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Old 03-21-2009, 05:04 AM   #43
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Originally Posted by Roadking View Post
As stated when your RV EMS detects L1 and L2 are in-phase it configures your EMS as either a 30amp source or a 20 amp source (depending on switch settings). A 50amp to 30amp adapter ties L1 and L2 together and hence applies the same phase to each of the legs on the RV, and hence the EMS senses the source as 30 amps, or 20 amps.

Many RVs with 50 amp connections carry a 7.5KW generator. Assuming voltage and current are in phase (which is not true for inductive loads such as AC motors, but lets assume they are) then POWER = Voltage times Current. So a 7.5KW generator operating at 125Volts will provide approximately 60 amps of current. 7500 Watts = 125Volts times 60 amps (again, assuming voltage and current are in phase).

Rarely will the generator actually produce a full 60 amps, and hence, the neutral wire will not be overloaded in generator mode. However, "cheating" the EMS into thinking it is in generator mode, and pulling a full load on a power source that has both L1 and L2 in phase could overload the neutral. As previously stated.. you could potentially pull 100amps on the neutral if both legs L1 and L2 had 50 amps available and you were indeed pulling full capacity. Therefore you should not be creative by "faking " your EMS in generator mode unless you know for certain you are never pulling more than 50 amps Overloading the neutral could cause a fire anywhere along the neutral path all the way back to the main breaker panel where adequate sized neutral wires exist.

On the other hand, if you know the maximum loads your coach pulls on each leg then perchance you may be able to manage the risk. For instance, when I was on my own meter at one particular resort I used a cheater/adapter to get the "out of phase" L1/L2 power. My L1 source had a 30 amp breaker and my L2 source had a 20 amp breaker. The only time I would trip a breaker (the 20 amp) is when the Dryer, Hotwater Heater, and 2nd AC compressor were all running at the same time. So normally, I rarely pulled more than a total 50 amps as I recall only tripping a breaker once.

I also know under normal common sense conditions with my 7.5KW generator running I have never pulled more than 45 amps total. I'm sure I could add an electric heater while running the AC and pull more, but besides me, who would do that?? This means it would be highly unlikely that I would overload a neutral wire as my max draw would typically be only 45 amps. Your coach may be different and if you don't know you should not risk any fires or potential damage.

I totally agree that this is not a good practice and should only be done knowing what you are doing. It is also highly unlikely that you would pull more than 50 amps and if so for a very brief period of time. As with any service the National Electric code allows for de-rating of service conductors for this reason.

Mike, RVIA & RVSA Certified Master RV Technician
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Old 03-21-2009, 08:19 AM   #44
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The Onan 7.5k and 8.0k Quiet Diesel generators produce only 120v, with the two legs in phase. If your motorhome rejects in-phase L1/L2, then you certainly don't want that model of generator, because you could never use the genny!

Total output in these generators is about 63A - 31.5A on each leg in the 7.5 and and more in the 8.0k version. That means the neutral carries up to 63A if the generator is fully loaded. Presumably the neutral feed back to the generator is sized for that.

My cheater box has two 30A connectors with the option of using a 30/20 or 30/15 adapter on each. Therefore the max load I can get is 60A if two 30A sources are available or 50/45A if using a 30A plus a 15 or 20A source.

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Old 03-21-2009, 08:41 AM   #45
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Originally Posted by VACMAN View Post
Am I right or wrong in my assumption? If I plug the MH 50 amp plug into a 240 receptacle, that will be bringing 2 legs of 120 volt each to the MH, and the setup on the MH will handle these 2 120 volt lines? Vacman
Well, I think you are both right and wrong... (Your thinking is good your language is a bit off)

Not two legs but one leg divided into two circuits.

This works because there is nothing in the rig that is 240 volt, it's all 120 volt and the appliances do not care if they are plugged into L1 or L2 (the two legs) so they work just as well with one leg.

The issue though is with only 20 amps you can not run much at one time, In fact if my batteries are low 20 amps is JUST the converters, nothing else. in my house, Once the batteries top off I can run other things
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Old 03-24-2009, 04:08 AM   #46
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Originally Posted by David K View Post
The legs for the 120v need to be "out of phase" or from different sides of the breaker box -- not from the same bus bar as the most of the 240v in our houses are done. In the house, phase does not become a consideration as the appliance will use the "in phase" 120v and the neutral wire does not have to worry with the "additive" effect of the 220v -- it don't happen.
If the 240V wiring in your home is wired off of the same buss bar you won't have 240V.
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Old 03-24-2009, 08:30 AM   #47
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My coach is currently plugged into 20 amps. I can run.

Dead batteries (2 banks) the converters. And very little else

Full batteires: The converters, Television or radio and one from the following list

Heater (2,000 watts)
Heaters (1500 + 150 watt) Plus soldering iron
Water heater
Air conditoiner

NOTE: Frig at 250 watts is, like the 100 watt soldering iron, a "nothign load" and can be run most any time.

I use the rig most every day, summer and winter, as it's my "Ham Shack" (See sig) and thus I will use either 1AC in the heat of summer or the two small heaters in the winter (I only heat the smallest occupiable room in the rig) I normally run the big electric heater off the generator once a month

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