Single phase 240V - what happens if one plugs an RV into 2 phase power?

[JoelP]

It is my understanding that all RVs have single phase systems for 50 A hookups. While there are two legs of power, unlike most 240V equipment and appliances in your home that use these legs out of phase to generate 240V, in an RV these 2 legs are really only two 120V sources that are in phase with each other.  I was about to install an older Level 2 charger for a plug in hybrid car for my son-in-law and was surprised to see that it was also 40A single phase, requiring a special 40A circuit breaker that put both halves on the same busbar of the home's service panel. My own level 2 charger runs from a 2 phase 240V outlet which has a plug that looks just like the standard 50A plug we use in RV parks. 

It seems to me that I had read that it would do damage to plug my RV into such a 2 phase 240V circuit, but I am perplexed as to why this would be a problem. My impression is that the L1 and L2 legs of my RV power simply power different circuits and that there is rarely if ever any use for 240 V in any RV, even to the high power devices like my heat pump.  So, if one circuit were out of phase with respect to another going into my RV, what harm could this do if they never are required to interact?  Would my Progressove surge protector react adversely to having L1 out of phase with L2 and shut down one or both legs of power?

 

[John From Detroit]

Ok, I am a certified electronics and you have confused me

240 volt is normally single phase.. (Though ther eis a 208 volt system that is.. different)

120/240 volt though really a single phase centertapped is often called two phase.

WHich is to say the NEUTRAL is the center tap and the two hot legs are either side of that With respect to neutral they are "Out of phase" but together they are "in phase"

By calling 240 volt "2 phase" you have confused me.

What happens if you plug a 240 volt device into a 50 amp park outlet using a properly wired plug.. IT WORKS.. (the neutral is ignored)

What happens if you use a proper adapter to plug a 120 volt device into a 50 amp park outlet. It works. one of the "Hot Legs" on the outlet is ignored.

But your phrasing of yoru question leaves me confused.

What happens if you plug in with the WRONG adapter. or plug into a 240 volt outlet you think is a 120 volt outlet..  The magic Smoke escepes from your electronics and they have to be repalced. (Bad things).

 

[JoelP]

Ok, I am a certified electronics and you have confused me

240 volt is normally single phase.. (Though ther eis a 208 volt system that is.. different)

120/240 volt though really a single phase centertapped is often called two phase.

WHich is to say the NEUTRAL is the center tap and the two hot legs are either side of that With respect to neutral they are "Out of phase" but together they are "in phase"

By calling 240 volt "2 phase" you have confused me.


What happens if you plug a 240 volt device into a 50 amp park outlet using a properly wired plug.. IT WORKS.. (the neutral is ignored)

What happens if you use a proper adapter to plug a 120 volt device into a 50 amp park outlet. It works. one of the "Hot Legs" on the outlet is ignored.

But your phrasing of yoru question leaves me confused.

What happens if you plug in with the WRONG adapter. or plug into a 240 volt outlet you think is a 120 volt outlet..  The magic Smoke escepes from your electronics and they have to be repalced. (Bad things).

John- thanks for rthe quick reply and apologies for getting the nomenclature wrong.  Let me try again.

In my home the 120/240 two phase service panels has 2 bussbars each has 120V at 60 Hz but each bussbar is out of phase with each other enabling a tap from each bussbar to produce 240V relative to the neutral center tap.


Your statement:  With respect to neutral they are "Out of phase" but together they are "in phase"  Not sure how this is possible.


Isn't it true that L1 and L2 circuits in an RV are both in phase with each other relative to neutral?

My key questions is what happens if I plug the 50A shore line from my RV into my homes 50A 120/240 2 phase outlet? It seems to me that this would not cause any problem.  One leg relative to neutral would power L1 circuits and the other leg relative to neutral would power the L2 circuits, right?

 

[lynnmor]

You would do well to remove the word "phase" from your vocabulary, that is the source of your confusion.

 

[Lou Schneider]

In my home the 120/240 two phase service panels has 2 bussbars each has 120V at 60 Hz but each bussbar is out of phase with each other enabling a tap from each bussbar to produce 240V relative to the neutral center tap.

Maybe this was a typo, or it may be the crux of your misunderstanding.  In a 120/240 single phase connection each leg produces 120 volts from it to the neutral center tap.  From one leg to the other you get 240 volts because at the instant one leg is at +120 volts to the 0 volt neutral and the other is at -120 volts.  Like two batteries connected in series the voltages add when you go from one buss to the other (120 volts + 120 volts = 240 volts).

Your statement:  With respect to neutral they are "Out of phase" but together they are "in phase"  Not sure how this is possible.

Isn't it true that L1 and L2 circuits in an RV are both in phase with each other relative to neutral?

When one buss is at +120 volts and the other is at -120 volts, with respect to their polarity they are out of phase with each other but the two legs form a single phase because there's only one path from one to the other, with neutral (0 volts) at the center point.

If their polarity was in phase with each other (both legs going positive or negative at the same instant) you'd get 120 volts from each to neutral, but 0 volts difference from one leg to the other.

There's a difference between being in-phase or out-of-phase with respect to polarity, versus talking about single phase or three phase power.

My key questions is what happens if I plug the 50A shore line from my RV into my homes 50A 120/240 2 phase outlet? It seems to me that this would not cause any problem.  One leg relative to neutral would power L1 circuits and the other leg relative to neutral would power the L2 circuits, right?

Yes, but be sure the outlet has a neutral conductor capable of handling the full breaker rating (50 amps).  The purpose of the neutral is to keep the center tap voltage at zero volts so each leg stays 120 volts above it. It does this by carrying the difference in current drawn from the two hot legs.  The worst case is drawing current only from one leg so it all returns via the neutral.

If the socket is designed for a 240 volt load, the load will be connected from one hot leg to the other and the neutral may not be there or it may be severely undersized.

Without a neutral, the center point voltage will swing wild in proportion to the current drawn by each leg.  If you draw 9 amps on Leg 1 and one amp on leg 2, you'll have 24 volts from Leg 1 to the center tap and 216 volts from Leg 2 to the center tap.  These are the voltages that will appear on your 120 volt loads on each leg.

 

[NY_Dutch]

Here's a simple drawing of a single 60 hz cycle that may help explain the "single phase" reference. Note that one 120v leg is measured from the zero crossing "common" or "neutral" line to the positive peak of the cycle, and the other 120v leg is measured from the zero crossing line to the negative peak, and the 240v measurement is peak to peak. This is exactly the same regardless of whether it's for an RV or a residential kitchen stove. In your house, some 120v devices are wired to the positive half of the cycle, while others are wired to the negative half of the cycle, and 240v devices are wired peak to peak to the full cycle. A 50 amp RV is wired identically, although 240v appliances are rarely used in RV's.

 

[Gary RV_Wizard]

Isn't it true that L1 and L2 circuits in an RV are both in phase with each other relative to neutral?

Short answer: NO!
50A RV power is exactly the same as the 120/240v power in your house.  Relatively few RVs actually have 240v appliances, but the possibility is there.

 

[Tom]

Not wishing to add confusion, but we have a couple of articles on this subject in our forum Library:

• My attempt to explain what Lou and others have done well - click here.
• An article submitted by Winnebago's technical staff comparing 30A and 50A service - click here.

 

[JoelP]

Maybe this was a typo, or it may be the crux of your misunderstanding.  In a 120/240 single phase connection each leg produces 120 volts from it to the neutral center tap.  From one leg to the other you get 240 volts because at the instant one leg is at +120 volts to the 0 volt neutral and the other is at -120 volts.  Like two batteries connected in series the voltages add when you go from one buss to the other (120 volts + 120 volts = 240 volts).

When one buss is at +120 volts and the other is at -120 volts, with respect to their polarity they are out of phase with each other but the two legs form a single phase because there's only one path from one to the other, with neutral (0 volts) at the center point.

If their polarity was in phase with each other (both legs going positive or negative at the same instant) you'd get 120 volts from each to neutral, but 0 volts difference from one leg to the other.

There's a difference between being in-phase or out-of-phase with respect to polarity, versus talking about single phase or three phase power.

Yes, but be sure the outlet has a neutral conductor capable of handling the full breaker rating (50 amps).  The purpose of the neutral is to keep the center tap voltage at zero volts so each leg stays 120 volts above it. It does this by carrying the difference in current drawn from the two hot legs.  The worst case is drawing current only from one leg so it all returns via the neutral.

If the socket is designed for a 240 volt load, the load will be connected from one hot leg to the other and the neutral may not be there or it may be severely undersized.

Without a neutral, the center point voltage will swing wild in proportion to the current drawn by each leg.  If you draw 9 amps on Leg 1 and one amp on leg 2, you'll have 24 volts from Leg 1 to the center tap and 216 volts from Leg 2 to the center tap.  These are the voltages that will appear on your 120 volt loads on each leg.

I really do understand the concept of phase and how 2 out of phase 60 Hz 120V signals can produce a 240V potential. Perhaps it's my choice of words to state my question.

The neutral in my 50A home circuit has the same gauge as the hot leads so even if the load is quite uneven it cannot be any worse than what happens in the shore line to the RV.  I can imagine that the load between L1 and L2 could be quite uneven in an RV with the large draw from the heat pump, but wouldn't this be the same issue for the circuit at the RV park which also has a neutral to which we are connecting our 3 prong plug.

Anyway, Gary has answered my question that there is no real concern for hooking my RV power to my 50 house circuit.

 

[Isaac-1]

Household power in the US is not 2 phase, it is 120/240 center tapped, split single phase.  See https://en.wikipedia.org/wiki/Split-phase_electric_power

 

[xrated]

Household power in the US is not 2 phase, it is 120/240 center tapped, split single phase.  See https://en.wikipedia.org/wiki/Split-phase_electric_power

Nailed it ....  ^^^^^^

 

[John From Detroit]

Isn't it true that L1 and L2 circuits in an RV are both in phase with each other relative to neutral?

AH, now I understand. thanks for  clearing it up.

You have an error. ON GENERATOR. then the L-1 and L-2 are basically the same. (No voltage between them) but on SHORE ... Unless you are in a cheap old park that just put a 50 amp outlet on a 30 amp feed, (Fairly rare) THEN.... you have 120/240 same as you have at your house. And everything works normally.

WHY do it that way?

Well for the wire run to the generator it really does not much matter.. but you can easily have many many yards between you an the big transformer at an RV park.. By using the 120/240 volt arrangement you reduce the voltage drop in the wires. because the neutral only carries the DIFFERENCE between the two legs.

But where I'm at now  is just like your hosue  L-1 to neutral or L-2 to neutral 120 volts

L-1 to L-2 240 volts

Oh  WHat happens if you plug a 240 volt device into an outlet where L-1 and L-2 are the same phase.. NOTHING there is no voltage so nothing happens.

Many RV' have a device called an Energy Management System made by Intelletec (not to be confused with the Progressive device of the same name) This device sometimes called a load shedder. can turn off an air conditioner. or a water heater or other devices in order to keep your power consumption UNDER 30 amps when on a 30 amp site.

HOW does it know if you are on a 30 or 50 amp site.. 240 volts present L-1 to L-2 on a proper 50 amp site.

 

[Gary RV_Wizard]

Some further items that may confuse what people think they know about RV power:

Some parks may provide non-standard 50A (in phase), and a 50A RV also receives in-phase power when using an adapter to connect to 30A or 15A service.

Some inverter generators, notably the popular Onan 7500 & 8000 models, produce in-phase power, i.e. two 120v hot legs that are in-phase.

Some RVs actually employ 240V appliances. For example, Tiffin & Newmar offer a 240v clothes dryer and a induction cooktop, and all-electric Marathon & Newell coaches use all 240v appliances.

 

[Lou Schneider]

One more power option is 120/208 volt park power because the park has incoming three phase Wye service.  It's not common but you can occasionally find an RV park wired this way.  The Escapees' Park Sierra co-op near Yosemite is one example.  Or it may be an older park that originally had only 30 amp hookups and later added some 50 amp sites.

The campsites are distributed evenly across all three legs of the incoming power, with each 50 amp site getting two out of the three incoming hot legs.

Similar 120/208 volt Wye connections where two out of the three legs are used in each unit are also fairly common in apartment houses and even in some stick 'n brick neighborhoods.

On a Wye connection, each hot leg is 120 degrees out of phase with the adjacent leg.  If you look at a 360 degree vector representation, the three legs form a Y shape with the neutral at the center junction and each of the three hot legs at the tips of the Y.

You still get 120 volts from each leg to neutral, but only 208 volts between any two hot legs because each leg is 120 degrees out of phase with it's neighbor, not 180 degrees out of phase like single phase power.

Most if not all appliances that use leg to leg power are rated to use either 208 or 240 volts so this makes little or no difference to your RV.

 

[John From Detroit]

I mentioned the 208 above (i think) but did not explain it. Thanks.

One of the more amazing things to me is the lack of understanding... But then.. Somewhere I've a piece of college type wallpaper (Actually it is about 1 foot to my left).

 

[JoelP]

Household power in the US is not 2 phase, it is 120/240 center tapped, split single phase.  See https://en.wikipedia.org/wiki/Split-phase_electric_power

Thanks Isaac. Now that I read the Wiki I see my error.  The industry only refers to two phase when it is 120 deg out of phase, not for the "split single phase" we mostly use in our homes today. That was my point of confusion. 

Actually I have degrees in Physics and Materials Science so the concept of phase is abundantly clear to me, but nomenclature can sometime trip me up.  I can readily explain the concept of cirucular polarization that relies on a quarter wave retarder to introduce a phase between electric and magnetic vectors, but when it comes to home electrical nomenclature I still have a few things to learn about the semantics.

My questions on this string have now been fully answered:  As John stated, if I were to plug a 240V home appliance into the RV power the 120V on each side of neutral would not produce 240V, but if I were to plug in my RV to my home circuit it would work just fine since I don't have any 240V devices in my RV, L1 and L2 would see just what it needs.


My RV has a Progressive surge protector and the only time it has blocked power is when I plugged it into a circuit or extention that was improperly sized and the Progressive detected that the voltage was dropped by the resistance in these conductors.


Not to prolong this much further, but I have noticed that my level 2 charger at home tolerates both 240V as well as 208V supply.  I have to wonder what they did in designing  this to condition this device to allow that variety of supply.  It would be interesting to see the schematic.

 

[Isaac-1]

I suspect the guts of your level 2 charger is a switch mode power supply, which tends to be highly tolerant of input voltage and frequency, much like your typical noteboook computer power brick which will accept anything from 100-240VAC at 50-60 Hz (or at least mine does), so virtually any wall outlet in the world with the right plug shape adapter, from Japan which uses 100V nominal 60 hz, to the north American 120V 60hz, to the UK with 230V at 50 hz

 

[Gary RV_Wizard]

As John stated, if I were to plug a 240V home appliance into the RV power the 120V on each side of neutral would not produce 240V,

Forgive me for belaboring this, but I don't think John said exactly that.  Many RVs do have 240v available and at least theoretically can have 240v appliances.  It actually depends on the type of load center installed and its bus layout, since the standard incoming 50A service has 240v between L1 & L2.  The load center may or may not be designed to allow a 240v breaker (bridges the two buses) to be utilized. The common Intelletec Smart EMS load center does not, but some others used in RVs do.  I mentioned previously that some Tiffin & Newmar models have that capability, as do all Marathon & Newell coaches.

 

[JoelP]

Apologies, thanks for the correction.  I realize it was you that mentioned this and clarified what such high end coaches can have in the way of 2 busses for wiring. For me I don't have to worry about such high end coaches anytime soon.

 

[John From Detroit]

Household power in the US is not 2 phase, it is 120/240 center tapped, split single phase.  See https://en.wikipedia.org/wiki/Split-phase_electric_power

Isaac... You do not know how glad it makes me to see you post that. I have been saying EXACTLY that for years and getting put down right left and center... I've had to modify it a bit to avoid flames.. But for years I've felt well named.. Like John The Baptist "A lone voice crying in the desert" on this issue.

And yes. I fully agree with you (Clearly).