Electrical Systems

I think I MAY have RV electrical systems figured out.  PLEASE correct my errors.  I?m sure other newbies will also appreciate your help.

A 30 Amp RV plugs into 30Amp - 120 Volt power
DO NOT use Dryer circuit to power RV!!  It is 30 A  240 V!!!!!
A 50 Amp RV plugs into 50 Amp 240 Volt power, or  100 Amps at 120 Volts
This explains why the 50 A service provides over THREE TIMES the power of 30 A service

(Okay so far?)

RV?s have two (almost) fully independent electrical systems
A 12 VDC system  powers all 12 V lights, slideouts, leveling systems, audio systems and water pumps. (other items????)

A 120 VAC system powers higher consumption systems like A/C, microwave oven, furnace blowers, TV, some lights, refrigerators, exhaust fans, hot water heater and home appliances brought along.   (add / delete????)

A 240 VAC system May be present in larger RV?s with 50 Amp service, primarily for A/C
(true??)

The 12 VDC system is powered from House batteries
The batteries can be recharged from any of the following sources
On board or portable generator  OR
120 VAC shore power through a transformer / rectifier  OR
Class A or C Alternator when engine is running  OR
SOLAR Panels      (Hello MoparMan)
These sources usually will ONLY charge batteries.  The batteries function as a buffer between the power source and the 12 VDC system.  Power can be drawn during the charging process.

The 120 VAC is powered from:
Shore power  OR
On board or portable generator  OR
12 VDC batteries through an inverter

240 power is supplied only from shore power or larger generators.

In other words, MOST electrical devices can be run WHETHER OR NOT the RV is plugged in or is on  generator power.  HOWEVER high draw appliances may require too much power to start or run on batteries.  If they do run, they will QUICKLY drain the batteries.  This includes A/C,  microwave and coffee maker.

Generators CORRECTLY SIZED will power everything.  Smaller units may power everything, but not at the same time.  Even smaller units may not be able to run the A/C.

If you are on shore power or generator, the inverter from the batteries providing 120 V  power is deactivated, usually automatically.  When this 120 V or 240 V power is disconnected, the inverter is reactivated.  The two systems are completely independent beyond this switch / relay.

When troubleshooting electrical issues, it helps to know if the device draws 12 V or 120 V.  Then you know which power system to focus on.

Some appliances will run on propane or 120V, depending on the specific model.  These include:
Refrigerator
Hot water heater
(True???  others???)

Thanks for the help getting me straightened out.
Gordon

grashley said:

A 30 Amp RV plugs into 30Amp - 120 Volt power
DO NOT use Dryer circuit to power RV!!  It is 30 A  240 V!!!!!

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Yep

A 50 Amp RV plugs into 50 Amp 240 Volt power, or  100 Amps at 120 Volts
This explains why the 50 A service provides over THREE TIMES the power of 30 A service

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Nope. 50 amp is two 120 volt circuits, not 240 volts.

RV?s have two (almost) fully independent electrical systems
A 12 VDC system  powers all 12 V lights, slideouts, leveling systems, audio systems and water pumps. (other items????)

A 120 VAC system powers higher consumption systems like A/C, microwave oven, furnace blowers, TV, some lights, refrigerators, exhaust fans, hot water heater and home appliances brought along.   (add / delete????)

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Nope, three independent systems, two 12 volt and one 120 volt

A 240 VAC system May be present in larger RV?s with 50 Amp service, primarily for A/C

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Nope, no 240 systems in RVs except the occasional highly expensive custom RV.

The 12 VDC system is powered from House batteries

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Nope, there are two 12 v systems, house and engine.

The batteries can be recharged from any of the following sources
On board or portable generator  OR
120 VAC shore power through a transformer / rectifier  OR
Class A or C Alternator when engine is running  OR
SOLAR Panels      (Hello MoparMan)

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Nope, generally the alternator does not charge the house batteries only the engine battery.

These sources usually will ONLY charge batteries.  The batteries function as a buffer between the power source and the 12 VDC system.  Power can be drawn during the charging process.

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Nope, the gen will power the charger which can produce 12 v even without batteries present.

The 120 VAC is powered from:
Shore power  OR
On board or portable generator  OR
12 VDC batteries through an inverter

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Yep

240 power is supplied only from shore power or larger generators.

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No 240.

In other words, MOST electrical devices can be run WHETHER OR NOT the RV is plugged in or is on  generator power.  HOWEVER high draw appliances may require too much power to start or run on batteries.  If they do run, they will QUICKLY drain the batteries.  This includes A/C,  microwave and coffee maker.

Click to expand...

Yep

Generators CORRECTLY SIZED will power everything.  Smaller units may power everything, but not at the same time.  Even smaller units may not be able to run the A/C.

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Yes and no. Very few RVs have a large enough gen to power everything.

If you are on shore power or generator, the inverter from the batteries providing 120 V  power is deactivated, usually automatically.  When this 120 V or 240 V power is disconnected, the inverter is reactivated.  The two systems are completely independent beyond this switch / relay.

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Nope, it depends on the way the RV is wired. Not all RVs are wired equally.

When troubleshooting electrical issues, it helps to know if the device draws 12 V or 120 V.  Then you know which power system to focus on.

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Not really. If the inverter is involved then both systems are being used.

Some appliances will run on propane or 120V, depending on the specific model.  These include:
Refrigerator
Hot water heater
(True???  others???)

Click to expand...

yep

Thanks for the help getting me straightened out.
Gordon

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You are welcome.

You have it almost completely right.  Even though a 50A wired RV has 240VAC available, only the very high end, all electric, RVs will have an 240V appliances.  Most have just 120V devices powered from the 2 120VAC legs of the 50A service.  Your batteries are charged from the converter or charger (if equipped with an inverter/charger) and power all the 12VDC devices, including most of the appliances like the furnace and lights.  The water heater and refrigerator will normally run on either propane or 120VAC, but still need 12VDC for the control electronics.  If equipped with an inverter, most outlets will be powered from it, but it will usually have a transfer switch built in so when on shore or generator power the inverter just passes that through to the outputs.

Many, but not all, motorhomes will charge the house batteries from the engine alternator, and many, but again not all, will charge the chassis batteries from the built in charger when on shore or generator power.

Just that make it any clearer?

Please clarify:

50A service is provided by a 240V circuit, but almost all RV's use this as two 50 amp 120 volt circuits????

Class A and C have two 12V systems.  TT and 5er have only one 12 V system????

SOME  motorhomes  will charge house batteries, others will not??

Gordon

grashley said:

Please clarify:

50A service is provided by a 240V circuit, but almost all RV's use this as two 50 amp 120 volt circuits????

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It is not 240 v. It is two 120 v circuits. Both 120 v circuits can originate from the same 120 v phase and therefore you could never get 240. Or they can come from different phases and you would have 240, but the RVs never need nor use 240. Just forget the entire idea of 240.

Class A and C have two 12V systems.  TT and 5er have only one 12 V system????

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Correct.

SOME  motorhomes  will charge house batteries, others will not??

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Most all motorhomes will charge house batteries. But not all of them will charge the house batteries from the alternator.

It is not 240 v. It is two 120 v circuits. Both 120 v circuits can originate from the same 120 v phase and therefore you could never get 240. Or they can come from different phases and you would have 240, but the RVs never need nor use 240. Just forget the entire idea of 240.

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Tom, that's just plain wrong. A properly wired 50A RV outlet will have 240v available, whether any given RV utilizes it or not. And there are a few that do, though mostly ultra high end rigs like Prevosts and Newells. I've seen them with my own eyes! A 50A outlet that has both hots in phase is not wired  per specification, even though it may work with the majority of RVs.

SeilerBird said:

It is not 240 v. It is two 120 v circuits. Both 120 v circuits can originate from the same 120 v phase and therefore you could never get 240.

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This is statement is wrong.
They must be from different phases! If not the neutral wire could be seriously overloaded. That's way the power supply is 240v. This way the neutral only carries the current difference form Hot 1 to Hot 2. EXM. If hot 1 is drawing 40 amps and Hot 2 is drawing 22amps then the neutral is carrying 18amps (40-22). IF they were from the same phase then the neutral would be carrying 620amps, well over its capacity.

I don't know of any motorhome that does NOT charge the house batteries from the engine alternator, but there may be one somewhere. I've learned the hard way not to say "none" or "never", cause any physically possible design has been used in some RV brand/model at some point in time.

Gary RV Roamer said:

Tom, that's just plain wrong. A properly wired 50A RV outlet will have 240v available, whether any given RV utilizes it or not. And there are a few that do, though mostly ultra high end rigs like Prevosts and Newells. I've seen them with my own eyes! A 50A outlet that has both hots in phase is not wired  per specification, even though it may work with the majority of RVs.

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You are correct Gary, I should have mentioned that not all RVs and pedestals are properly wired. I did mention the high end exceptions.

"Nope, no 240 systems in RVs except the occasional highly expensive custom RV."

This is statement is wrong.
They must be from different phases!

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Yep, I should have mentioned that they should come from different phases but they don't always come from different phases.

The yes/no list above is good I would only make two mods.

I call 50 amp 120/240 volt serivce because just like your house both voltages are present..
The only thing I have ever seen in an RV that needed 240 volt is a sepcial device called a load shedder that looks to see if it is there.. If it is the load shedder goes into standby and displays the 50 amp indicator.. It does not actually USE 240 volts.. Just checks to see if it's there.

If not it assumes 30 amps and limits use of heavy loads like Air Conditioenrs, Microwave and water heater.
Not all RV's have this device (Mine does not).. but if you got it.. Very handy.

Gary RV Roamer said:

I don't know of any motorhome that does NOT charge the house batteries from the engine alternator, but there may be one somewhere. I've learned the hard way not to say "none" or "never", cause any physically possible design has been used in some RV brand/model at some point in time.

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My 83 Allegro did not charge the house batteries from the alternator.

I knew there would be an exception somewhere! Though it is possible that the isolator was merely defective, rather than a lack of design capability.  Back in the day, a coach was more likely to have an isolator diode module than a relay.

Even my old 1976 Dodge Jamboree Motorhome had a on board isolator that routed alternator charging voltage to the house battery. Even my Jayco get chraging voltage from the trailer cord connector and has a 40 Amp service for 12 volt power. I would dare to say 99.9% of all RVs get charging voltage from the engine alternator.  Also 99.9% of all RV have a power converter that take 120 VAC and charge the batteries as well.

As for 50 Amp service I've never dealt with it. Even if I owned one I could not plug it in at home being the house is only a 30 Amp service from the inverter. 

Just basic loads from a inverter on 12V batteries is basically a t ten times change. So if the A/C draws 15 Amp @ 120 VAC it will convert to 12 VDC @ 150 Amps. Not even a vehicle alternator could keep up. As for my conversion that just a rule of thumb and not exact number. There is losses to calculate in so the draw would be higher yet. This why most house system use 24 volt and 48 volt battery banks instead of 12 volt. The alters the amp draw on the batteries.

Like I've got generators... I've got a 6,500w that powers my 2 story sticks and bricks. I've got a 4,000 watt I could pack for the RV but its rather loud, heavy to pick up and even load to the truck, heavy fuel consumption. Basically 3-5 gallons a day for fuel. I've also got a smaller portable 2000 watt witch is much lighter but the little 1.5 gallon tank only last for about 3 hours and done. But it too small to power the A/C or big devices. So instead of packing all all the fuel I went solar. Being when I am camping it like 1-2 hours to a town to get fuel. Not around the corner.  Basically where I go camping there is no power, no cell, no nothing.

My RV text doesn't talk about a 240 service that a limited number of top end coaches apparently use, maybe it should.

But, it does say that a 50 amp 120/240 vac power supply system has three current-carrying conductors as compared to two current-carrying conductors in 30 or 15 amp systems. That you get two hot legs in parallel supplying 50 amps each = 100 amps of 120vac current available for use in an rv.

"Because the voltages in the red and black hot legs are 180 degrees out of phase with each other, all the current can safely pass through the neutral  (white) wire without overloading or causing other electrical problems."

You manual is correct, but every 50a coach has 240V behind the power panel available and it can be used as such by a knowledgeable electrician but.... It is probably just as well that the subject is glossed over based on the KISS principle... None of the appliances used in most RV's can handle it anyway, we seldom have 240V driers, heaters, welders that will require it.

Whether or not the two legs are out of phase depends on the park wiring, they normally are, but.... If you can measure 240V across the two hot pins, they are... other wise 0V will say they are not. Either pin measured to the return (neutral) will measure 120V regardless of phasing.

Alfa38User said:

Whether or not the two legs are out of phase depends on the park wiring, they normally are, but.... If you can measure 240V across the two hot pins, they are... other wise 0V will say they are not. Either pin measured to the return (neutral) will measure 120V regardless of phasing.

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And if it doesn't read 240 volts, inform the park manager. The outlet is not wired to code and can cause problems by the neutral carrying too much current.

kdbgoat said:

And if it doesn't read 240 volts, inform the park manager. The outlet is not wired to code and can cause problems by the neutral carrying too much current.

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This is true and a good idea. But in the real world it you would be extremely rare that there would be enough appliances running at the same time to cause a problem with the neutral. RVs just don't have that much equipment. Unless you were going to get several welders going at one time.

Why take the chance because somebody else did sub-par and non-complying work? It's not worth taking the chance on burning your RV to the ground.

... you get two hot legs in parallel ... Because the voltages in the red and black hot legs are 180 degrees out of phase with each other

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For clarification, two legs 180 degrees out of phase cannot be paralleled. If you tried it, you'd know why in a hurry, but please don't.

SeilerBird said:

This is true and a good idea. But in the real world it you would be extremely rare that there would be enough appliances running at the same time to cause a problem with the neutral. RVs just don't have that much equipment. Unless you were going to get several welders going at one time.

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Tom S,

I can easily run enough that the neutral would be overload if the service was 2 hot in the same phase. The same would be true for most all electric coaches.
But any coach with 2 ACs and an electric water heater could be at 45 amps before anything else they might use - Micro etc.

I would know right away if the service was split phase because the Intellitec EMS would read that it was 30 amp service.

ken

btw, if the service provided isn't 240/120 (ie split phase), the it would be a violation of the National Electric Code to wire it to 50amp Outlet used by RVs (same outlet used by Electric ranges)

In 28 years of RVing, over 10 fulltime, I've never seen 50amp service that wasn't 240/120 - Except a park that provided 50amp 120 service, single 50 amp breaker but wired to both side of a standard 50amp outlet. This was perfectly safe because the max current provided was 50 amps.