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billknny avatar image
billknny asked

High Lines Voltage with Isolation transformer and Multiplus

System Description on sailboat:

  • Isolation Transformer 3600VA
  • Multiplus 24/300/70
  • Shore power 120V (nominal)
  • System Power 220V (nominal)

The only load on the transformer is the Multiplus.

We currently are in a location where the lines voltage is running right at the very top of the USA grid specification (127V). Coming out of the transformer we have 272V. This is over the maximum high voltage cutoff for the Multiplus (270V), so it stays in "Inverting" mode.

My understanding is that applying a load on the transformer output will drop the voltage to something like (127*2*1.05)= 267V which is still high, but would allow the Multiplus to make its line connection and charge our batteries and pass through AC power. Unfortunately, without extensive rewiring there is no way to apply a non-voltage sensitive load to the transformer.

Is there any other way to make this connection work? I have reviewed previous posts on similar issues, and it seems the answer is "no," but maybe something specific to my situation is possible? It frustrating to have to run the generator while shore power is available, but just 1 or 2 Volts too high (but still within local grid power specification!)

I appreciate that the usual issue with marina shore power is LOW voltage and this might be the reason for the 5% voltage boost out of the isolation transformer, but this is not the first time we have had this issue and the high voltage problem has been much more often the issue for us.


voltageIsolation Tranformer
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billknny avatar image billknny commented ·
I'm guessing the loud silence on this topic is an answer...


I am guessing that maybe I can rewire the domestic water heater to draw power directly from the transformer instead of from the secondary output of the Multiplus. Then I can HOPE that the 750W power draw of the water heater is enough to drop the voltage below 269V where the Multiplus will start working so at least I have battery charging and pass-through power when the water heater is energized...


Kind of a miserable half baked "solution" to a problem that shouldn't exist...

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Mike Dorsett avatar image
Mike Dorsett answered ·

This is where you need the voltage tapings on the isolation transformer for +10 and +20% so you can tolerate the high line voltage. These usually have -5% to +15 or 20% - maybe you can find a ups transformer to do the job..

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pwfarnell answered ·

There are quite a few threads on here about this. These say that you can swap the primary and secondary coils around. They also state that this is a service centre change. Nobody has posted any details of how this is done to allow diy swapping of the coils or any results of having done it.

In my marina, nominally 230V the supply is around 253V so with the step up at low load I have 265V which just sneaks in so I have not changed mine around.

The downside to changing around is if you also hook up in other locations when you travel. The transformer is +5% voltage at low load, but when it is at high load the voltage drops down by 6% or so. If you ended up somewhere with a low voltage and you had reversed the coils you could be down to <200V under high load as you have 5% step down plus another 6% droop. This is another reason why I have not changed mine. However, if you only hook up in the one place then this would be a valid change (if you could confirm how to do it or just tried it out).

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logins-portal answered ·

To cope with the high line voltage, you'll need the +10 and +20% tapings on the isolation transformer. These typically range from -5 to +15 or 20%; perhaps an ups transformer would suffice.


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