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Old 08-07-2014, 08:17 AM   #15
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With all due respect to TG Twinkie above, a licensed electrician once explained to me that a properly configured 50 amp service has four leads, two positives, a neutral and a ground. A 30 amp service has only one positive, one neutral and one ground. Despite the arithmetic suggestion otherwise, a 50 amp service is really two 30 amp services in parallel. The 50 to 30 amp adapter we are discussing (i.e., the dogbone) has a dead termination for one of the positive leads in the part that connects to the RV park's 50 amp plug, therefore the current that is flowing through your 30 amp wire is really only 30 amps. If the breaker trips in your trailer after hooking up to a 50 amp service with the dogbone adapter, it's not because you are getting 50 amps, there is something else terribly wrong that would be going on.
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Old 08-07-2014, 09:04 AM   #16
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A 30amp RV setup uses one hot (L1), one neutral (N), and one ground (G). It is fused (breaker) only on L1 with a 30 amp breaker. Voltage between L1 and N is 120vac.
The 50amp RV setup uses two hot (L1) and (L2), one neutral (N), and one ground (G). It is fused (breaker) on L1 and L2 with a 50 amp breaker on each of L1 and L2. Voltage between L1 and N is 120vac and between L2 and N is 120vac and between L1 and L2 is 240vac.
Being that most RV's only use 120vac, if yours has a 50amp plug, have of your loads (electrical items in your RV) will use the L1 to N side at 120vac and the other half will use the L2 to N side at 120vac. Each protected by its own 50amp breaker.
The 50 to 30amp adaptor uses only one of the L1 or L2 legs. However it still has the 50amp breaker at the pedestal. So your 30amp power cord could have more than 30 amps running through it if you don't have a 30amp main in your trailer.
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Old 08-07-2014, 09:25 AM   #17
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Let me add the the above post by explaining amperage.
Your breaker panel has one main breaker rated at 30 amps and 3 branch circuits rated at 20 amps each.
Branch circuit 1 protects the A/C.
Branch circuit 2 protects the microwave.
Branch circuit 3 protects all the outlets.
You turn on the A/C which draws 12 amps. Well below its 20 amp breaker.
You then turn on the microwave which draws another 12 amps. Well below its 20 amp breaker.
However 12 plus 12 equals 24 amps which is now flowing through the main 30 amp breaker.
You now turn on a blow dryer (which I would never use due to lack of hair the past 10 or so years) which draws 8 amps. Well below the 20 amp breaker it is on.
However 12 plus 12 plus 8 equals 32 amps. The main will trip.
Now if you don't have a main in your trailer and you are using a 30 amp cord pluged into a 50 amp service with an adaptor, your cord will over heat.
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Old 08-07-2014, 10:01 AM   #18
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This is a very informative thread. Thanks Geocamper for the accurate explanations. You were right on when you explained that current is not the same as voltage. Here is the way I see it.

When you use an adapter and plug into a 50 amp service your trailer "can" draw more than 30 amps. I have one AC and no microwave. We don't use many appliances and when the wife wants to dry her hair we are not running the AC so I am not worried.

If I was powering up a bunch of things, once the current went above 30 the main breaker in the trailer would trip. So, to answer the OP's question, you can't damage your trailer when you hook up to the 50 amp post. You could damage your power cord and that would be the worst of it but I think that this would be a very long shot as well.
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Old 08-07-2014, 11:12 AM   #19
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Good explanations and clarifications, thanks!

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Old 08-07-2014, 11:57 AM   #20
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Newbie question about 30 amp service

With all due respect. Just because an electrician has a license. It does not mean he/she is knowledgeable in all aspects of an electrical system.
I am glad others chimed in her to add to my explanation of how the power system works when it comes to 120/240 volt 4 wire systems and how that system is wired to an RV.
In most RV systems only one hot leg is required. However there are a number of the large motorhomes and 5th wheel trailers that use both legs of the 240 volt 4 wire system. These units are equipped with multiple air conditioners as well as microwaves and washer and dryer combos etc.
An example of a 30 amp 4 wire system in your home would be the circuit that provides power to the electric dryer. The 240 volt circuit L1 & L2 power the heating element. The motor a controls are 120 volt using one leg and the neutral.
The electric range requires a 50 amp 4 wire system. The oven elements are 240 volt in most cases while the stove top elements and controls are 120 volt.
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Old 08-07-2014, 02:26 PM   #21
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Question - when I have our 1980 31' connected to our garage for shore power, the inverter is charging our single battery and all lights and fantastic fans are operational. I will need to recheck, but I think a few of our outlets are not operational. If we try to turn on the AC fan, it works, but to turn on the AC to cool, it blows the circuit. If we try to turn on the original Dometic gas/electric fridge, it blows the circuit.
How can we check our fridge to see if it works on AC power? (since we are more concerned with the fridge working than the AC)
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Old 08-07-2014, 03:28 PM   #22
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What a great thread

I was advised to put in a separate 30Amp (TT-30R) receptacle and separate 30Amp circuit breaker at the house. Not to use a dog bone or any type of adaptor to plug into the 30Amp 240 dryer outlet or the 50Amp 240 outlet I have set up for the woodworking/power tools. Because it can over heat the 30Amp, 8 gauge, 3 wire, 120 line used as the main electrical line to the trailer. Going to any 240 outlet could over heat the 120 wire at either connection point (breaker box inside the trailer or house), causing a fire.

The way it was explained to me was, I could plug into a 30Amp outlet as long as that outlet is a single pole breaker. Could also go down from a 30Amp single pole to a 15Amp single pole but would have to sacrifice the use of the A/C. But never plug into a 240 outlet even with an adaptor.

They also said the same thing applies to hooking up to a generator or at any camp site. Hooking up to a 240 outlet, double pole breaker, may work sometimes but all it takes is once for it not to work to cause a major problem.

But I am by no means any kind of expert. I can only go by what information was told and explained to me. If I am wrong, besides being a little miffed by the extra expense of the new breaker, wire run and receptacle. Okay it is just that, the new in ground line, conduit, receptacle and work done was a rather expensive thing and those funds could have gone into the renovation project instead. I guess I'm going to feel rather stupid if all I really needed was a 10-30 dollar adaptor.
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Old 08-07-2014, 05:52 PM   #23
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Never use a 240 volt anything. No one told you too. You need a thirty amp 120 volt outlet. Jim
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Old 08-07-2014, 06:58 PM   #24
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What a great thread


The way it was explained to me was, I could plug into a 30Amp outlet as long as that outlet is a single pole breaker. Could also go down from a 30Amp single pole to a 15Amp single pole but would have to sacrifice the use of the A/C. But never plug into a 240 outlet even with an adaptor.

But I am by no means any kind of expert.
Not rocket science, just electric that can kill you, don't need to be an expert, just carefull and be able to read a meter.
The adapter 50 to 30 they are talking about may look like a 50 amp plug but one of the positive legs doesn't get a connection. Just because you are plugging in 4 prongs only 3 come out of the adapter. Put a multimeter on the adapter and check before you plug it in, if someone wired it wrong you could have 240 v.

In all of these possible problems nobody mentioned another test item, a thermometer, IR thermometers are getting cheap enough to keep track of electric overheat, tires, wheel and motor temp. never travel without one. Saved the day when diagnosing a brake problem.
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Old 08-08-2014, 07:07 AM   #25
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The 50 amp service provides 50 amps of current to each leg. Since you are using 1 leg, there is still 50 amp capacity. Even when you plug in the adaptor. It doesn't know the difference between your rig and one that has 50amp requirement.
You should have a main 30 amp breaker in your coach. Which will trip if overloaded. The only part that is not protected is the shore power cable. If it were damaged to the point of causing the camp sight breaker to trip. It would not matter if it were 30 or 50 amp.
Absolutely correct, I would like to add that the available 50 amps in 30 amp capacity cord that has a faulty connection could overheat even with the 30amp breaker in place at the camper. One reason why I switched over to 50 amp wire and transfer switch then put in 50amp breaker in the rv box and a separate 20 amp breaker for the second a/c that doesn't go to the breaker box in rv.
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Old 08-08-2014, 08:30 AM   #26
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Never use a 240 volt anything. No one told you too. You need a thirty amp 120 volt outlet. Jim
Thanks Jim, that makes me feel better about putting in a separate set up at the house.
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Old 08-08-2014, 08:51 AM   #27
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Not rocket science, just electric that can kill you, don't need to be an expert, just carefull and be able to read a meter.
The adapter 50 to 30 they are talking about may look like a 50 amp plug but one of the positive legs doesn't get a connection. Just because you are plugging in 4 prongs only 3 come out of the adapter. Put a multimeter on the adapter and check before you plug it in, if someone wired it wrong you could have 240 v.
Thank you.

Since I am new to all this, I want to make sure I don't make a huge mistake that could jeopardize anyone's safety.
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Old 08-08-2014, 09:02 AM   #28
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Here's a good book on the subject

Check out RV Electrical Systems by Bill and Jan Moeller. It starts out with a simple explanation of basic electricity and then goes into details about how things work in an RV.
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