In the USA a typical large RV needs a 50A 120/240 split-phase connection, and the autotransformer can be used to load balance. Very cool.
But what happens if that same RV encounters a 30A single-phase pedestal and has to use a 30A-to-50A adapter? Those adapters internally connect that single 30A hot leg to both L1 and L2 for the RV’s 50A connection.
If L1 and L2 (now internally joined together by the adapter) are being fed into that same autotransformer, can the autotransformer handle it? I’m assuming it can’t load balance, but will this cause fireworks inside the autotransformer or elsewhere?
Assuming you have a 240 volt inverter and you are using a Victron autotransformer to supply 2X120 legs to the panel, you will need a second autotransformer to supply 240 volts to the inverter. The second autotransformer will convert the 30 amp 120 volt to 240 volts (at 15 amps). This is the setup we use in our motorhome and it works very well. You will need to adjust the input current on the inverter to 15 amps, I usually set it to 14. I like having a Digital Multi Control, it makes this very easy although you can do it with a Cerbo.
As far as using an autotransformer in an RV, it’s my understanding NEC defines an autotransformer as “a type of electrical transformer with a single winding acting as both the primary and secondary winding.” As the Victron autotransformer uses two windings, both a primary and a secondary winding, it is not an autotransformer as defined by NEC and therefore acceptable for use.
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
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P.S. It occurred to me that maybe you are referring to an isolation transformer that does indeed have separate primary and secondary windings?
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
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You are correct - though in the 2026 NEC it is 551.20(f). I believe the intent is to prevent open grounds or improperly bonded neutrals and grounds on the input to the RV. It’s also to prevent boosting voltage from sagging campground distribution networks, which causes higher currents on the campground circuits. Using an AT on the outputs of two 120V inverters to balance them, so long as the ground relay functions in both inverters + the AT are configured and installed correctly would not present an electrical hazard (repeat: if properly installed). However, in my opinion, the NEC leaves no ambiguity because of how common it is for RVs to not be properly wired AND campgrounds having lots of electrical problems. The use of a quality EMS device between the pedestal and shore power input will monitor for open grounds, open neutrals, neutral & phase reversals, low voltage, high voltage, etc. should address the concerns of hot-skin conditions.
However, since they leave no wiggle room, they are not allowed. I also don’t think AT’s are necessary unless the power needs inside the RV are high.
If you need more power than Victron’s 2x-120V Multi and Quattro (limited to just 3kVA), then parallel two of them for 6kVA or 4.8kW continuous and then you won’t need an AT. This should also solve any issues with generators that output 240V/120V and not just two outputs of 120V in phase with each other.
In my experience, tiny homes, mobile offices, etc. are generally covered under RVIA standards too (as my customers tell me) and we have used AT’s inside them along with Progressive Industries EMS’s hardwired between the short power input and inverters. Apparently that is a code violation but we’ve not failed any inspections so far from any RVIA consultants and inspectors that have been hired to qualify our designs. I will have to investigate this further!
Installing 2x 5kVA inverters with an AT is a common configuration and necessary because Victron doesn’t supply any split-phase inverters for the USA or anything above 3kVA in their 2x-120V line up.
2026 NEC adds the requirement to have GMI Ground Monitoring interrupter on all “RV” shore power over 20 amps 120V, this is a separate safety issue the NEC is resolving with this requirement not related to ATs (this does help with Victrons leakage to ground issue, as this item was specifically written to address this)
the ban on Autotransformers stays (for many good reasons)
2023 added an exemption for plug and cord AT that can be connected between park pedestal and RV, they can not be installed in the RV, The AT must be UL listed for that purpose, note none of victrons transformers are UL listed, and should not be used in US markets.
RVIA has a very limited standard written for low voltage otherwise everything else is covered under NEC 2023, my personal thoughts on RVIA is they want to insure fast and complete combustion so cause is hard to find to limit liability for manufactures. RVIA is for manufactures not for consumers so don’t put too much faith in them.(ever seen and RV burn, they go fast and complete)
just because you can do it doesn’t mean you should
just because you have always done this doesn’t mean it is safe
each item in the NEC is there for a reason and unfortunately some of time the reason is lives lost.
The manual mentions the seperate windings in a rather roundabout way, this blog explains it better: “The wire coil in the drawing appears as a long continuous coil, but in reality the part of the coil serving leg A is wound over the top serving leg B and the two parts of the windings interact with each other electromagnetically so they exactly mirror each other. That is how the center tap is always at zero volts even if leg A is loaded up to the max and leg B isn’t loaded at all.”
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
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And the RVIA has put off requiring GMI’s until 2027 (at the earliest) due to not having enough listed components on the market to mandate it. shrug
Yes, we know this. The NEC is written in the blood and tears of many EE’s and electricians. But, it’s also true that the NEC is written by committee and that standards can and do change or are clarified. Take, for example, 690.31(D)(1). In 2023 it required that ANY DC cables entering a building must be brought in with metal raceway. The enhanced content in the 2023 NEC Handbook gave a paragraph explanation saying why metal was a better idea, blah blah blah. In 2026 they dropped the requirement if a listed PVHCS has been implemented as is REQUIRED by the NEC in the first place (since 2017 I think, but don’t quote me on that). Why do I point this out? It’s just to simply say that if it’s written in the NEC, it’s not always gospel and there are sometimes errors. And it’s why AHJ’s are also given quite a bit of latitude to decide what makes sense in a specific installation.
But for a layman, I would suggest they do NOT use an AT for the reasons you mentioned.
I wasn’t arguing with you. I just wanted to add some color to the conversation. I even mentioned that using an AT is not necessary so long as the person is willing to parallel multiple 2x-120V inverters. Mostly, I was agreeing with you that even though we know what we’re doing and the installations are safe, I will have to reconsider this recommendation for our specialty vehicle manufacturers and let them make the final call. I also asked my Victron rep what his OEM vehicle manufacturers are doing in light of Victron not offering ANY split-phase inverters for North America.
