question

michael-rustemeyer avatar image
michael-rustemeyer asked

Autotransformer with MultiPlus-II 2x 120v

I have a newly installed MultiPlus-II 2x120V in my 5th wheel. I usually have 50amp inputs, so it has been running well. But this last weekend I was at a campground where I had to dogbone to 30amp inputs into the 5th wheel. The MPII only draws from line 1 though, which limited me to 30 amps plus battery. We needed more AC than that could support. When split phase 30 amps are available, if they feed the autotransformer first and then the MPII, would that equalize the draws in on L1 and L2 so that I could pull 50amps total, 25 from each? If I then plug into single phase 50amp normal campground with the autotransformer still in line, would it work like it does now? What if I end up on a 30 amp campground and only have one 30a single phase in, can I keep it in line for that scenario? Basically I'm trying to understand if I can add this in to fix my one problem w/o breaking the stuff that works well now...

Multiplus-IIAutotransformer
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

1 Answer
Kevin Windrem avatar image
Kevin Windrem answered ·

If you have 30 amps split-phase available, this should work without changing things. But don't used the typical TT30 to NEMA 14-50 dogbone. You will need a NEMA 14-30 to 14-50 adapter. You will only get assist from the Multi on L1 since it's a single inverter core. An autotransformer on the output won't work because when inverting you only get a single phase output (both L1 and L2 are in phase).

If you have 30 amps 120 volt (one leg only), then you'll never get more than 30 amps / 120 volts into the RV even if you add an autotransformer.

You can not safely combine two separate 30 amp 120 volt feeds to get split phase power.

4 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

derrick thomas avatar image derrick thomas commented ·
I have seen many RV pedestals (especially in parks with older infrastructure) that have only 30 amp outlets. Sometimes you can get lucky and find two 30 amp outlets at the same pedestal that are 180 out of phase, in which case you CAN connect with the appropriate adapter and provide 30 amp split phase 240 to the RV. However, I imagine that most management would frown upon this behavior.
0 Likes 0 ·
michael-rustemeyer avatar image michael-rustemeyer commented ·
I was able to use the dogbone adapter before the MPII, what I gather from your last sentence is that there isn't a safe way to pull equally from two 30 amp receptacles to get my 50 usable amps by the MPII? I gather I need split phase power coming into the MPII and can't get that from the two 30 amp plugs? The NEMA 14-30 to 14-50, is that adapter just to use one of the 30 amp pedestals?
0 Likes 0 ·
Kevin Windrem avatar image Kevin Windrem michael-rustemeyer commented ·
The 14-30 is a 4-wire split-phase receptacle just like the 14-50 you currently have except that it is for 30 amp service. It uses an L-shaped neutral pin rather than a straight pin. I doubt you'd find one in an RV park because it is not a standard hook-up. ( Common hook-ups in the US are TT30 for 30 amp 120 volt and NEMA 14-50 for 50 amp 120/240 split-phase.)

The TT30 is single phase and attempting to connect two of them to a split-phase inlet to your RV isn't approved. There is no guarantee the two receptacles are on different phases and/or a common neutral, especially if you are pulling from different pedestals. If they are on the same phase then you won't double the power into the RV. Plus it would trip GFCIs in the pedestal if present.

Pedestals typically have a 20 amp receptacle as well as a TT30 and/or a NEMA 14-50. You may find that the TT30 and 20 amp receptacle are on different phases if a 14-50 is present but are likely on the same phase (0 volts between them) if there is no 14-50 receptacle. Even if you are lucky enough that the TT30 and 20-amp are on different phases, trying to combine them is unsafe and will trip the 20 amp GFCI.

RVs that are designed to use 50-amp split phase power will require manual power management when running on anything less. Typically this means only running one air conditioner and/or not running water heater, microwave or electric fire place at the same time.

You may get away with running two air conditioners on a 30-amp service if you install soft starts on both of them but it's not guaranteed.

0 Likes 0 ·
michael-rustemeyer avatar image michael-rustemeyer Kevin Windrem commented ·
Thanks for the time you put into this, it's extremely helpful as I try to wrap my head around everything.
0 Likes 0 ·