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Old 08-05-2019, 02:11 PM   #1
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ACs, Amps and EMSs (Someone hep me!)

I'm not stupid, but sometimes electrical details just confuse the hell out of me.

First question: On the two-AC models, will both run off the single shore power connection? If so, when I buy an EMS, don't I only need the 30Amp EMS, as long as I'm planning on plugging into a 30 A shore power?

Second question: if this is so, why is there discussion elsewhere about having a second, 50 A, shore power cord? Are the trailers (so to speak and only metaphorically) AC/DC,* in the sense that they'll plug into either one?

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Old 08-05-2019, 02:16 PM   #2
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ACs, Amps and EMSs (Someone hep me!)

Two run both AC units you need a 50 amp circuit and cord. If there is only 30amps available you can run 1. You would also need a 30 amp to 50 amp adaptor to use that circuit as the plug is different.

Not sure what an EMS is, but if you mean a surge protector then it needs to be matched to the largest current (amp) load.

And the only place you can normally get DC from is your Tow vehicle. Our trailers have both AC and DC circuits. The batteries and charger run the DC side.
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Old 08-05-2019, 05:06 PM   #3
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EMS is an electrical management system, which I take it is a smart surge protector. I think.

But to your point and my question. If I buy a 50 A surge protector, can I use it with a 30 A supply?

And if I need a 50 A supply (and surge protector) (and cord) to run 2 ACs, does that mean I'm using 2 cords, one for each unit, at the same time, or just one 50 A cord to run the two?
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Old 08-05-2019, 05:57 PM   #4
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Originally Posted by Belbein View Post
EMS is an electrical management system, which I take it is a smart surge protector. I think.

But to your point and my question. If I buy a 50 A surge protector, can I use it with a 30 A supply? Yes

And if I need a 50 A supply (and surge protector) (and cord) to run 2 ACs, does that mean I'm using 2 cords, one for each unit, at the same time, or just one 50 A cord to run the two? 1 50 amp cord

And I looked up EMS it’s a sophisticated load management system from what I read. My trailer has a mechanical EMS - me
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Old 08-05-2019, 06:26 PM   #5
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NO, NO, NO. Buy the 50 EMS surge protector from Southwire with the wireless remote display. It saved my trailer twice in the last two weeks with one park with an open ground, and the second with low voltage.
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Old 08-05-2019, 07:39 PM   #6
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50 amp

I believe all the 2-AC trailers have a 50A socket in the side of the trailer, and (at least mine did) come with a 50A power cord. You can plug this either directly into a power pedestal that has a 50A socket (which is a different connector than on the trailer), or into a 50A surge protector, which has a socket for the cord, and a short pigtail to plug into the pedestal (alternatively, you can install a permanent surge protector in the trailer, in which case you plug directly into the pedestal).

With the 50A cord plugged into a 50A power pedestal, you can run both AC units at the same time.

If the pedestal only has a 30A (or, shudder, 20A) socket, you need to have a 50A -> 30A pigtail - this has a 50A socket for the power cord or surge protector, and a 30A plug to go to the pedestal. You can still use your 50A surge protector in the circuit, it just goes between the 50A plug and the pigtail. In this case, you can only use a single AC unit, and need to do careful power management to ensure your consumption doesn’t exceed the available 30A supply. Examples of high-current appliances include the hot-water heater, toasters, the microwave, hair dryers and such.

A surprising tidbit is that the cord power ratings are actually peak, not constant load. Your constant load shouldn’t exceed 80% of the rated value, or you risk burning up one or the other connectors on the power cord. This happened to us on a large boat in Costa Rica...and resulted in a small fire at the power pedestal.

Finally, you need to keep the contacts clean and corrosion-free. Otherwise, you end up with an unanticipated high-resistance circuit...which runs hot and may damage the cord and/or connectors.

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Old 08-05-2019, 08:00 PM   #7
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Quote:
Originally Posted by Belbein View Post
I'm not stupid, but sometimes electrical details just confuse the hell out of me.

First question: On the two-AC models, will both run off the single shore power connection? If so, when I buy an EMS, don't I only need the 30Amp EMS, as long as I'm planning on plugging into a 30 A shore power?

Second question: if this is so, why is there discussion elsewhere about having a second, 50 A, shore power cord? Are the trailers (so to speak and only metaphorically) AC/DC,* in the sense that they'll plug into either one?

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HI

1) For normal running of two A/C's you need a trailer wired up with a 50A service. Indeed it supplies 100A at 120V compared to the 30A at 120V the 30A version supplies. Both A/C's will run off of a single 50A shore power connection.

1a) Get the 50A surge suppressor / EMS for a 50A trailer. Get the 30A version if you have a 30A trailer. The EMS matches the trailer rating not the pedestal you happen to be hooked to at this or that campsite.

2) 50A cables are big and heavy. When it gets cold out, they are stiff, big and heavy. That makes them a bit interesting to lug around. Many people carry a "short" and a "long" version. The short one gets used unless you just *have* to pull out the long one.

Since your trailer is set up for 30A service, it only needs a 30A EMS and a 30A cable. There is no need to fiddle with a 50A EMS or a 50A cable.

Bob
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Old 08-06-2019, 12:30 PM   #8
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If this helps, Great, If not, Oh Well.

For the OP, if this helps, then great:

The 50 amp has four wires and the 30 amp has three.
For the 50 Amp it is:
50 amps 120 volts Hot to Neutral (Leading)
50 amps 120 volts Hot to Neutral (Lagging)

Which also supports 50 amps 240 volts (Hot to Hot) and in this case, under perfect world, the neutral would be carrying zero. 50 amps between the two hot wires and nothing on the neutral.

For the 30 amps it is:
30 amps 120 volts Hot to Neutral
And the campgrond predetermines whether that 30 amps is on the Leading versus Lagging when they wired up that pedistal


The concept of leading and lagging might raise many replies to this description:
Think of it as the leading is 90 degrees leading phase relative to neutral producing 120 volts, and the lagging is 90 degrees lagging relative to neutral also producing 120 volts but out of phase to the former, and between the two is 180 degress producing the 240 volts.


And if for some crazy reason, a pedestal is wired completely wrong with both 50 amps HOTs on the same leading or lagging, then the Neutral would be potentially carrying 100 amps, and likely melt something in the circuit. This arrangement is unlikely, but even licensed electricians have been known to make mistakes.
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Old 08-06-2019, 01:12 PM   #9
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GeeSag, you have described all these items absolutely correctly! Great post.
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Old 08-06-2019, 02:10 PM   #10
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Quote:
Originally Posted by Belbein View Post
I'm not stupid, but sometimes electrical details just confuse the hell out of me.

First question: On the two-AC models, will both run off the single shore power connection? If so, when I buy an EMS, don't I only need the 30Amp EMS, as long as I'm planning on plugging into a 30 A shore power?

You have not provided enough information. The answer depends on what electrical distribution panel is in your trailer. Normally, if there are two air conditioners on the trailer, it has 50 amp, but not always. Some people have added a second 20 amp cord to run a second air conditioner. Assuming your trailer has a 50 amp service, you need a 30 to 50 amp adapter when you are on a 30 amp service. You would plug the adapter into the 30 amp pedestal outlet, plug in the 50 amp EMS to the adapter, plug the trailer cord into the EMS.
Whether a second air conditioner will run or not also depends on whether your trailer has one or two power cords. If you have the 50 amp and are plugged into a 30 amp pedestal outlet, either air conditioner will run, but not both. Two air conditioners will trip a 30 amp breaker. You have to manage electrical use to keep the load appropriate for the supply.
If your trailer has a 50 amp service, you need a 50 amp EMS. If your trailer has a 30 amp service, you need a 30 amp EMS.


Second question: if this is so, why is there discussion elsewhere about having a second, 50 A, shore power cord? Are the trailers (so to speak and only metaphorically) AC/DC,* in the sense that they'll plug into either one?

The only reason to carry a second 50 amp cord is when the power supply is not near the trailer so that a single cord will reach the pedestal.
Some people who have a 50 amp service might carry a 30 amp cord to use between the pedestal and the 30 to 50 adapter so they do not have to handle the heavy 50 amp cord. This is about the weight of the cord, not about electricity.

ps:
30 amp 120v and/or 50 amp 120/240 volt supplied from the pedestal are both alternating current (AC).
The converter converts 120 volts AC to approximately 12volts direct current (DC)
If you have an inverter, it converts 12 volts DC from the battery to 120 volts AC.


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Old 08-06-2019, 02:40 PM   #11
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Quote:
Originally Posted by GeeSag View Post
<<snip>>

And if for some crazy reason, a pedestal is wired completely wrong with both 50 amps HOTs on the same leading or lagging, then the Neutral would be potentially carrying 100 amps, and likely melt something in the circuit. This arrangement is unlikely, but even licensed electricians have been known to make mistakes.
I ran across a different scenario a few years ago. The lady at the desk told me the site was "50 amp". When I looked at the pedestal there was a 50 amp single pole breaker supplying a 30 amp RV outlet. Being suspicious about this, I got out my multimeter and checked before plugging in my trailer. Ground and neutral were bonded in the secondary panel (hot trailer skin). After I warned the lady about the danger, I moved on to another site
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Old 08-07-2019, 07:55 AM   #12
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Hi

Indeed if you *do* have 50A in phase on the supply, the neutral gets more current than it should. There's no debate about that. It's not got a breaker on it so this it is un-protected for this fault. You have a very real chance of meltdown / fire. It is *not* the way it should be done.

The more interesting point is - does it matter in a typical AS?

Assuming it's wired in phase: If you pull less than 25A on each leg, you can't get more than 50A out the neutral, there is no problem. Same if you pull any combo between the two that comes up below 50A total.

Ignoring low voltage and other weird issues, your A/C's each pull right around 15A each when running. The combo is just above / just below 30A. To get above 50A, (with both A/C's running....) the "rest of the trailer" would have to pull 20A. The microwave is about 13A and the stock converter is about 3 to 5A, hot water is abut 10A, and the fridge a bit over 2A. It *is* indeed possible. It's not very likely. Yes, those numbers are from a stock 2017 30' Classic. Each model will be a bit different.

In the example above you are at about 60A on the neutral. The plug will get warm, but probably will not melt down. Once the microwave is done heating up the pizza, the load drops and the plug cools down some. If somebody grabs the giant hair dryer .... all bets are off

Bob
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Old 08-07-2019, 08:59 AM   #13
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Quote:
Originally Posted by Belbein View Post
First question: On the two-AC models, will both run off the single shore power connection? If so, when I buy an EMS, don't I only need the 30Amp EMS, as long as I'm planning on plugging into a 30 A shore power?
IF your trailer has two ac units, you have 50 amp wiring and so I'd buy the 50 amp EMS.
Quote:
Second question: if this is so, why is there discussion elsewhere about having a second, 50 A, shore power cord? Are the trailers (so to speak and only metaphorically) AC/DC,* in the sense that they'll plug into either one?
Some people, including me, have a second power cord, a 30 amp one, for when I want to plug into an outlet that only has 30 amp service OR I want to plug into my generator. It's just easier than dragging out the huge 50 amp cord and adapting with 'dogbones".
However, that creates a minor problem in that you have a 50 amp EMS and a 30 amp cord now. I finally bought a second EMS to use with 30 amp service.
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Old 08-07-2019, 11:26 AM   #14
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Quote:
Originally Posted by Belbein View Post
I'm not stupid, but sometimes electrical details just confuse the hell out of me.

First question: On the two-AC models, will both run off the single shore power connection?

Recent Airstream trailers with two air conditioners are wired for 50 amp service. If you are parked someplace where 50 amp service is available, you can easily run both air conditioners.


If you only have 30 amp service available, you can probably run both air conditioners, as long as both fans are on low, you switch the fridge to propane, and don't run any other electrical things. The actual amp draw of the air conditioners will depend on the outside air temperature (higher temperature, more amps) and the voltage at the pedestal (higher voltage, fewer amps). In some cases the pedestal breaker will trip, or the fuse will blow on an older pedestal. It will also trip/blow if someone forgets and turns on the microwave. I carry extra 30 amp fuses.




Quote:
If so, when I buy an EMS, don't I only need the 30Amp EMS, as long as I'm planning on plugging into a 30 A shore power?
Most EMSs are designed to limit you to running one air conditioner at a time if there is only 30 amp power, so that you don't blow fuses or trip breakers.



Quote:

Second question: if this is so, why is there discussion elsewhere about having a second, 50 A, shore power cord? Are the trailers (so to speak and only metaphorically) AC/DC,* in the sense that they'll plug into either one?
There are "adapter" 30 amp cords that connect to the 50 amp twist lock inlet on the Airstream. It is wise to carry one if you have a trailer that has 50 amp service. The 50 amp cord is big, heavy, and expensive to replace once it fails ($300). I only use the 50 amp cord when we are at a campsite with 50 amp service (most around here don't have it but that varies regionally), and even then only if we think we're going to run the air conditioner.


Otherwise if you're at a campsite with only 30a power, you have to use the big heavy 50a cord and a "dogbone" adapter. I carry one just in case but hardly ever use it.
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