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greekislandlover

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Going to the US next year, and am going to hire an RV for part of the trip. Doing all the research at the moment, and am reading some forums on the topic.

One of the threads is where a lady managed to plug a load of 110v appliances into 220v and sh*gged the lot. My question is how is that possible to do? Surely 110 and 220 outlets are different shapes and colours to stop this happening as they are here?

From my understanding of the ensuing thread, in the US final circuits are fed from a centre tapped transfprmer and comprise of a neutral to the centre tap, and two 110v supplies from the transformer. So connecting between the neutral and each of the line connections gives you 110 but at opposite phases of the cycle, and connecting between the two lines gives 220 for dryers and welders and such. So if a neutral goes missing it's possible to get 220v by mistake, if some numpty doesn't know what he's doing you can blast appliances with double the voltage and so on.

So my understanding of the US electric supply is now confused. Anyone able to offer a proper insight into how it works over the pond?

I'm mainly concerned that I'll plug my rented RV into the wrong voltage socket and blow it up!
 
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T

tony.towa

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This sounds like a job for mdshunk. He might even be able to help with the RV side too. Sounds like a neat idea for a holiday though.
 
M

mdshunk

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120 volt and 240 volt plug configurations are quite different. It would be downright difficult to send 240 into your 120 volt cord on your RV...

but....

RVers are an industrious lot, and they often have "cheaters" of all manner to plug anything into anything else. They make unsafe connections on a regular basis with these home made adaptor cords. RV campsites, where you might plug in, are sometimes also cobbled up over the years by their non-electrician owners. You can't always be 100% guaranteed that a certain receptacle that you're about to plug into is the voltage and ampacity that you've expect for that particular plug configuration.

I don't camp by RV, but if I did I'd have a meter handy to be sure that what I'm about to plug into is what I think it is. As an electrician, I might have a variety of adaptor cords made up for myself too, but I'd have enough brain in my head to know what I'm doing with them.

Yes, US supply is 240V, center tapped neutral, with 120 volts from the center tap to the line connection. One popular "cheater" cord would be two 120 volt plugs to connect to two different receptacles on opposite "phases" to bring 240 to the RV where the campsite doesn't have a 240V receptacle available. Another popular cheater is a 240V plug with only one line and the neutral connection wired so that you can feed just 120V to the RV. I suspect that such an adaptor was used in the original poster's case, and it was improperly fashioned.

By the way, some RV's are 120V and some are 240V. Depends on the size of the RV, mainly. When I say 240V, I really mean 240/120, since we're talking about both lines, the neutral, and the ground, to give the ability to serve 240V appliances and 120V appliances inside the RV.

That might be clear as mud to you all, but I can explain more if it's interesting to you.

You started to talk about a particular condition called "open neutral" which can cause problems with our 240/120 center tapped supply. Here's a graphic to explain how this trouble comes about. Can put a real squash on your day if the electric company's drop wire to your house has a neutral conductor go open:

 
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G

greekislandlover

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Thanks for the reply - much appreciated.

I did rather think that there were some DIY electrics going on, but I wasn't going to suggest this on the other forum. I think that's what others are thinking too. Interesting bunch of people from the rest of the forum!

Anyway - electrics. Am I right in thinking that each outlet in a domestic property has two separate phases, and to get 120v, you can use either of these phases? Is some requirement or advisory that the load should be shared over the phases meaning that some appliances would be connected to neutral and L1, and some to neutral and L2? Or are all of the appliances supplied with a plug wired in the same way?
 
M

mdshunk

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Anyway - electrics. Am I right in thinking that each outlet in a domestic property has two separate phases, and to get 120v, you can use either of these phases?
That is correct. The breakers "plug in" the panels, so if you plug them in a row, each one is on a different phase. Breaker 1, a plase; breaker 2, b phase; breaker 3, a phase again, breaker 4, b phase again...

Is some requirement or advisory that the load should be shared over the phases meaning that some appliances would be connected to neutral and L1, and some to neutral and L2?
Actually, in residential work, there's not really any way to "balance" the load, due to the varying way the electrical appliances and lighting in a home is used. What runs coincidental this minute might be quite different than what's running coindidental an hour from now. For that reason, as long as you have roughly the same number of breakers plugged on both phases, you've done as good as you can do. In commercial work, where the load profile is a little bit more continuous and known, some effort is put into balancing the load, often among all three phases in commercial work, to the best of one's ability.
 
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