Lots of inaccurate and incorrect statements in this thread... Instead of listening to forum members who have no experience in the electrical industry it would be better to go to the Mike Holt electrical forum and ask there.
1. The NEC (the applicable electrical code in the US) does not prohibit the use of portable autoformers like the Hughes or Frank's units. First, NEC article 551.20 covers "Combination Electrical Systems", ie., what we often call a converter/panel. An RV manufacturer cannot provide an autoformer as part of the OEM electrical system. Article 551.20 can be found
here. Secondly, the NEC has no jurisdiction over portable electrical devices and equipment that plug into a permanent electrical system (building, RV park, marina, etc.). Saying the NEC prohibits autoformers is like saying they have jurisdiction over an extension cord, toaster or hair dryer - these things come under the jurisdiction of standards like CSA, UL and others. Of interest is that Hughes sells a kit to install their autoformer inside an RV.
2. Autoformers do NOT steal power from a CG - period. However, if enough CG users had autoformers plugged in at the same time, it is possible to cause a feeder breaker or CG main breaker to trip. I have never heard of this happening, but technically it is possible. If a lot of users were using autoformers, the current draw may be higher for each user than it would otherwise be and overload a sub-feeder serving a run of pedestals. Autoformers can in theory affect other users due to increased current to a pedestal but unless a lot of users had autoformers on a run of pedestals, the effect would be negligible.
3. When it comes to any transformer or autotransformer/autoformer, power out = power in, less losses - in accordance with Ohms law. A transformer/autotransformer normally has specs. on the internal no load and full load losses. For a 30 amp pedestal, the max. an RV can draw is limited by the 30 amp breaker in a pedestal. Say the CG voltage dropped to 95 volts (boosted on output by 10 percent to 104.5V), the most you can draw on the output of an autoformer (neglecting losses) would be 2,850 watts and the current would be reduced accordingly.
4. I'm not sure where the 4 percent "overhead" figure came from as posted by one person. Hughes or other autoformer manufacturers do not provide "overhead" figures, they have "no load loss" and "full load loss" figures. "No load" losses are independent of the load or time of day and "full load" losses take into account the losses in the windings and core when running at max. rating. Autoformers being single winding have lower losses compared to a two-winding transformer partly due to less copper in the windings. The losses also vary depending on what step an autoformer is operating at. I'm not sure without doing a bunch of searching online what an equivalent rated autotransformer would have for losses (ie., not an RV unit). The typical user could care less what losses an autoformer has and what it will limit a pedestal to, they will just keep plugging things in until a breaker trips.
5. The current draw from AC motors does NOT always increase as the nameplate voltage goes down. This is too complex to get into here and has little bearing on RV AC units, autoformers and voltage drop.
6. With AC units in RVs, the current draw always goes UP as voltage goes down. This is one reason why voltage at a CG pedestal is very important for RV-ers. Further, low voltage in an AC unit
will, not "maybe" or "never" damage an AC unit. If you look at specs from AC manufacturers, the highest voltage that the can run on before damage occurs is 104 volts. I seem to recall one set of specs stating 105 volts was the threshold. Since low voltage causes increased current in an AC unit, the windings in the motors are subjected to abnormally higher temps. which will ultimately cause an AC unit to fail. It does not happen immediately, but is cumulative over time, unless maybe you were to run on very low voltage.
7. When voltage goes down, the output of any resistive item (coffee maker, toaster, hair dryer, electric heater, etc.) goes down as the square of the voltage. At say 104 volts (the voltage where a PI EMS will drop out) the output of a 1,000 watt heater will be reduced to 104/120 x 104/120 x 1,000 = 750 watts. At 100 volts it would be 690 watts. Getting a toaster or coffee maker to work properly can be difficult as a result or take longer. A fridge running on electric may have cooling problem because the electric element is putting out less heat.
8. Low voltage in a CG is NOT caused by an owner intentionally undersizing or cutting cost of any of the wiring or equipment. It is because the NEC has not adequately provided for all the 50 amp RVs and generally higher use of power by 30 & 50 amp users. The NEC only requires 20 percent of a CG to have 50 amp pedestals. Up until 2005 it was only 5 percent and far enough back in time, it did not require any. There are simply waaay more 50 amp RVs around today than in the past and there just aren't enough 50 amp pedestals. Look around a CG and you can find lots of 50 amp RVs using 50-30 adapters. In some cases, some CGs/RV parks have installed electrical systems above the min. code requirements such as upscale RV parks and some gov't CGs. We have yet to stay in a CG built after 2005 and I would say most CGs out there pre-date the NEC 2005 code edition.
9. In some cases, bad electrical maintenance in a CG can be a factor in low voltage due to poor connections and tired pedestal receptacles. Most of the time, you can blame the NEC and/or power utility company.
10. Utility voltage CAN be low in some cases due to long distribution lines and heavy draw at peak usage such as in hot weather and/or certain times of the day. Power utility companies in the US and Canada comply with ANSI standard C84.1 within a specified voltage tolerance boundary. PG&E has a good explanation
here. They are allowed to operate within +/- 5% of nominal (120 volts). On top of that, the NEC allows a specific amount of voltage drop in branch circuits and feeders. Together, the end user's voltage can be as low as 13 percent below nominal (104.4 volts). Therefore, a CG that has voltage at 105 volts is in compliance with ANSI C84.1 and the NEC and there is NO WAY it is the CG's fault. But when you consider higher demand of RVs than the NEC provides for and the inadequate number of 50 amp pedestals, the voltage can easily dip below 104.4 volts and sometimes by quite a bit.
11. When it comes to the quantity of 30 & 50 amp pedestals and demand factors on pedestals and overalls system, it is covered by NEC article 551.71
here. For those willing to read through this section, it will explain why CGs often have voltage issues. Note that the NEC is continuously evolving over the years and decades and they do make some improvements to CG systems, but they are still too far behind on what's happening nowadays.
12. On the "41 percent" demand comment, this is not entirely accurate - 41 percent is for a total of 36 or more sites in a CG. You cannot get 36 pedestals on a sub-feeder run serving one section in a CG. For "modern" loop-fed pedestals, the most you can get on a 200 amp sub-feeder is 9 x 50 amp pedestals or 13 x 30 amps. For 9 pedestals, the min. required demand factor is 55 percent and for 13 is 48 percent. (Refer to table 551.73 in the NEC.) Also to note that a single 30 amp pedestal has a demand rating of 30 amps while a 50 amp is 40 amps (120/240). The problem here is that along come a bunch of 50 amp RVs plugging adapters into 30 amps and run the 30 amps to the max., which they would otherwise draw with 30 amp RVs. And on an overall CG basis, the total demand can be higher than the 41 percent max. demand required by code.
13. To say that a CG is going to be concerned about the extra losses caused by an autoformer and the consequent additional power utility costs does not make sense. That would be like saying they don't want you to use a hair dryer, electric heater, coffee maker or other electrical items. When you pull up to a 30 amp pedestal, users would expect to have full use of what's available from a 30 amp pedestal. I would guess that CG owners are either misinformed or more likely, they concerned about too many autoformers being used that could cause one or more feeder breakers to trip. Up and down the west coast, I have rarely seen others using autoformers. In some parts of the US with high summer temps. and thus more AC use, I can see more of a concern for autoformers.
14. The fact is, low voltage in a CG is common. If you travel around to enough to various CGs, you will run into it sooner or later. It can often get really low in some cases and close to the 105 volt damage threshold of AC unit even before you've turned anything on. We've been to enough CGs now with low voltage that we decided to get a Hughes autoformer for this season and bought a lightly used one for just over $200. In the beginning, I was dead set against autoformers but now won't travel without one. Have only had to use it once so far this season but expect to use it more on the 3,000 mile road trip we've just started.
If you want to reduce your chances of running into low voltage, get a 50 amp site in a section of 50 amp pedestals and if you're a 30 amp RV use an adapter. Also, look for newer CGs as they will have somewhat better wiring and more 50 amp pedestals. Having an autoformer is an excellent accessory to have. Would you rather let your AC unit get damaged, have your EMS cutting out or have appliances and devices have problems? The only thing I wish RV autoformers had is a CSA certification but that is another discussion for another day.
