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Old 03-01-2012, 10:37 AM   #29
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Originally Posted by HD4Mark View Post
Some "ifs" here like hopefully both circuits you use are not actually one in the same.
...and in hooking to 2 different pedestals how do you determine if you are on 2 different circuits? Do the two pedestals have to be on different phases?
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Old 03-01-2012, 10:58 AM   #30
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Marty,
Your correct. Thanks for bringing that out. Main point I was making is that load is shared between L1/L2 leads so that the Nuetral lead does not have to be sized for 100 amps. If, the same phase was used on each L1/L2 leg, then the Nuetral lead could be having to pull twice the number of amps it is designed for. This is why it has to be a a normal 220VAC style circuit (2 110VAC leads that are 180 degrees out of phase). People grasp and retain the idea of excessive current load (L1 50A + L2 50A = 100A total) on Nuetral lead easier than phase relationship as the reason why it has to be 220VAC.

Dave
Exactly. That is the reasoning behind generator output such as mentioned earlier in this thread of single phase generators tapped to output 20amps and 30amps to L1 and L2. There the combined neutral wire rating will not be exceed.

Hooking a cheater cable to two adjacent 30 amp pedestals may or may not be on 2 Legs. If not, then the possibility exists to exceed the neutral wire rating by 10 Amps or 20%. Care and knowledge on power consumption of motor home appliances can keep you below an overload condition.

Marty
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Old 03-01-2012, 10:58 AM   #31
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Originally Posted by mgscott4 View Post
Your right cbeierl, I used and ohm meter and checked out my 50-30 adapter and the hot lead is connected to both hot leads on the 50 end.

I wonder how the coach knows when I'm on 30 amp and won't turn on the rear AC?

I do know the 20 amp breaker on my 5.5kw Onan generator feeds my rear AC and electrical water heater and I can't turn them both on at the same time or it trips. The 30 amp breaker on the generator feeds the rest of the coach.
If you have the read A/C turned on, it will come on at the set temperature unless you have EMS (Electrical Managment System) to watch for you. You'll just end up tripping the 30 amp breaker at the post because the coach is asking for more than the breaker will allow through.

Bad design to have the A/C on a circuit with any other item.

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Old 03-01-2012, 12:56 PM   #32
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The problem with discussions like this is that there are many differrent implementations of electrical systems. There is no one generic setup. Because a 50 setup provides 50 amps on each leg (L1 & L2) you have 12,000 watts of power available. Only the largest of rigs can carry a generator that is that big. Many have just a 5.5KW generator that is a less than half of the system capablility. Users with smaller generators have to simply learn to adapt to the reduced capability when on generator. By comparing the L1 to L2 lead voltage, an EMS system can detect the presence of 220VAC across L1/L2. If it does not detect 220VAC then it will revert back 30 amp single phase operation. It does not however know, you are connected to a 20 or 15 amp source so that has to be a manual selection. If, the generator does provide 220 split phase but is only rated at 7KW, the EMS has no way of knowing you are limited to 7KW (not 12KW) unless there is a seperate control line provided to the EMS informing it your on generator (few and far between). You the user have to be savy about the abilities of your setup.

Many RV's (mostly older) had a generator that provided a 30 amp (L1) and 20 amp (L2) output. The L2 output was dedicated to only supporting the rear AC and was not even routed through the main AC breaker panel. 30 Amp shore power (or L1 generator supply) supplied the Main Power panel and front AC. Rear AC power was from generator (20 amp breaker at generator). Confusion was brought in by inserting a Front/Rear AC switch which allowed re-direction of the Front AC 110VAC supply (from the L1 Main Supply Panel) to the Rear AC (No front AC power). This follows the use of only 1 AC on 30 amps philosophy (yes, some have rewired there systems around that limitation and manage the load themselves)

There are several different implementations of the 30 to 50 amp doglegs and cheater box's. You have to know your system well to use these devices.

A standard dogleg (L1 jumpered to L2) splits 30 amps between each L1/L2 side so an EMS does not see 220VAC therfore the EMS system will go into 30 amp mode. Should work OK on a GFCI Breaker protected campsite.

Like Marty indicated, a double pole dogleg (2 seperate 30 amp plugs) joins the Nuetral and Ground leads from each campsite box together but keeps the L1 and L2 seperate. The EMS unit may or may not see 220VAC depending on the wiring of the campsite supply. If both campsite posts are on the same phase (ie L1) then the EMS will not see 220VAC and revert to a 30 amp mode. If the 2 campsite posts are on seperate phases (L1 and L2) then it will see 220VAC and not know you are still limited to 30 amps for each leg. You the user will have to manage that manually. You cannot use a double pole dogleg on GFCI Breaker protected campsite because splitting the nuetral/ground leads creates an unbalanced condition.

The 30/20 amp cheater box can be OK if you have a 30 amp rig and are using it to provide a seperate shorepower supply to the rear AC. It basically is a seperate shore power cable supply that alllows you to either run the rear AC on either the generator or shorepower.

A 30/20 amp dogleg connected to 50 amp rig joins the Nuetral and Ground leads from each recepticle together but keeps the L1 and L2 seperate. However an EMS will typically still see this as 30 amp service as both of these outputs are normally connected to the same phase. Therefore, the rear AC may not work (EMS shed) even though you have a seperate L2 supply. On a non-EMS 50 amp rig you should most likely be able to use the rear AC. YOu cannot use 30/20 amp dogleg on a GFCI Breaker protected campsite because splitting the nuetral/ground leads creates an unbalanced condition.

For the user that has the rear AC and water heater connected to the L2 leg on a 50 amp system with a small 5.5KW generator, you will simply have to revert to propane water heater mode when using the generator if you desire both AC and water heater.

As I said, you have to know how your 50 amp rig is setup and then determine the capabilities you can implement when using either a generator or a campsite with only 30 amp (or less) capability available.

Dave
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Old 03-01-2012, 01:30 PM   #33
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As I said, you have to know how your 50 amp rig is setup and then determine the capabilities you can implement when using either a generator or a campsite with only 30 amp (or less) capability available.
You guys with the electrical knowledge blow me away sometimes. It's like asking a former tire engineer what psi tires should be inflated to or, worse yet, asking my BIL, the former transmission designer, what temperature my Allison should be. My simple brain has taught me well. If the breaker trips, too much stuff is on that circuit. Move some stuff to another circuit or unplug it completely. If the electric line catches fire then my simple brain did not listen and left too much stuff plugged in.
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Old 03-02-2012, 08:25 AM   #34
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Great stuff!

Ok, this is a really helpful thread. The expertise of the ppl on here greatly exceeds any expectation I had. If I am correct, I have picked up two very helpful pieces of information. One, running a typical 50 amp motorhome on a 30 amp hookup does not mean certain areas in the motorhome won't have power at all. It simply means that I will have to manage what appliances can be run at the same time and that this will vary from MH to MH based on the power requirements of each appliance (including water heaters and AC). Two, if the MH I am looking at has two AC's, I will need to investigate how they are wired to see if this will fit my needs. For example, if I am in a hot weather area, connected to 30 amps, I would probably run the living area during the day, turn it off at night and run the bedroom AC while sleeping. I would want to be able to do this without running the generator. From reading the above posts, some MH's would not allow this since they are set for the bedroom AC to run only on generator unless hooked up to 50 amps.

If any of my thinking above is incorrect, please feel free to correct. Thanks again to all of you for taking the time to comment.
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Old 03-02-2012, 09:44 AM   #35
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Don't ask me how I know this, but I know for certain if you run simultaneously 1 heat pump, a coffee pot, the clothes dryer, the microwave and a hair dryer, you will soon learn more about your breaker box than you ever did before.
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Old 03-04-2012, 04:55 PM   #36
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Our EMS panel has a digital amp meter which keeps us informed about where we are with our amp "draw" at all times. It has also educated us to "how much" various things draw. The easiest things to eliminate while on 30amp power is the water heater, the fridge & space heaters. Toasters, kettles, coffee pots & microwaves all at the same time for bfast will also push you over the edge!

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Old 03-04-2012, 07:08 PM   #37
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Good thread - like it an in some cases actually factually correct. I especially like to see the retired Marines light up. Sound a lot like Gunny Rose I took abuse from in 1961 - 1966. "I don't know what you Navy guys call it but it looks like a bird just s**t on your uniform!!", and it had.

Sorry, just though this thread could use a little levity. Funny, the system won't let me post what the bird actually did.

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Old 03-06-2012, 09:53 AM   #38
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.... For example, if I am in a hot weather area, connected to 30 amps, I would probably run the living area during the day, turn it off at night and run the bedroom AC while sleeping. I would want to be able to do this without running the generator. From reading the above posts, some MH's would not allow this since they are set for the bedroom AC to run only on generator unless hooked up to 50 amps.

If any of my thinking above is incorrect, please feel free to correct. Thanks again to all of you for taking the time to comment.
I think you will find that it is run the back one during the day and then the front one at night. You don't want to sleep under an A/C that is going if at all possible. Now when it hits 100+, we run both at the same time - - but that is a real rarity for us, we try to be in areas that don't get that hot in the summer.

Barb
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