’s always 30A RVs that have this problem?

The answer to this question has several components.

Age of the Receptacle

Many RV parks opened their doors years ago, with only 30A service on some or all of their sites. Many receptacles in service in those locations may be decades old depending on how the campground has been used. Destinations with large numbers of seasonal residents can expect longer service life from a receptacle, with campers plugging into it fewer times each year.

Every season in the elements also adds some corrosion, pitting and/or deposits, on the electrical contacts. Obviously, this is climate dependent, but temperature extremes, humidity, particulate matter, corrosive atmospheres (e.g. salt air) can accelerate the degradation.

But there’s not an obvious indicator as to how old a receptacle is. Even if there was a date code you could read, a receptacle decades old that’s been in the campground’s shop could easily be in better condition than a receptacle with more use made in the last year or two.

Number of RVs using the Receptacle

For any given RV site, on average, there will be more RVs capable of using a 30A receptacle than a 50A receptacle. If you recall the discussion about power adapters, it’s possible and safe to plug a 50A RV cord into a 30A receptacle, but not as safe the other way around.

Even if we banish the power adapter for the purpose of the discussion here, there are still far more RVs equipped with 30A shore power connections. Virtually anything with 1 or fewer air conditioners, including most travel trailers, smaller fifth-wheels, pop-ups, hybrids, most class B and C motorhomes, and smaller class As.

Every time someone uses it, the spring action in the receptacles degrades just a bit. Same for usage, with thermal cycling and physical wear.

Usage at the Limit

30-amp receptacle with burned contacts
From this Facebook post, you can see what happens when the 30-amp NEMA TT-30 plug and receptacle don’t make sufficient contact.

Even when an RV is configured for 30-amp service, it’s usage is much more frequently close to the limit. Remember that at 120V, a 30-amp receptacle supplies a maximum of 3600W, where a 50-amp receptacle can supply up to 12,000W.

Now think about what kinds of things are running in a typical RV. Air conditioners generally draw in excess of 1,200W, same for hair dryers, space heaters, electric cooking appliances (crock pots, pressure cookers, immersion cookers, microwaves, etc.), converters, water heaters…you get the idea.

Let’s take a medium-size travel trailer on a hot day. The air conditioner is running most of the time, along with the converter keeping the batteries charged and supplying DC loads. Add a few hundred watts for TVs and other electronics, and even that travel trailer is hanging at more than two-thirds of the rated power for the circuit.

On a bigger RV with 50-amp service, you probably add a second air conditioner, electric water heater, more electronics, bigger microwave, and might have more people using more stuff. But even then, and assuming you average double the load of the smaller RV, you only use 40% of the rated current (assuming a load balanced between the two hot legs). Where a hair dryer would trip the breaker in the 30A trailer, the receptacle doesn’t break a sweat with 50A service.

What can you do to prevent it?

First of all, let’s get a few things out of the way: this isn’t the result of a power surge or miswired receptacle, so surge protectors and other EMS systems don’t do anything to prevent this.

NOTE: There is one power protection device on the market that claims to detect a 30-amp plug/receptacle overheating. However, there is no indication of the trip temperature. They also lack any indication that it protects at both the campground pedestal AND Surge Guard receptacles–only protecting one still leaves you as vulnerable as you would have been without the device. None of the hardwired models have this functionality, as there’s no way to integrate a thermistor to monitor the temperature in a standard plug. It’s also not a UL-listed device, and lacks an enclosed receptacle for use outdoors.

Your first indication with EMS monitoring will be low voltage. Most trip at approximately 105V. If you’re using a pedestal close to the 30A rating at 120V, we’re talking about 450W of heat at those contacts. In other words, before you get to that fault condition, the plug melts.

Inspect the Receptacle

The single best thing you can do is inspect the receptacle when plugging in. Does it fit tightly? I know I’ve seen campgrounds with 30A receptacles so loose the plug would barely stay in. When you see this, stop. Notify the campground. Check any 15 or 20A circuits available on the pedestal, and consider using one of them for bare-bones power needs until they’re able to make a repair (i.e. running converter and refrigerator).

If the receptacle seems fine when you connect, that’s good, but not the end of the story. Particularly when running heavy loads for extended periods of time, check the plug. Put your hand on it. If it feels unusually warm, reduce your load and keep a close eye on it. Notify the campground, and if you can’t use necessary appliances, see that they fix the problem.

If identified early enough, the campground can simply replace the receptacle and move on. If not, the heat can damage your plug, like we see in the photos here, but it can also damage wiring leading to it, including your shore power cord, and the wiring inside the pedestal. A little bit of attention can save both you and campground operators a lot of money and hassle, not to mention avoiding fire or injury.

But don’t do this…

  • Sand or file contacts — making them thinner will make the problem worse, not better. If the contacts have visible corrosion, clean them.
  • Buy a “surge protector” — this isn’t a power surge at all, and won’t be prevented by it. There are reasons to have one, but this isn’t one of them.

Simple Summary

Pay attention. Pay attention to loads on the circuit. Feel plug connections, and look for signs of heat. Pay attention to the condition of wiring, receptacles, and your shore power cord. Pay attention to which campgrounds maintain, or fail to maintain, their electrical equipment. Supervise, or observe, repairs to your pedestal, and make sure the work is properly completed by a qualified person. The first person to plug in to a newly installed receptacle is most likely to find out whether it was wired correctly.

And don’t spend money on stuff that doesn’t prevent the problem you’re trying to solve.

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