Question on Autotransformers in RV use for electrical guru's

[wagwar]

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?

[Sean]

...
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...

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.

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

[TedsBUSted]

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