I’m trying to better understand how people are successfully using autotransformers in RV/mobile applications, specifically when dealing with both 120/240 split-phase and 120/208 wye shore power.
In a typical RV setup, we’re expecting 120/240 split-phase two 120V legs that are 180° out of phase, giving us 240V line-to-line and 120V line-to-neutral. That all makes sense and is what most of our equipment is designed around. The confusion starts when plugging into 120/208 service (common at commercial parks or pedestals fed from a 3-phase wye system). In that case, you still have two “hot” legs, but they’re only 120° apart, so line-to-line is 208V. That part is understood. Most mass production RV have only 120v appliances unlike most higher end, custom coach builders which do have 208/240V appliances.
Where I’m getting hung up is this:
If you look at the math, in a 120/208 system, each leg is still 120V to neutral but the effective split-phase behavior isn’t the same as true 180° split-phase. Now, I’ve seen multiple installers recommend using a Victron autotransformer to “balance” or “support” split-phase loads in RVs, even in parks where 208V is present. But I don’t fully understand how that works in this scenario.
What I would expect loads are balanced via the autotransformer but on 120/208 the output from the center-tap to L would be ~104V.
What I’m not seeing explained:
How installers are avoiding this voltage drop to 104v on 120/208 campgrounds
How the autotransformer is actually being wired in these RV applications, specifically the natural.
How to transition with UPS mode enabled, will you need floating phase to allow 120/208 shore and 120/240 on inverter mode for the Multiplus?
I also understand that Victron historically has not supported autotransformer use in mobile/RV applications (likely due to code, bonding, and safety concerns), which adds another layer of confusion when I see it being recommended in the field.
When new or updated campgrounds in US are built, they often use two legs of a three-phase wye to evenly distribute the park’s loads. These parks typically run on 120/208 volts. However, mass-produced RVs usually have separate 120V L-N legs, resulting in a measured L-L voltage of either 208 or 240 volts.
Custom coaches are designed with 208/240 compatible appliances to accommodate this setup.
Worth noting that US NEC (and by extension, RVIA) expressly forbids the use of autotransformers in RV installations (has been the case for quite some time now), so I’m not sure who you’re seeing recommend these in the field, but probably best to talk to someone else.
I know NEC doesn’t allow autotransformers, but it’s hard to deny that Hughes, who sells them specifically for RVs, uses them.
Many installers are promoting them as a way to handle load balancing with split-phase inverters in North America.
I’m wondering if I’m missing something. It seems like it would be impossible for a Victron Autoformer to work with shore power from 120/208 or 120/240, especially with UPS functionality, and still keep the correct L-N voltage on 120/208 service. The only way I can think they can make it work is with a relay to switch the autoformer in and out depending on shore vs inverter.
No matter the code or whether this is a good idea, I just want to understand the technical details. I know three installers who are using them, and I’m still trying to figure out how they’re doing it with MPIIs in split-phase.
The Hughes isn’t an autotransformer - and indeed they’ve been fighting this for years, on that basis.
Any installer who’s promoting the use of a Victron Autotransformer for an RV installation in the US is not doing anyone any favors.
The correct, legal way of doing this is either a single MultiPlus-II 2x120, if inverted split-phase is not required, or qty 2 MultiPlus-II 1x120 units programmed in parallel-split phase mode, if inverted split-phase is required.
I get the legal side of things. I’m just curious about the technical details.
I’ve seen with three YouTubers and two trained Victron installers who have done this. For years, I’ve been saying that when 120/208 is passed through an autoformer and a new center-tap, the new 0v reference should be exactly halfway between the L-L voltage, which is 104v. If you use one in a modern campground without manually switching or isolating it to a single leg, you’re likely to cause a brownout.
Here is a example on this very forum:
Also, I don’t see how you could be using 120/208 shore power and have the inverters switch from 120 degrees to 180 degrees of phase separation. I’m not sure if the floating phase return option would allow that. I thought that was only for smaller deviations.
This is pure theory because I keep getting told that this is possible and being done and I feel like I am incorrect about my knowledge to have so many people claiming it works.
just because you watch it on youtube or found it on the internet does not make it right.
if a Victron dealer in the US is using autotransformer you should not listen to them or copy anything they do. (unfortunately Victron features this on their blog so be very careful when picking a Victron dealer there are some that have no idea what they are doing.)
two(or more) Victron inverters if setup correctly are capable of adapting to 120 or 180 phase separation, including only one leg for when shore power is only 120v.
seek a qualified individual (in the US a EC (electrical contractor) or EE (Electrical engineer) if you need help with this.
you can find more of my rants on this topic on this forum
Again I am not asking how to do it. I know how it works in theory.
What I want is a pure technical explanation that am I not missing something. I install the gear now, I have friends in the field that are installers. I agree that just because YouTube or someone on this forum did it does not make it right.
What I want is someone confirm that if you have a pair of MPII’s in split-phase and then wire a Victron Autotransformer on the output if you plug into a 120/208 shore power (2 legs of a 3-phase wye) the new neutral from the center-tap will be 0v reference between L-L which is 208V. Thus the L-N becomes ~104V and they will cause a brownout.
If I am wrong then explain to me why.
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
10
This is oversimplified and incorrect. AT’s make good sense in certain fixed installations and in specialty vehicles where the operator knows for certain that they will never, ever plug into shore power with 208V two-phase voltage. But for consumer vehicles I won’t argue your point, especially since Article 551 of the NEC explicitly forbids it.
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OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
11
I’m not up for teaching any EE theory this morning but I will simply say this: the AT will make a helluva racket if you do this. An AT has a single winding, not two, and the center tap is expecting to see 0° relative to one phase when fed with two phases 180° apart. Feed it something else and it likely won’t last long.
Precisely what I thought. I find it troubling that we have so many installers attempting to both sell and install autoformers for RV applications where they will encounter both 120/240 and 120/208 services. I had had some heated debates and I know this is dangerous but so many friends/installers/YouTubers have recently recommended them to the point where I have been questioning myself that I was missing something.
that just because YouTube or someone on this forum did it does not make it right.