50 amp to a 30 and 20 out how to do? HELP!

[htwheelz67]

I'm adding a second ac to my WW trailer, it has a 30 amp service, I am wiring the 2nd ac with seperate wiring and load center and breaker out of the load center will be a plug for the ac. I was looking at this....splitter cable

plus a 50 amp ext cable vs 2 cables, I figure I can make my own but at the end of a 50 amp cable. I am unclear how to do it. I want one 30 amp outlet and one 15/20 amp outlet, I'm using 2 dometic hi eff a/c's so they will each run off 15 amp breakers. I'm not too proficient in 240 volt 4 wire wiring.

[CDP]

I have sent an email to my friend asking which park he found two 25A circuits for his 50A service. I'll post it when he replies.

[westernrvparkow]

myredracer wrote:

wnjj wrote:

Incorrect. They are 180 degrees out. Only one phase of the 3 available phases on the power lines are used for a residence. The single 240V phase is center tapped for the neutral to provide two 120V circuits that are 180 degrees out from each other. 120 degrees wouldn't cancel the neutral current anyway.

Concur... However there a few CGs out there that have a 120/208 volt system derived from a 3-phase system and have the hot legs 120 degrees apart. Typically it would be 120/208 volts connected in "wye" configuration like in the 1st diagram. While there are other configurations, a wye secondary is normally what you'll find since the center is connected to ground and is similar to a 120/240 volt system except the hot legs being 120 degrees apart. Homeowners and RV-ers are normally connected to a transformer with single phase primary and secondary like in 2nd diagram and the hot legs are indeed 180 degrees apart but it's not referred to as "phases" in the industry.

Some info. is here and here.

A 120/208 volt (single phase) system for RVs in CGs is normally not a problem because all loads are typically 120 volts. If you had a dryer or other appliance with resistive heating elements in it, you only get (208/240)x(208/240) = 75 percent of the normal output. Voltage drop in the system would make that even worse. The same standard RV 50 amp plug & recept. works on both a 120/240 or 120/208 volt system in a CG.

I stand corrected. You are absolutely correct. No wonder people have trouble with electrical terminology. Home wiring is single phase, yet can have two "phases" created by a transformer that are 180 degrees apart, but it is still considered single phase. 50 amp service delivers 100 amps of potential 120V power in it's two separate legs, but we still call it 50 amp service. And I am sure there are numerous other terms that defy logic. Go means stop and stop means go. Thanks for the clarification.

[Rick_Jay]

myredracer,

Those were informative links. Thanks for sharing them. 🙂

~Rick

[myredracer]

wnjj wrote:

Incorrect. They are 180 degrees out. Only one phase of the 3 available phases on the power lines are used for a residence. The single 240V phase is center tapped for the neutral to provide two 120V circuits that are 180 degrees out from each other. 120 degrees wouldn't cancel the neutral current anyway.

Concur... However there a few CGs out there that have a 120/208 volt system derived from a 3-phase system and have the hot legs 120 degrees apart. Typically it would be 120/208 volts connected in "wye" configuration like in the 1st diagram. While there are other configurations, a wye secondary is normally what you'll find since the center is connected to ground and is similar to a 120/240 volt system except the hot legs being 120 degrees apart. Homeowners and RV-ers are normally connected to a transformer with single phase primary and secondary like in 2nd diagram and the hot legs are indeed 180 degrees apart but it's not referred to as "phases" in the industry.

Some info. is here and here.

A 120/208 volt (single phase) system for RVs in CGs is normally not a problem because all loads are typically 120 volts. If you had a dryer or other appliance with resistive heating elements in it, you only get (208/240)x(208/240) = 75 percent of the normal output. Voltage drop in the system would make that even worse. The same standard RV 50 amp plug & recept. works on both a 120/240 or 120/208 volt system in a CG.

[wnjj]

westernrvparkowner wrote:

Rick Jay wrote:

CDP wrote:
...discovered his two 25A circuits were on the same phase...

Ok, so there's TWO problems here: same phase (0V differential between L1 and L2) and 25A circuits. They should be dual 50A circuits.

Would you mind mentioning this campground? Sounds like a good place to stay away from. If true, I'd also recommend making a post on RVParkReviews.com.

mx727 wrote:
Both 120v legs are the same phase and the system is better referred to as split phase with two legs.

Yes, the 240V/120V service is a single "split" phase service, but if you view L1 and L2 on an oscilloscope with respect to the neutral, they WILL be 180 degrees out of phase.

~Rick

They will actually be 120 degrees out of phase. Electrical service is actually three phases, but only two are generally used at any one time in residential and light commercial applications such as RV parks.

Incorrect. They are 180 degrees out. Only one phase of the 3 available phases on the power lines are used for a residence. The single 240V phase is center tapped for the neutral to provide two 120V circuits that are 180 degrees out from each other. 120 degrees wouldn't cancel the neutral current anyway.

Look at those gray transformers up on older poles. They have a single wire from one of the overhead lines and one neutral/ground as inputs.

[westernrvparkow]

Rick Jay wrote:

CDP wrote:
...discovered his two 25A circuits were on the same phase...

Ok, so there's TWO problems here: same phase (0V differential between L1 and L2) and 25A circuits. They should be dual 50A circuits.

Would you mind mentioning this campground? Sounds like a good place to stay away from. If true, I'd also recommend making a post on RVParkReviews.com.

mx727 wrote:
Both 120v legs are the same phase and the system is better referred to as split phase with two legs.

Yes, the 240V/120V service is a single "split" phase service, but if you view L1 and L2 on an oscilloscope with respect to the neutral, they WILL be 180 degrees out of phase.

~Rick

They will actually be 120 degrees out of phase. Electrical service is actually three phases, but only two are generally used at any one time in residential and light commercial applications such as RV parks.

[westernrvparkow]

CDP wrote:
I don't know what the code is for RV site services. A friend of mine ran into this fairly recently after a discussion on electrical services. Just out of curiosity, he checked the service he was plugged into at the time and discovered his two 25A circuits were on the same phase. He had voltage from hot to neutral on both circuits and "0" between the two. Of course you know this must mean they're in phase since any phase difference would show some voltage difference between the two. He didn't look into this any further before they left the next morning, and since it was of no consequence to him at the time anyway. He just thought it odd enough to comment to me during a recent conversation on the same subject.

I don't know how the pedestal in question was wired internally. Nor do I know if others in the same park were like this one. I made an estimate on how it might be wired to get the results of the same phase on both circuits of a 50A service.

Perhaps there have been changes in the code that might allow this, something the park manager did himself without knowing the code, or perhaps an honest error by someone else that should never have been working on a power pedestal. It is only 50A at 120V which really isn't that much for one conductor to the pedestal. I don't know. My friend is a retired Radar Engineer and I trust what he told me to be accurate. He's actually sort of anal that way. He probably took his oscilloscope to the pedestal for a better investigation. lol

There is no such thing as two 25 amp circuits on a 50 Amp pedestal. If you mean he got a reading of zero volts between the hot circuits on a 50 amp pedestal, then yes, the pedestal is incorrectly wired and out of code. Such wiring could lead to the neutral being overloaded, since the neutral in a 50 RV outlet only carries the difference between the usage in the two legs, so the theoretical limit is 50 amps (50 amps one leg, zero amps 2nd leg). For this to actually happen the two legs must be 120 degrees out of phase. If they are both on the same phase, there is no cancelling effect and the neutral will carry the sum of the usage of both legs. That would mean the neutral could theoretically be carrying 100 amps, which would eventually lead to it failing.
All that information is in theory, in practice both legs seldom carry 50 amps each, 50 amp neutrals do not fail instantly fail at 51 amps and most 50 amp RVs have an energy management system that would see zero volts across the two legs and automatically switch to 30 amp mode (since that is what those adapters from 30 amp source to 50 amp RV actually show). So normally no serious threat actually occurs. That being said, if I saw that wiring at a park, I would be wheels a rolling. Who knows what else has been jury-rigged.

[Rick_Jay]

CDP wrote:
...discovered his two 25A circuits were on the same phase...

Ok, so there's TWO problems here: same phase (0V differential between L1 and L2) and 25A circuits. They should be dual 50A circuits.

Would you mind mentioning this campground? Sounds like a good place to stay away from. If true, I'd also recommend making a post on RVParkReviews.com.

mx727 wrote:
Both 120v legs are the same phase and the system is better referred to as split phase with two legs.

Yes, the 240V/120V service is a single "split" phase service, but if you view L1 and L2 on an oscilloscope with respect to the neutral, they WILL be 180 degrees out of phase.

~Rick

[CDP]

As always, thanks for the comments my friends.

Dave

[mx727]

Phase gets thrown around a lot, but 120/240 residential service is single phase. There are not two separate phases. A house and the power mast at your RV site has 240V single phase power coming in.

There is a center tap ground on the secondary transformer that puts 120V on two separate legs which allows you to get 120v by utilizing only one leg. Thus, we get 240v by going line to line and 120v by going line to neutral which is the grounded center tap.

Both 120v legs are the same phase and the system is better referred to as split phase with two legs.




Here is some more in depth explanation for those that care:

Split Phasehttps://en.wikipedia.org/wiki/Split-phase_electric_power

[myredracer]

CDP wrote:
I don't know what the code is for RV site services.

RVs and RV Parks are covered under article 551 of the NEC. Up until recently there was a free and easily accessible copy of the 2104 NEC online. Now it looks like you have to register with NFPA before you can get it for free. Maybe a free online copy of the NEC isn't currently available because of the new 2017 edition coming in?

There is a draft copy of the 2014 edition of the NEC here. I saw a draft of proposed changes for the 2017 edition recently and there was a proposal to require 40 percent of pedestals to be 50 amp but it looks like it stayed with 20 percent. Too bad because the NEC is waay behind on all the 50 amp RVs today. However, you won't find many new RV parks being built.

FWIW, a copy of the 2005 edition of NEC is here. Have not yet found the previous 2002 edition or others prior to that. The 2005 edition is when they increased 50 amp pedestals from 5 percent to 20 percent of the total in a CG.

[CDP]

I don't know what the code is for RV site services. A friend of mine ran into this fairly recently after a discussion on electrical services. Just out of curiosity, he checked the service he was plugged into at the time and discovered his two 25A circuits were on the same phase. He had voltage from hot to neutral on both circuits and "0" between the two. Of course you know this must mean they're in phase since any phase difference would show some voltage difference between the two. He didn't look into this any further before they left the next morning, and since it was of no consequence to him at the time anyway. He just thought it odd enough to comment to me during a recent conversation on the same subject.

I don't know how the pedestal in question was wired internally. Nor do I know if others in the same park were like this one. I made an estimate on how it might be wired to get the results of the same phase on both circuits of a 50A service.

Perhaps there have been changes in the code that might allow this, something the park manager did himself without knowing the code, or perhaps an honest error by someone else that should never have been working on a power pedestal. It is only 50A at 120V which really isn't that much for one conductor to the pedestal. I don't know. My friend is a retired Radar Engineer and I trust what he told me to be accurate. He's actually sort of anal that way. He probably took his oscilloscope to the pedestal for a better investigation. lol

[myredracer]

CDP wrote:
Gil made some good points. But here is a newer point that may be rather rare yet.

You may not have 240V anyway. In some parks, you may run into a 50amp service that is two, 25amp, protected circuits of the same phase giving you "0" volts between the two.

I've never heard of that but wouldn't be surprised. If any park/CG is doing that, it wouldn't comply with the NEC which is quite specific on a what a 30 amp 120 volt and 50 amp 120/240 volt recept. is and what demand loads are for them. Some parks have general purpose handymen that also do electrical work and I'd worried there could be safety issues at stake as well.

[CDP]

Gil made some good points. But here is a newer point that may be rather rare yet.

You may not have 240V anyway. In some parks, you may run into a 50amp service that is two, 25amp, protected circuits of the same phase giving you "0" volts between the two. In residential services, the 240V comes from the voltage between two different phases. In commercial power distribution, the power companies change phase paring in residential areas to equalize load management. But every house has two different phases (AB, BC or CA). Within RV parks, it seems they're starting to do their own oddball phase load management since 240V isn't generally needed in RVs that I know of. I don't know about the small RV electric cloths dryers, I don't have one. But I suspect they run on only 120V.

The parks might just bring one phase to a slow blow, 50A breaker that's imbedded in the pedestal and feed the entire pedestal from that. If it's a dual pedestal, they may feed one side with one phase through an imbedded 50A breaker and the other with the other phase through another imbedded 50A breaker. They might feed one phase to odd number pedestals, or spaces, and the other phase to even pedestals.

If you check on the pedestal with a multimeter from one hot side to the other on the 50A receptacle and read "0" volts, you have the same phase. Then read from one side to the hot terminal on the 30A service and so on. If you read "0" volts between all hot sides, you have all of one phase. This assumes of course, the pedestal is hot as it should be. You can measure hot to neutral and/or ground first to make sure.

The benefit to the park of doing it this way is they can easily prevent guests from pulling any more than 50 amps no matter how a guest tries to get around it. The guests would not have access to the imbedded 50 amp breaker and would have to call park management to have it reset, if it isn't automatic. Since larger units today are becoming more and more power hungry, some folks trip a breaker when using the microwave on a hot summer night and try to get around that by "creative electrical engineering" and plugging one AC unit into the 30A or 20A service. They turn on the microwave and will still trip the imbedded 50A breaker.

I would expect to see more and more new parks wired this way. If guests are abiding by park rules, they should not have any problems except for the rare weak breaker. Breakers do wear out and get weak over time. But other than that, no worries mates.