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Author Topic: Question on Autotransformers in RV use for electrical guru's  (Read 2060 times)
bevans6
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« on: November 26, 2012, 03:19:55 PM »

Another forum came up with this:

NFPA 70 - National Electrical Code® 2011 Edition

Chapter 5 - Special Occupancies

Article 551 - Recreational Vehicles & Recreational Vehicle Parks

Section 20 - Combination Electrical Systems

Paragraph E - Autotransformers

"Autotransformers shall not be used."

Autotransformers are the basis of many, if not most, voltage adjusting setups for RV use in parks with low pedestal voltage, the Hughes Autoformer being one popular version.  They sense the voltage and step it up 10% if needed with a relay that selects the appropriate tap on the transformer.  An autotransformer is a single coil transformer with taps to take off stepped up or stepped down voltage.  The question is - why does NEC not allow them for use in RV's and RV parks?  All sorts of ideas, like they "suck current from other users" and overload the pedestal wiring, which I find a tad sketchy.  Does anyone know a real answer?

Brian
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« Reply #1 on: November 26, 2012, 04:50:46 PM »

That's why I don't always follow codes that are written for people regulated by codes.  My Trace SW2512MC senses campsite low current and kicks in with power from the battery bank to match the same wave length if I overload the campsite plug (i.e. running two air conditioners off a weak 30 amp plug in). 

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« Reply #2 on: November 26, 2012, 06:30:55 PM »

This may be related or completely bogus and irrelevant, and comes from a conversation I had just a couple of days ago with someone about grid-tie alternative energy set-ups for homes (ie. having a solar panel array or wind turbine, and selling any excess power you produce back to the power company).

Apparently, when people live in rural areas a long way from other houses, an otherwise perfectly standard grid-tie set-up can cause expensive & dangerous damage. In those cases, where there are no other houses in the locality which can use the power you're feeding into the grid, the voltage being produced by your grid-tie inverter has to step-up significantly to overcome the resistance it meets when sending the power a long way into the grid. But the inverter is supplying your own house as well, and the high voltage it's producing can be sufficient to burn-out your own appliances.

But I've no idea if this phenomenon has anything to do with why the code says you shouldn't have a transformer in an RV that's connected to a park electrical system.


Jeremy



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« Reply #3 on: November 27, 2012, 03:16:17 PM »

I presume anything that increases voltage is to prevent high amps which burns out all kinds of electric motors, especially on ACs.

Hi amps is the bad guy, not high voltage except in extremes.
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Sean
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« Reply #4 on: November 27, 2012, 03:42:56 PM »

...
Autotransformers are the basis of many, if not most, voltage adjusting setups for RV use in parks with low pedestal voltage, the Hughes Autoformer being one popular version. ...


Brian,

You (and others in this thread) are incorrectly supposing that the code's proscription against autotransformers is related to "buck/boost" transformers such as the Hughes model -- it is not.

Note that buck/boost transformers do not need to be made with an autotransformer -- they can be (and sometimes are) made from a standard isolating transformer instead.  Likewise, many autotransformers are used simply to convert voltage, rather than in variable buck/boost applications, the most common being to derive, for example, 120vac power for a device from, say, a 277-volt source because that was what was available.  I blame Hughes for this confusion, since their marketing materials tend to associate the two technologies, when in fact there is no association whatsoever (except in their own product).

The reason the code forbids them in RVs has to do with safety issues related to the way an autotransformer works.  In particular, certain kinds of fault conditions can cause the full input voltage to be applied to the output, which is hazardous if the transformer is being used, for example, to convert 240vac to 120vac.  These devices also have different ground fault considerations.  For this reason the code forbids them in many places (not just RVs).  For example, they may not be used in most wet location applications.

Note that the code was not looking at the type of buck-boost devices you are talking about, but rather autotransformers in general.  Putting one in-line in a properly grounded shore service does not present the same safety issues as having one built-in between the coach's main panel and an appliance or receptacle.

All that said, the code proscription means that an autotransformer can not be built into an RV, even wired ahead of the main panel at the shore input.  So if a manufacturer wanted to include a voltage-boosting system (which is allowed under the code), it simply needs to be the type that uses a regular, isolating-type transformer.  Neither does the code prohibit use of a free-standing unit in-line in the shore connection, such as the Hughes.

-Sean
http://OurOdyssey.BlogSpot.com
« Last Edit: November 27, 2012, 03:53:49 PM by Sean » Logged

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bevans6
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« Reply #5 on: November 27, 2012, 05:25:51 PM »

Sean - thanks.  I don't have such a device nor do I  anticipate having one, but I just couldn't figure out what the issue was.  Now I have a follow up - how do you know this stuff?  I presume you have a senior degree or degrees in engineering around electricity/electronics, but this is just not mainstream...

Brian

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« Reply #6 on: November 27, 2012, 06:28:13 PM »

... how do you know this stuff?  I presume you have a senior degree or degrees in engineering around electricity/electronics, but this is just not mainstream...


It came to me in a vision after consuming psychedelic mushrooms....

Actually, neither of my degrees is in electrical engineering, although I do have an engineering degree.  Most of what I know about electrical codes and construction comes from OJT and the school of hard knocks.  I spent 20 years designing and supervising the construction and operation of major technical facilities.  Along the line I had to learn everything I needed to know about power systems, HVAC, plumbing, and more or less every construction trade, and I came to be on a first-name basis with more than one code inspector.  To put our buses in perspective, most of the facilities I built consumed a minimum of 400kW of electrical power, just for the equipment, and went up to several times that amount.  As a side note, I will say that my cell phone is now more powerful, in every respect, than mainframe computers consuming 15kW apiece back when I started in that business:
http://ourodyssey.blogspot.com/2011/04/mountain-view-downtime.html

Obviously, I did not learn Article 551 of the code when I was in that business.  But I knew my way around the code very well, and I even worked for an electrical contractor for a number of years, even going so far as to prepare for the California C10 license.  Naturally, when I started designing my bus nearly ten years ago, I pulled out my copy of the code and studied up on all the relevant provisions.  Most of them made perfect sense to me.  Though I, too, wondered at the time why there was a proscription against autotransformers.

Unlike our good friend Geoff, I am a firm believer that every provision in a code is there for good reason.  Unfortunately, those good reasons usually involve a number of fatalities or serious injuries.  (Someone once did a study on how many folks had to die before a typical code provision made it in -- I don't have it in front of me but the number would shock most people.  Similarly, most cities do not erect a traffic control at an intersection until some number of fatal accidents occur there.)  So when I saw this (and some other parts of 551), in typical engineering fashion, I went off in search of the answers.  It helps to know a little bit about how codes work and how codemaking bodies deliberate.

Nowadays I keep up on 551 and the code in general not because it is part of my job (I am retired), but because I get so many questions here and elsewhere.  My way of giving back to the bus community, if you will -- it would take most bus nuts years to attain the proficiency with the codes to know how to interpret them and the whys and wherefores, and it's really not necessary for most folks.  They are written for inspectors, engineers, and post-journeyman professionals -- even the average electrician in most firms would have trouble with some sections.  Most of the journeymen I know carry around a little pocket-sized book with all the relevant tables, charts, and code requirements in plain English.

-Sean
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Rick 74 MC-8
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« Reply #7 on: November 28, 2012, 05:48:23 AM »

Plus he stayed at a Holiday Inn Express last night.
 Sorry just couldn't help myself
   

                                  Rick 74MC-8
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« Reply #8 on: November 28, 2012, 06:17:32 AM »

My way of giving back to the bus community,


Just a huge thanks to Sean and the others who regularly volunteer their expertise to our benefit.  We all know that if we picked up the yellow pages we would be paying at the $$$.  Thanks again and look forward to hearing about your new boating adventures.

 
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« Reply #9 on: November 28, 2012, 07:12:49 AM »

   Just a huge thanks to Sean and the others who regularly volunteer their expertise to our benefit.  We all know that if we picked up the yellow pages we would be paying at the $$$.  Thanks again and look forward to hearing about your new boating adventures.

    Amen.  Thanks, Sean.
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« Reply #10 on: November 28, 2012, 07:46:56 AM »

Thanks Sean! You are one of the good guy's of Bussin. Magical Mushrooms hmmm, got a link? Kiss
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« Reply #11 on: November 28, 2012, 08:05:58 AM »

Sean,

We all fervently hope that you have satellite internet on the boat.
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« Reply #12 on: November 28, 2012, 08:45:27 AM »

Regarding code compliance, I have a friend who fervently believes that the electrical code is written solely to make money for manufacturers of wire and electrical products.  His garage has a number of code violations.  He thinks wire sizes are over-sized so wire manufacturers make more money.

We wired a trailer with electrical power for lights and outlets.  I insisted on following electrical code.  He might have used under-sized wire if it was up to him.  The entire project cost about $500 with all the lights and everything.  We might have saved $20 with undersized wire.

The thing is that for his job he designs refrigeration systems and he has to follow electrical code in his designs.
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« Reply #13 on: November 28, 2012, 03:57:21 PM »

You guys give homage to someone who comes up with an answer nobody can understand.  I seen this repeated several times on this board over the years.  Not that Sean is wrong, its more like WTF?

--Geoff
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« Reply #14 on: November 28, 2012, 05:07:25 PM »

I appreciate Sean's and others info.  Has saved me from making some costly and/or possibly unsafe errors. 

He could have just said it does not apply.  Some of us (me) would surely want to know why. The explanation lets the skeptical do their own research.  I find it very helpful and informative. 

After all, is this not what the forum is all about, the exchange of info (knowledge)? 
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« Reply #15 on: November 28, 2012, 05:23:04 PM »

I'm no expert, so go easy....

It sounds like these may be a valuable tool in our electrical arsenal, esp. if we might venture into Mexico! However, from my limited understanding, it sounds like these devices must 'sacrifice' amperage for voltage. Is that correct?  Is there any way to know how much amperage you have 'lost' when one is in operation?

Also, transformers generally create heat. Is it safe to hardwire one of these autoformers into the electrical bay? OR would it be better to use it 'stand alone' outside of the RV?
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« Reply #16 on: November 28, 2012, 08:45:55 PM »

...
It sounds like these may be a valuable tool in our electrical arsenal, esp. if we might venture into Mexico!


FWIW, we don't have one, having found virtually no need for such a device anywhere in the US.  However, we have traveled extensively in Mexico and that is the one place where we wished we had one.  We made do without, and I'm not sure the limited number of times, even in Mexico, where it would have helped would have justified the fairly high cost, plus the space and weight in the bay to haul it around...

Quote
However, from my limited understanding, it sounds like these devices must 'sacrifice' amperage for voltage. Is that correct?  Is there any way to know how much amperage you have 'lost' when one is in operation?


Yes.  The formula is very simple.  Remember that the amperage coming from the pedestal is fixed at the handle rating of the breaker (instantaneous, or about 80% of that handle rating continuous).

If you multiply the handle rating of the breaker by the voltage at the pedestal (under load), that will be the total watts available.  Divide that by the voltage output from the buck/boost transformer, and that will be the maximum amps available at the output.

For example, let's say you have a 30-amp park receptacle, and you measure the voltage under load at 90 volts.  30*90=2,700 watts available.  If your transformer is producing 120 volts, then you have 2,700/120=22.5 amps total available.

If you had a 50-amp park receptacle, and you measured the hot-to-hot voltage at 200 volts, and your transformer was producing 240 volts, you'd have (50*200)/240=41.67 amps available.

Remember that the transformer itself dissipates some of the power as heat.  I would figure around 5% loss for the typical autotransformer, so knock 5% off those values.

Quote
Also, transformers generally create heat. Is it safe to hardwire one of these autoformers into the electrical bay? OR would it be better to use it 'stand alone' outside of the RV?


Depends on the make and model.  You should follow the manufacturer's recommendations for installation and ventilation.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #17 on: November 30, 2012, 02:42:57 PM »

My book's a '99 edition, but still,  I wonder if 551-20 is being interpreted correctly here?.

The topic of 551-20 deals with RV Combination ("mixed") Electrical Systems, that is, AC and DC systems sharing components. Whereas the topic of 551-40 addresses  RV ("straight") 120/240 AC systems.

While 551-20 (e) clearly prohibits autotransformers, autotransformers don't seem to be prohibited under 551-40, which addresses ("straight" non-AC/DC combination) 120- 0r 120/240 Volt Systems.  And of course if 551-20's intent was to protect from dangers of a fault condition with a non-isolated autransformer at an RV's supply source, the dangers would seem to be  no less with a "straight" (non-combination) AC system. Therefore if the intent of 551-20 were to prohibit autotransformes used to "tweak" shore power, it would seem that 551-40 would also prohibit autotransformers.

Also, since 551-72 clearly specifies that distribution to RV vehicles "shall be derived from 120/240-volt, 3-wire system" the danger of a fault causing some other voltage (208, 277, 480...) to feed to an RV would seem extremely remote.

With that, I wonder if the intent of 551-20 (e) may be to prohibit autotransformers only on the "combination" circuits of an RV, rather than autotransformers used to "tune" "shore supply" voltage by a few volts. Possibly the focus is autransformers on the supply side of AC to DC converters? Or could the subject be very small autotransformers, such as seen with lamp dimmers?

Again, I'm not certain, but to me the code doesn't read as cut-and-dried prohibiting all autotransformers.

Ted
« Last Edit: November 30, 2012, 03:01:56 PM by TedsBUSted » Logged

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