By Mike Sokol
Our surge suppressor failed, and when we unplugged the 50-amp cable, we found this. Our cord was hot to the touch. What could have caused this? Could we have somehow caused it? We do have a new suppressor and power cord! See the attached picture (below right).
Thanks for any advice. —Maryanne Perry
I have a pretty good idea what caused this, and no, it wasn’t your fault. Let’s take a close look at the picture you sent to diagnose exactly what got hot, and that will allow us to determine just how it could have happened. Take a look at the picture on the right. As always, you can click on any picture to enlarge it for a better view.
You’ll notice there are four wires on your 50-amp shore power plug. I’ve labeled them as Hot 1, Hot 2, Neutral and Ground. You can clearly see that the overheating occurred on the Neutral contact at the bottom of the picture, and it began to melt the rubber insulation – which is very dangerous. But in reality, the Neutral contact is not to blame. It’s simply the victim of a pair of misbehaving Hot 1 and Hot 2 connections. Let’s see why that is.
If you take a look at my diagram on the left, you’ll see that the campground pedestal (and all NEMA 14-50 outlets in general) are supposed to be wired so that Hot 1 and Hot 2 are connected to two different legs (poles) on the incoming power transformer from the power company. And that’s the reason we can get 120 volts by connecting an appliance between the neutral and either Hot 1 or Hot 2.
You can also see from the diagram that connecting any appliance between the Hot 1 and Hot 2 contacts will result in 240 volts, which is how your home electric water heater and oven are wired. However, very few RVs actually use 240-volt power for anything, so their appliances are divided up between the two Hot Legs of 50 amps each. And that’s why a 50-amp RV connector can supply a total of 100 amps of current: It has 50 amps of current available on each 120-volt leg and 50 + 50 = 100 amps. It’s simple addition.
The hat trick is that when this 50-amp connector is fed by two different legs (poles) the current in the neutral subtracts rather than adds. So if you’re drawing 40 amps of current on the Hot 1 leg, and perhaps 30 amps of current on the Hot 2 leg, then instead of the expected 70 amps of current on the neutral (40 + 30 = 70), you only have 10 amps of current on the neutral (40 – 30 = 10). And that’s why the neutral wire and contacts in your shore power cord are sized to handle a maximum of 50 amps of current rather than 100 amps of current. If you force a 50-amp rated wire and connector to carry 70, 80 or 90 amperes of current, then it will overheat and eventually burn up.
What causes this? It’s non-code compliant wiring at the campground. Instead of running two separate wires (poles) of 50 amps each, they simply jumped a single pole (wire) of power to the Hot 1 and Hot 2 contacts on the pedestal outlet. They probably did this to save money by not rerunning a 4th wire when they upgraded to newer 50-amp pedestals from older 30-amp pedestals. And while your RV appliances will function normally because they only need 120 volts (not 240 volts like your home appliances), you won’t know that the neutral is being overloaded with up to 100 amps of current until it’s too late. This is a clear code violation that should have been caught during electrical inspection. Problem is, campground wiring is almost NEVER inspected, which is why you need to protect yourself from this sort of miswiring.
So here is a solution to protect your RV wiring from this kind of assault. The latest line of 50-amp surge protectors from Surge Guard actually monitor the neutral wire for excessive amperage draw and will shut power down if the current exceeds 70 amps. And that should prevent your neutral connectors and wiring from burning up if you happen to plug into a miswired pedestal with a single-pole 50-amp outlet.
I’ll note that you should also inspect the neutral wire inside of your RV where it connects into your circuit breaker panel to make sure it hasn’t overheated there as well. You want to make sure your neutral wire hasn’t been overheated inside of your RV because an open neutral will cause all sorts of very bad things to happen. I’ll write about what happens if your 50-amp shore power neutral becomes compromised in a future article, but for now lets just say it’s a very expensive failure.
RV Electricity Seminar Update
I’ll be presenting my first Advanced RV Electricity Seminar on June 8th in Funkstown, MD. There will be a 1 hr Basic RV Electricity seminar starting at 10:30am, followed by a 3 hr Advanced RV Electrical Troubleshooting seminar beginning at 1PM. Cost is $30/$20 for the Basic seminar and $125/$100 for the Advanced seminar. RVtravel Member/Readers get the discounted price. For more information on seminar content and to pre-register, please click HERE.
For the entire list of all RV Electricity Seminars I have scheduled this summer, please click HERE.
See you next week. In the meantime, let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.