50 Amp Service Question

[NoDirectionHome]

Are the two 120VAC legs of a 50 Amp RV sevice pylon in phase or 180 degrees out?

[rpsinc]

I know a bit about electrical but dont understand your question.  Can you explain?  

 

[Darryl&Rita]

Neither, really. They come off the middle of a centre-tapped transformer, and are referred to as single phase.

[NoDirectionHome]

Are the two hot legs in phase or out of phase like a home is wired?  

[rpsinc]

In this case, the 180 degree shift is nothing magical. It is simply polarity. It means that the ends of the coil are equal and opposite at any instant of time. In this exact same sense, the poles of a battery are "180 degrees" opposite. North and south are "180 degrees" opposite. In other words, somebody used the term 180, and it happens to have the same phrasing as the 120 degree shift used in 3-phase terminology, except it really doesn't mean a phase difference any more than north or south are out of phase.

The device that creates it is the transformer. At any instant of time, current is flowing in one direction, and therefore the magnetic lines of flux have a specific polarity, driving a current in the secondary a certain direction, creating a positive potential on one side and negative on the other. In the next part of the cycle, the polarity changes, so negative becomes positive and positive becomes negative.

In other words, at any instant, one wire is positive and one wire is negative, thus the idea that they are "180 degrees out of phase".

 

Found this explanation.  Hope it helps with your question.

[Darryl&Rita]

In the drawing above, the power line ties to Vp. Your house, or RV uses the right hand side of the transformer connections. More here.

[Chad Heiser]

1 hour ago, NoDirectionHome said:

Are the two 120VAC legs of a 50 Amp RV sevice pylon in phase or 180 degrees out?

The two legs of 120 VAC coming into your RV (L1 & L2) are the same as the two legs going into your home.  They are not in the same phase.  This is what allows you to make 240 VAC by combining the two legs.  In other words, you can get 240 VAC in an RV panel if you need it.  For instance, I run a 240 VAC clothes dryer in my RV.  I can do this by taking 120 VAC from L1 and 120 VAC from L2 and combining them for 240 VAC with a double pole breaker.

[NoDirectionHome]

ok, thanks!!! that's what i was looking for.  I'm considering the options for wiring up my Victron Quattro.  Just needed to be sure.

[Ronbo]

They are supposed to be 180 out to balance the load. Some parks will not wire it this way and rum both legs from the same tap. If you are trying to run something that is 240v, check the voltage first. There are some rv’s now That need 240.

[oldjohnt]

No Direction, good question, here's my answer:

An RV power pedestal (if wired correct) is similar to typical household wiring which in the trade is referred to as  120/240 Volt Single Phase Three Wire. Its derived by use of a Single Phase Transformer which on the LV secondary is 240 VAC end to end, L1 to L2. The transformers LV secondary winding, however, is tapped in its center (Neutral) resulting in 1/2 or only 120 VAC from EITHER end to the center tap. At the transformers LV secondary as well as the homes service entrance the Neutral is earth grounded typically by a No 4 bare copper wire (Grounding Electrode Conductor) to certain "Grounding Electrodes" such as but not limited to a copper rod driven into mother earth. ALSO at the service entrance there is a Neutral Ground BOND since the homes service Neutral is BONDED NOT FLOATING like many small generators.

The two legs L1 and L2 are 180 out of phase with respect to each other, when one is 120 + the other is 120 - thus its 240 line to line which are the ends of that 240 volt transformer.     

Typically something like 1/2 of  50 amp RV circuits use 120 VAC L1 to Neutral, while the other half uses 120 VAC L2 to Neutral. If each leg was drawing the same current there would be 0 amps of Neutral return current. Its best if the two legs of 120 VAC L1 & L2 are balanced.

John T Longgggggggg retired Electrical Power Distribution Design Engineer and rusty as an old nail on this stuff so no warranty but believe the above is true ??? heck I've been retired so long lol this may be wrong as rain grrrrrrrrrrrrrrrrrr consult the NEC or trained competent professional and current engineers and electricians if in doubt versus myself or some others.  

[Kirk W]

3 hours ago, NoDirectionHome said:

Are the two 120VAC legs of a 50 Amp RV sevice pylon in phase or 180 degrees out?

 

1 hour ago, Chad Heiser said:

The two legs of 120 VAC coming into your RV (L1 & L2) are the same as the two legs going into your home.  They are not in the same phase.  This is what allows you to make 240 VAC by combining the two legs.

Chad is correct. If the two legs were in phase the connections would be in violation of electrical codes. 

[oldjohnt]

Just to clear up any remaining confusion, the service via the service transformer IS SINGLE PHASE, NOT TWO PHASE. In the electrical trade among electricians and engineers and the NEC its referred to as SINGLE PHASE THREE WIRE. If you looked at a sine wave trace you would see it swing + and - and complete one full cycle 60 times per second IE 60 Hertz. While L1 & L2 are 180 degrees opposite from each other its still a SINGLE PHASE service, NOT Two Phase NOT Three Phase. .   If one leg is at 120 + and at the same time the other 120 - (180 degree difference) its 240 across both.

If wired correctly (how I would design it) a 50 amp RV service pedestal would have BOTH L1 & L2 legs IE 120/240 SINGLE phase three wire, with 120 VAC L1 or L2 with respect to Neutral and 240 VAC across L1 & L2. However it could be wired (perhaps older park or one scabbed) with ONLY ONE 120 VAC LEG in which case a 50 Amp RV may still "work" (subject to its wiring) but there's no 240 VAC available.

Hope this helps, I haven't designed power distribution systems in years but think this is still correct ?? Post back any questions and I will try my best to help if I'm not out in the RV or still unpacking lol. To be safe and sure, consult the latest NEC and CURRENT practicing engineers and professional electricians versus what's posted here including by myself.

Best wishes n God Bless

John T        

[Dutch_12078]

If a park pedestal is wired with a "scabbed" non-split phase 50 amp service using one leg from the source (120, no 240), there's a potential to severely overload the neutral in the RV shore power cord. If both L1 and L2 were drawing the theoretical max 50 amps, that would put a 100 amp load on the neutral that's only rated for 50 amps. With a proper split-phase service (120/240), the neutral only carries the difference in draw between L1 and L2, for a theoretical max of 50 amps if one leg is at max with no draw on the other leg.  Any draw on the other leg will reduce the draw on the neutral.

[Bill Joyce]

There are some older parks that are wired in 3 phase, since it was once legal for 50AMP RVs to only get 208 volts between the legs.  A friend with a bus conversion found a couple like that since he had a 240 volt stove.  Our 2002 Dolphin said 208 to 240 volts on the sticker by the power cord, our 2004 Newmar says 240 volt.

[Jack Mayer]

57 minutes ago, Dutch_12078 said:

If a park pedestal is wired with a "scabbed" non-split phase 50 amp service using one leg from the source (120, no 240), there's a potential to severely overload the neutral in the RV shore power cord. 

1

Exactly right, Dutch.  I used to see this from time-to-time in my travels. I saw it a few times in parks I worked at - in particular when I was installing wifi in parks. It is interesting trying to explain why this is not acceptable to an RV Park owner....especially if they "bought into" the problem. While I don't get into the power in parks as much as I used to, I don't really see this issue much these days.

The three-phase parks are even rarer, but I've seen those, too.  

It used to be that the NEC pretty much (effectively) ignored RV Parks. Not so anymore - and I have mixed feelings about that. In some ways it is good, but like the rest of the Code, there are parts of it that are not really that "helpful". Overall, though, I'd say the progress made in the RV areas is positive. In particular, getting parks to wire using a reasonable power factor is always difficult.

[Dutch_12078]

I agree, Jack, the park power problems have diminished significantly over time as parks have upgraded or gone away. Newer parks that were subject to more stringent inspections are much less likely to have problems beyond individual pedestal mis-wiring like reversed hot-neutrals at the 30 amp sockets.

[oldjohnt]

4 hours ago, Dutch_12078 said:

If a park pedestal is wired with a "scabbed" non-split phase 50 amp service using one leg from the source (120, no 240), there's a potential to severely overload the neutral in the RV shore power cord.

  VERY GOOD point Dutch, good info, let me expand on that for the benefit of some of the non electrical gents on here. Of course, it depends on just how a mis wired RV park pedestal is configured. If instead of two Legs L1 & L2 there was ONLY ONE LEG feeding the 50 amp (intended for 2 legs) RV receptacle, if the two hot conductors were each loaded at 50 amps, that's 100 amps on the Neutral and the RV power cord is only 50 amp rated  YIKES !!!!!!!!!!   

That's sort of like a somewhat similar risk and NEC violation when a person uses a 50 to 30 dog bone adapter (absent additional 30 amp overcurrent protection added) to supply an only 30 amp rated RV power cord YIKES !!!!!!!!!

 

3 hours ago, Jack Mayer said:

It used to be that the NEC pretty much (effectively) ignored RV Parks. Not so anymore - and I have mixed feelings about that. In some ways it is good, but like the rest of the Code, there are parts of it that are not really that "helpful".

 Jack, back when I attended NEC Seminars I was an avid RV user and dealer at the same time, I agree with you, the NEC didn't address RV parks and service to that much extent. However, as its been expanded in later years in my experience it actually is "helpful" to trained  professional electricians and engineers who practice power distribution, even if NOT much help to other less trained and experienced individuals. For those who aren't fluent with the NEC and don't study and practice it on an almost daily basis IT CAN BE SOOOOOOOOOOOO HARD TO READ AND UNDERSTAND grrrrrrrrr lol. You can't get through a paragraph without it referring back to two or three more exceptions each of which also refer you back to other sections grrrrrrrrrrrrrrrrrrr.

 

3 hours ago, Jack Mayer said:

In particular, getting parks to wire using a reasonable power factor is always difficult.

I would say its even WORSE as most RV park owners (and even non engineers and electricians) have absolutely no idea what power factor even means lol

 

3 hours ago, Jack Mayer said:

The three-phase parks are even rarer, but I've seen those, too. 

 208 Y 120 Volt Three Phase Four Wire is completely different then 120 240 Volt Three Phase Four Wire Delta

 

In closing, FWIW I agree with you BOTH things have improved and are safer and more reliable as parks are designed up to modern NEC  standards.

Thanks for the fun sparky chat Dutch and Jack, even if were boring the others to tears lol God Bless you both, I enjoy your posts and am glad to help anytime I can.

John T

[Kirk W]

1 hour ago, Dutch_12078 said:

Newer parks that were subject to more stringent inspections are much less likely to have problems beyond individual pedestal mis-wiring like reversed hot-neutrals at the 30 amp sockets.

I think that most of the problems of the type you refer to came about because the owner or an amateur employee who thought that they knew about electricity did the work. With the increased numbers of 50A RVs and even the higher power demands from those using 30A have helped to get parks to make more efforts to do things right. I'd not be surprised if RV park insurance has played a part in all of this as well. Forums like this which educate the RV owners probably increase the pressure on park owners to get things done right as well. 

[oldjohnt]

 I can tell you definitely its easier to incorrectly wire the Neutral and Hot Conductors on a 30 amp RV receptacle then on a standard household 15/ 20 amp. An electrician can correctly wire a standard 15/20 amp household receptacle blind folded and even most amateurs can get it right. Some of the many different brands of 30 amp RV receptacles I've worked on are not quite as clean and clear and plain marked as on a standard 120 volt 15/20 amp household unit. Perhaps that contributes to occasional RV pedestals being wired incorrectly by maybe the park owner or Billy Bob and Bubba who work for him.  However it takes some serious mis deeds to incorrectly wire a 120/240 volt 50 amp receptacle with ONLY one leg of 120 grrrrrrrrrrrrrrrrrr

 Nice chattin with yall

PS  I just 15 minutes ago after this initial post wired up a new 30 amp RV receptacle in my new pole barn. I know which terminals are which HOWEVER I had to put my glasses on and look deep and hard to see the very teeny brass looking and silver looking tabs which identify the Hot and Neutral... Its no wonder amateurs get them wired wrong !!!!!   

John T   

[mptjelgin]

I've volunteered in one Texas State Park where I opened up the 50 amp box and looked at the outlet, only to discover a three wire circuit with a short jumper added from L1 to L2. Turns out it had once been a 30 amp circuit, but the the increasing number of 50 amp  RV's they'd "updated" it to a 50 amp plug. I did my best to explain the problem with what had been done, but doubt that anything was ever changed. 

This same park had a travel trailer set up for use by seasonal rangers, interns, etc. Stapled across the back of their maintenance building were no fewer than four, 30 amp extension cords to make the 100+ foot run from the outlet to the trailer.  I wonder what the voltage drop was on that setup with the A/C running?

[57becky]

On 9/25/2018 at 7:20 AM, Bill Joyce said:

There are some older parks that are wired in 3 phase, since it was once legal for 50AMP RVs to only get 208 volts between the legs.  A friend with a bus conversion found a couple like that since he had a 240 volt stove.  Our 2002 Dolphin said 208 to 240 volts on the sticker by the power cord, our 2004 Newmar says 240 volt.

The Escapees Park Sierra south of Coarsegold, CA is one of these parks that feed their pedestals off of three phase, 208Y/120 volt transformers.  Many residential appliances can be run on 208 or 240 volts, but rarely are homes served with 208 volts.  Our 2012 Montana has the sticker next to the shore cable connector that says either 240 volts or 208 volts is acceptable.

 

[oldjohnt]

  

4 hours ago, 57becky said:

Many residential appliances can be run on 208 or 240 volts, but rarely are homes served with 208 volts. 

  EXACTLY, a household typically uses 240 VAC relatively high current inductive (AC unit) motor loads (plus dryer, water heater or stove resistive loads) and mostly 120 VAC Single Phase, and 208 would indeed be rare. In my years of power distribution I never designed any 208 volt household systems. In many locations three phase isn't even available.

 Good and informative post 57. As many 50 amp RV's typically (not all) use two legs of 120 VAC with respect to Neutral and if there's NO 240 VAC appliance use anyway, RV park power distribution using a 208 Y 120 Volt Three Phase Four Wire system can (subject to loads) make sense.

 HOWEVER  if the RV uses some "240 Volt" appliances, 208 "may or may not" work as well, SUBJECT TO the specifications of the particular appliance. It's more relevant a motor be served with sufficient voltage then a pure resistive load like a clothes dryer, water heater or stove. Many electrical appliances will work on EITHER 208 or 240 and have a typical voltage tolerance allowing for such variance.

 It comes as NO surprise to me, an engineer, to see an RV label such as 57 Becky posted that EITHER 208 OR 240 is acceptable, especially considering the 208 or 240 (if any at all) is used for pure resistive heat loads versus an inductive motor load like an AC unit. 

 When designing a power distribution system the 208 Y 120 Volt Three Phase Four Wire was preferred (subject to loads) and great for balance versus a 120 240 Volt Three Phase Four Wire DELTA in configurations where ONLY one of the three 240 volt transformers utilized a center tap Neutral. (In the trade referred to as  a Red/High Leg Delta)

 Soooooooo if a 50 Amp RV uses two legs of 120 VAC and even if it does utilize 208 or 240 for pure resistive heat loads (NOT AC units, although on some 208 or 240 is acceptable) and provided the system is appropriately balanced, the 208 Y 120 Volt Three Phase Four Wire system makes sense.

 UNFORTUNATELY if the designer or electrician doesn't understand all this and the RV park owner is concerned some patron may actually require full 240 versus 208 (certainly possible) a full 240 volt system will be installed. 

 NOTE I'm NOT advocating for EITHER system, I'm ONLY explaining how and why a 208 Volt Three Phase system  might suffice. RV park owners ITS YOUR CHOICE NONE OF OURS

 Best wishes n God Bless all

John T

[Jack Mayer]

John, You can argue the 208Y vs 240 back and forth. These days I'd probably never put in a 208Y system, given the direction that RVs are moving. More and more require at LEAST the 50 amp service (100 amps), and many are moving to larger service requirements. Also, some RVs do now have loads that truly require 240 volts. In particular mini-split Air Conditioning units. Some do require 240. So given that 208Y is not available in many places, and the other requirements, I'd certainly not design for that.  (I know you know this - it is mostly a comment for others   )

[Kirk W]

24 minutes ago, Jack Mayer said:

These days I'd probably never put in a 208Y system, given the direction that RVs are moving.

As one who has been reading voltages for a very long time, I can't remember the last time that I saw 208V in an RV park. In addition, if the RV owner uses either of the major brand EMS systems they won't connect to a voltage more than 10% low.

[oldjohnt]

33 minutes ago, Jack Mayer said:

John, You can argue the 208Y vs 240 back and forth.

 EXACTLY JACK, I sure agree, as I noted I'm NOT advocating for EITHER system, just explaining the two systems for the non electrical gents here. There are applications where the 208 Y is best and applications where 240 Delta is best. As 57 Becky pointed out, there are obviously parks where 208 Y is used but Id say the VAST MAJORITY use 240 which is a MUST if 240 ONLY loads are present. In my many years in the industry I came across SOME equipment clearly labled 208 OR 240 while others 240 ONLY !!!!!!!!!!!!!!!! Even where a 10% tolerance is built into the design, I prefer to operate a motor at the higher voltage.

PS With a proper design, a complex 240 Volt Delta system can still be balanced, but no sense in going into the whole 208 Y versus 240 Delta on this RV specific forum. Now if you scroll over to Mike Holts forum this topic might be covered ad nauseum LOL   

 As always I enjoy our sparky chats, thanks for your thoughts, wish we could meet up on the road and talk shop someday.   

John T