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Combining Generator L1 & L2 when using a MultiPlus II


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I discovered this issue after finishing my Victron MultiPlus II inverter/charger. A brief summary . . .

Since the MP II is 50A, you can run the entire load of a 50A RV through it. For example, unlike smaller inverters that can require a separate load center, the MP II can be directly connected to the output of a 50A panel. Since an RV’s transfer switch is upstream from the 50A panel, an MP II can be fed from either shore power or a generator.

But the generator input can be the rub.

The issue is a missing L2 from the genny. I noticed this on my Victron Touch 50 display when I was getting power from my generator - L2 would always be 0. I wasn’t getting full power from my generator as I discovered when I tried turning on both my ACs. Generator couldn’t do it.

Then I read this thread on DIY Solar Power Forum which recommended this solution (starts about 11 min into video) — From Changing Lanes.

Any MP II users out there experience this? What did you do?
 

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The MP II will only pass through power on L2 when it senses L2 is 180 degrees out of phase with L1.  In other words only when supplied with 240 volt power like you get from a 50 amp power pedestal (or a 240 volt capable generator).  When the power supplied on L2 isn’t 180 degrees out of phase from L1, the MP II will ignore any incoming power on L2 and combine L1 and L2 internally to supply both legs with the power coming in on L1 (like a 50 to 30 amp dog bone adapter).  A lot of on board Onan RV generators supply power on both L1 and L2, but they do it as independant legs that are not 180 degrees out of phase and do not produce 240 volt when combined.  

The work around to get all the generator power to pass through the MP II is one of two things.  The simplest one is to combine the L1 and L2 generator lines in the transfer switch (move L2 to the lug where L1 is connected).  This will push all the generators output onto L1 and the MP II will then spread that output over both L1 and L2.  This can be done as long as the generators total output isn’t greater than the 50 amp (single phase) pass through capability of the inverter.  The other option is to add an auto transformer to the generator output lines before the transfer switch.  The auto transformer will make the L2 output go to 180 degrees out of phase with the L1 output and in essence turn the generator into a 120/240 volt generator.  This will allow the two legs of the generator to pass through the MP II.

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On 7/7/2022 at 12:49 AM, Chad Heiser said:

The work around to get all the generator power to pass through the MP II is one of two things.  The simplest one is to combine the L1 and L2 generator lines in the transfer switch (move L2 to the lug where L1 is connected).  This will push all the generators output onto L1 and the MP II will then spread that output over both L1 and L2.  This can be done as long as the generators total output isn’t greater than the 50 amp (single phase) pass through capability of the inverter.

Probably do the L1+L2 combine. However, in the mean time . . .

When using the gen in its current config, do I set my AC Input Limit to 25A and just run a single AC?

SoftStart units are in definitely in my future.

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1 hour ago, Zulu said:

Probably do the L1+L2 combine. However, in the mean time . . .

When using the gen in its current config, do I set my AC Input Limit to 25A and just run a single AC?

SoftStart units are in definitely in my future.

25 A should work for the AC input current limit.  You are currently getting one leg of your generator output.  25 amp might be a bit high if you are pulling heavy loads.  I think the generator only puts out about 22 or 23 amps on each leg, even though there are 30 amp breakers.  With no load on the second leg, it should be able to put out a little more on L1 though.

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