SeilerBird
Well-known member
Ken - I know it is possible, I am just saying in the real world that situation would rarely come up. But it definitely is unsafe.
Electrical Systems
- Thread starter grashley
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Tom, I would appreciate an explanation and maybe a reference as I have provided, because I'm not sure we are talking about the same thing as parallel can be used many ways. Because if the RVIA Electrical Specialist text published in June 2012 was wrong on that subject, assumably they would have issued a correction by now.
Tom
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Any basic reference on AC (alternating current) theory will explain it in words, diagrams and math. A google search will bring up countless such explanations from our accredited schools of electrical engineering.
Since the two legs are 180 degrees out of phase, the (RMS) voltage on one leg is +120V while the (RMS) voltage the other leg is -120V. If you tried to connect the two legs in parallel, it would be the equivalent of a short circuit across 240V.
My first practical demonstration outside of school/theory was when I worked at a power generating station, bringing one of those large generators on line, effectively paralleling it with the others on the grid. If the add-on generator was not exactly in phase with all the others at the time of connection, the whole building heard the bang as the generator jumped to instantaneously bring itself in phase. That was circa 1967, but the physics and electrical theory haven't changed.
I've observed countless examples since then, including a friend who insisted on paralleling the two 120V legs on his boat to power from a single 120V inverter. He lived off the grid quite happily for a weekend but, when he got back to his berth and plugged his boat into 50A/240 service, the entire shed where he was berthed found out about it. He'd ignored my warnings, and effectively put a short circuit across the marina's 240V power supply.
Edit: I suppose that, in elementary math, we learned that two drawn lines are "parallel" if they never touch (when drawn to infinity). However, in the electrical sense, we learned that "parallel" meant something quite different. Since we are discussing electrical stuff, the RVIA electrical text should talk in electrical terms.
Quillback 424
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Tom,
In the late 1960's I was on a diesel-electric submarine and EVERY individual had to bring up, parallel and drop off a generator to qualify for the electrical signature on their qualifications card. Once I was fully qualified in subs (SS) I had the fun of teaching and eventually qualifying others to perform this procedure.
Once in awhile the student would get a little apprehensive and operate the switch before the phase meter was passing the correct position and, as you said, BANG!
In the late 1960's I was on a diesel-electric submarine and EVERY individual had to bring up, parallel and drop off a generator to qualify for the electrical signature on their qualifications card. Once I was fully qualified in subs (SS) I had the fun of teaching and eventually qualifying others to perform this procedure.
Once in awhile the student would get a little apprehensive and operate the switch before the phase meter was passing the correct position and, as you said, BANG!
John From Detroit
Well-known member
RodgerS said:
Not accurate.. They are in opposition not parallel... If they were in parallel then at max load the neutral would (Well for one thing it would not be neutral) hauling 100 amps.. Twice its maximum rating.
But since they are in opposition the neutral, at max load, is not doing anything but sitting there. zero current (If perfectly balanced) it only carries the difference between the two legs.
The transformer that provides power is center tapped.. The center tap is the neutral.
This is shore power.. On generator power they tend to be in parallel.. but my generator only puts out a total of 50 amps so the neutral is not overloaded.
Tom
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[quote author=Quillback]... the student would ... operate the switch before the phase meter was passing the correct position and, as you said, BANG![/quote]
Aye Larry, those "phase meters" were quite crude, but essential. I'd imagine that the 'bang' was exaggerated (aka VERY LOUD) in a submarine
Aye Larry, those "phase meters" were quite crude, but essential. I'd imagine that the 'bang' was exaggerated (aka VERY LOUD) in a submarine
Quillback 424
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Aye, Tom. We certainly didn't need to dust the M/G sets for awhile.
Quillback, been there with you on the quals and the parralling of generators. We used to the 64's on a regular basis. The meters on each would swing back and forth when the tie.breaker was closed. The farther out of phase they were, the more the meters would move. I was the LPO and my crew would make a contest out of it to see how steady they could hold the meters. Most non-quals were scared to death to close the tie breaker, but we had a striker that had watched it a few times. When it came to his turn, he casually slammed the the tie breaker handle up, opened the one to drop the off-going generator like he was turning on and off light switches and then just turned around and sauntered into the mess decks for a cup of coffee. The meters never even quivered. Needless to say I didn't have a problem giving him the sig.
Alfa38User
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Heh heh, On the generators we ran in our power plants you had to bring them on line one at a time watching very carefully for the flashing lights so that the phasing was correct, scary at first but after a while, second nature. I think later on, they had generators and circuits that would do that automagically, but I haven't seen that!! Yes, quite a bang if you screwed up!!
Alfa38User
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RodgerS said:
hmmm... From reply #13 (RogerS): "That you get two hot legs in parallel supplying 50 amps each = 100 amps of 120vac current available for use in an rv."
Had the two little words in bold been left out we could have save a lot of pixels..... 8)
Gary RV_Wizard
Site Team
It's not that rare a situation in modern coaches. With two a/c units, washer/dryer, fridge, water heater, and the usual run of 12v stuff on the converter/charger, I can readily pull 60-65 amps. If someone happens to use the microwave or a hair dryer for a couple minutes it shoots up from there. On a hot day when my a/c runs pretty much non-stop and the fridge is working hard as well, I often see sustained total amp loads in the 40-45 range, so flipping on any additional appliance (toaster, coffee pot, etc) immediately pushes the limits. Longer coaches with three a/c units can reach the limits quite easily.
Fortunately, they don't all operate simultaneously most of the time, so the risk of an neutral actually overheating from overload is not great.
SeilerBird
Well-known member
Gary, the reason I say it is a rare situation is because it would depend on the pedestal being wired incorrectly, and running basically every thing in the motorhome at the same time.
Gary RV_Wizard
Site Team
I'm not sure that is true. While I'm no NEC expert, I know of nothing that prohibits the use of oversized components in a circuit. As long as the circuit protection (the breaker) matches the lowest rated component, I think it conforms. A 50A outlet, served with only 30A/240v power from a twin 30A breaker, should "meet code". Just because the outlet is rated to handle up to 50A and 240v, it does not mean that much power is actually available for use. The NEC is concerned with safety, not false advertising.
The NEC also also allows the power source to be over-committed, e.g. 5x 50A outlets sharing a single 200A source. If each of the five tried to actually use the full 50A, the 200A breaker on the source panel would trip and nobody gets any power at all. Perfectly legit. That goes back to the 80% average load assumption that is the basis for most ratings and wiring design. Same is true of 15A household outlets, which is why breakers may trip even though the device in use is within the max range allowed.
Tom
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You're backpedaling
??? You have no knowledge of whether I'm in my RV or not.
Ken & Sheila
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Gary RV Roamer said:
Yes it would be perfectly OK to use a 240/120V 30Amp circuit. But not OK to use 2- 50amp hot circuits in the same phase as that could be a safety issue by potentially overloading the Neutral. Without the code in front of me I'm not sure it NEC would accept a 50amp240/120 Volt outlet wire with 2 hots in the same phase but with 25amp breakers (but it would be safe). The RV Parks section of the code does clearly state it should be 240/120 volt service, but it also references other parts of the code.
NEC has always allowed "overcommitment". Just look at your house breaker box.
The old NEC code for campgrounds allowed a huge overcommitment that is the reason we find low voltage problems in older parks. The newer codes reduced the amount of overcommitment but even it can be a problem if a park is full and it is hot (ACs running). Mostly an issue with 30amp service. Current code for a park with 36 or more sites only has to have design to provide 41% of total maximum. There is an informational note that this may not be adequate for areas of extreme hot or cold temps - potentially almost anywhere nowdays, northern Idaho over 100 this week.
ken
Gary RV_Wizard
Site Team
Agree, Ken. It is all but impossible to quote the NEC out-of-context, since what one paragraph gives another may take away. There is a lot of "Kentucky windage" in interpreting it as well, especially at the local enforcement level where practical (or sometimes political or personal) decisions get made.
OP
OP
grashley
Well-known member
THIS STRING HAS BEEN HIJACKED!!!
To refocus, if I am on battery power ONLY and the inverter is off, what in my 5th wheel will be powered, aka what actually runs on 12 VDC??
All lights? Most lights? TV? Satellite Receiver? Slide-out? Jacks? Water pump? Awning?
What requires 120 V to operate? Inverter power insufficient?? A/C? Refrigerator? Microwave? Satellite Receiver?
Assuming recent equipment, will my truck charge my 5er batteries while being towed??
Gordon
To refocus, if I am on battery power ONLY and the inverter is off, what in my 5th wheel will be powered, aka what actually runs on 12 VDC??
All lights? Most lights? TV? Satellite Receiver? Slide-out? Jacks? Water pump? Awning?
What requires 120 V to operate? Inverter power insufficient?? A/C? Refrigerator? Microwave? Satellite Receiver?
Assuming recent equipment, will my truck charge my 5er batteries while being towed??
Gordon
SeilerBird
Well-known member
Yesgrashley said:
Yes
No
Yes
no
a/c, microwave, tv, satellite, slide out
depends on the size of the invertor
maybe
Rene T
Site Team
My 5vr awning, slide out and front and rear jacks run off 12 volt. One of our TV's is also 12 volt.
