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Old 12-14-2005, 11:54 AM   #1
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Electrical Dummy

I’ve posted before that I’m electrically challenged, for some reason I just don’t get some of it. So I’d appreciate some help understanding what’s going on..

I redid the lower rear end and of course had to remove all the electrical to get to the floor. Now it’s time to put it back together. The original installation at the breaker box featured what I think is a 50amp line (about a 1” diameter set of wires) coming from a 15amp male receptacle (shore power plug) hooked up to a 30amp breaker. The breaker box has the 30amp breaker plus 2- 20amp breakers. The air conditioner was coupled to one of the 20amp beakers that it shared with a 110 line to the outlets and the other 20amp breaker feeds the rest of the stuff on board. I’m thinking it‘s a good idea to put the air conditioner on its own circuit and have found ½” breakers and a new breaker box that will add the additional circuit in the same space. So I got that goin’ for me. I replaced the Univolt with an Intellepower 9100 and a new fuse box from West Marine and everything worked fine before I removed it all to replace the floor.

As far as I can tell, the existing system has worked just fine since 1964, so it ain’t broke and it don’t need fixin’. The only thing OFF here is me! But I’d like to have the confirmed understanding that what I’m putting back in is correct.

So why, if the whole system starts with a 15amp rated shore plug, how come everything beyond it has a greater rating? The existing breakers add up to 70amps of distribution, but if you’ve only got a 15amp shore power plug allowing up to 15amp of juice coming in … ??? Lets say that everything 110 in the trailer in ON and it needs 53amps to make everything work. How does a 15amp power line make all this work? I just don’t get it.


Thanks,
Special Ed!
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Old 12-14-2005, 12:38 PM   #2
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Hi Ed:

First, it would be a great help if we knew or could find out somewhere, such as in your profile or under your avatar, what kind of RV you are asking about: travel trailer? motor home? 1948? 2004? model name and length? Having some context will help others better help you.

Second, is there a 15 amp adopter presently fitted on the end of your possibly larger capacity shore plug? Does the shore plug appear to have ever been replaced in the past however many (60?, 30? 2?) years since your RV was manufactured? Knowing the year and type of your RV will help determine what type and capacity of shore plug it originally had.

Kindly give us some preliminary facts to work with here so we can begin to narrow down the presently unlimited answers to your questions.
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Old 12-14-2005, 12:54 PM   #3
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Thanks Fred, my info used to be listed, I guess the new forum upgrade may have changed it. I have a 1964 Tradewind. The male shore power plug that provides exterior access is stamped 15amp. I assumed it was original but don't actually know if it is a relacement.
Ed
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Old 12-14-2005, 01:38 PM   #4
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Quote:
Originally Posted by Wardinbb
Thanks Fred, my info used to be listed, I guess the new forum upgrade may have changed it. I have a 1964 Tradewind. The male shore power plug that provides exterior access is stamped 15amp. I assumed it was original but don't actually know if it is a relacement.
Ed
So Fred, are you indicating that a much larger rated plug is normally installed?
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Old 12-14-2005, 01:53 PM   #5
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Ed,

First, i think if you look closely at the breakers you will find you have 2-20a circuits fed by a 30a main breaker; your total is 40 amps, not 70 amps.

The shore power line should connect to a terminal on the 30a breaker, from there energy flows to the buss, and then out through the 2-20a breakers.

In other words, the power flows 'backwards' through the main 30a breaker, through the buss, then 'forwards' through the 2-20a breakers.
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Old 12-14-2005, 02:09 PM   #6
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Now the juice is flowing! My 1964 Ohio built 19' Globe Trotter has an Indiana made shore power cord with a molded 30 amp/125 volt male plug at its end, but I too don't know whether is it original to the trailer or a later owner's upgrade. The 1964 sales catalog specifies 110 volt electrical service only without any amperage ratings, so that's no help.

I usually keep a 15 amp black rubber adopter fitted on my 30 amp male head so I can plug into my extension cord at home.

Ed - is there a 15 amp adopter on your shore power male plug end? I thought my shore power cord was only 15 amps until someone showed me it actually had a 15 amp adopter pressed on the 30 amp male head. Common black rubber construction made them look like one piece to me, until some one surprised me by pulling them apart!

If your male head end is only 15 amp, I'd replace with a 30 amp rated male plug to match the main breaker to which it is wired. I'm no electrical engineer, and I haven't looked at any RV electrical wiring code, but I don't think the land line has to be rated to carry the cumulative amperage of all the individual lines downstream from the breaker box. The line breakers are there to protect faults downstream from the breaker box. I don't think the wring code assumes all lines will operate at maximum amperage all at the same time. And if they did, or if you tried to do that, the main 30 amp breaker should disconnect to save the day.

But don't depend on my electrical musings. Instead, wait for Gerald to give you a much more qualified answer than mine.
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Old 12-14-2005, 02:16 PM   #7
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Well Ya,

Thanks Mark. Yes, I see that and its true, the 30amp breaker feeds the 2-20amp breakers.
But how come a 15amp line is run to a 30amp breaker when there's only 15amp available? And how does a 15amp energy input run the 2-20amp circuits when the air conditioner and other appliances in use draw, lets say 28 amps from the combined circuits? Thats what I don't get!

Here's some PICs of my set-up before I removed it to fix the floor. The thick power cord running up the wall is from the shore power plug (15amp) connecting to the 30amp breaker as you've said. But the shore power from my garage is only 15amp as well. So how the heck does the air conditioner and several other items plugged in that combined are drawing more than 15amps work when there's only 15amps of shore power available?

Thanks again,
Ed
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Old 12-14-2005, 02:27 PM   #8
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Gettin' closer

[quote=47WeeWind]

"Ed - is there a 15 amp adopter on your shore power male plug end? I thought my shore power cord was only 15 amps until someone showed me it actually had a 15 amp adopter pressed on the 30 amp male head. Common black rubber construction made them look like one piece to me, until some one surprised me by pulling them apart!

If your male head end is only 15 amp, I'd replace with a 30 amp rated male plug to match the main breaker to which it is wired. I'm no electrical engineer, and I haven't looked at any RV electrical wiring code, but I don't think the land line has to be rated to carry the cumulative amperage of all the individual lines downstream from the breaker box. The line breakers are there to protect faults downstream from the breaker box. I don't think the wring code assumes all lines will operate at maximum amperage all at the same time. And if they did, or if you tried to do that, the main 30 amp breaker should disconnect to save the day. "

Thanks WeeWind. Nope, all I've got is a 15amp male plug, no accessories attached. Upgrading to a 30amp receptical makes sense. I've posted a reply to Markdoane that still illustrates me 'not getting it.' Maybe there's nothin' to get!
Ed
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Old 12-14-2005, 03:54 PM   #9
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Ed,

I think what you're asking is how the total of the breakers can exceed the rating of the shore connection. The answer is that electrical loads don't all run at the same time.

Most trailers with 30 amp service have 5 breakers attached. A typical installation would be like this:

Circuit 1 - 20a
16 amp air conditioner

Circuit 2 - 20a
Bedroom recepts (2) - 2 amp
Credenza - 1 amp
Living Rm 2 recept - 2 amps
Converter - 8 amps
Total 13 amps

Circuit 3 - 15a
Water heater - 12 amps

Circuit 4 - 20a
Microwave - 12 amps

Circuit 5 - 20a GFI
Bathroom recep - 1 amp
Outside recep - 1 amp
Galley recep (2) - 2 amp
Refrig - 2.7 amp
Total 6.7 amps

The total 'calculated' amps is almost 60 amps. Obviously, all the connected loads cannot run at full load at the same time, but it works ok if you are careful.

My advice-I would replace the entire 120v box, or rewire it. It's somwhat a mess, and all the ground wires look hazardous. I don't think this is factory original.
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Old 12-14-2005, 05:12 PM   #10
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It Must Be Magic

Quote:
Originally Posted by markdoane
Ed,

I think what you're asking is how the total of the breakers can exceed the rating of the shore connection. The answer is that electrical loads don't all run at the same time.

Most trailers with 30 amp service have 5 breakers attached. A typical installation would be like this:

Circuit 1 - 20a
16 amp air conditioner
Circuit 2 - 20a
Bedroom recepts (2) - 2 amp
Credenza - 1 amp
Living Rm 2 recept - 2 amps
Converter - 8 amps
Total 13 amps
My advice-I would replace the entire 120v box, or rewire it. It's somwhat a mess, and all the ground wires look hazardous. I don't think this is factory original.
Thanks Markdoane for the input. But I still have the same question. Your example above is perfect. I've been plugged into shore power with a 15amp supply and all the items you've listed in circuits 1 & 2 have worked at the same time! Even though there's only 15amps available! I get the concept of the load to the breakers but how is all this stuff working when it draws, as per the example, 29amps and there's only 15amps hooked up to it?

Yes, the breaker box is scary. Probably when they added the air conditioner things were rearranged. And yes, I plan to replace the box and breakers, but I only figured on adding 1 new circuit for the AirCond. Thanks for the circuit list, its a great guide.
Ed
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Old 12-14-2005, 05:31 PM   #11
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If you have 15 amp service, that's all you get. The supply (shore power) in your case will be protected by a 15 amp breaker. In the above example, you would trip the shore power breaker when you turned on the air conditioner. If it did not trip the air conditioner would try to run (very bad) and your lights would be dim. The breakers only prevent excess amperage on a circut.
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Old 12-14-2005, 06:14 PM   #12
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Thanks AZ, thats what I would figure too. I still have the "how come" question because I was on shore power from my garage with 15amps of service. I had all the 12v lights on together with the air conditioner. Lets set aside the math on the amps those #1156 bulbs draw and just say the convertor was working at 8 amps plus the load of the air conditioner. I know it doesn't add up to less than 15amps and I originally asked "how's this working?" This occurred when I installed the Intellepower 9100 to see if my installation worked. It didn't occur to me at that time that I was getting more than 15amps of power, I just figured the 9100 was installed correctly for the test I gave it. Later when I removed everything and saw the shore power plug on the trailer rated at 15a I was confused as to why a 15amp plug is used to feed a 30amp breaker!? Then I added up the draw from my test run and couldn't figure how the heck all that stuff worked.

Physics is the key to the natural world. 15amps should only run 15amps worth of stuff. I don't know what else to say.

Thanks all for your input, I appreciate the time you've given for the help.
Ed
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Old 12-14-2005, 06:28 PM   #13
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Is it possible that the 15 amp plug is connected to a 20 amp circut? A 15 amp cord will plug into a 20 amp connector.
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Old 12-14-2005, 06:38 PM   #14
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The solution is inherent to the problem

But you've got to state the problem accurately!
Thanks AZ for stayin' with me.
I've looked at the situation again. During my 9100 installation test, the battery was hooked up as well. I thought the 9100 bypassed the battery and just charged it when the 9100 was on. I can only figure that the battery was actually running the lights and the shore power through the 9100 was only working the Air conditioner. If thats the case, it all makes perfect sense. Otherwise I'm still confused. But, hey, I told y'all I was challenged!
Thanks,
Ed
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