shaner07 avatar image
shaner07 asked

Multiplus ii 2x120 with onan 5500

Just ordered my multiplus ii 2x120 (currently being shipped to me) and now I found out it will be an issue with my onan 5500 in my toy hauler 5th wheel. I’m on shore power about 10% of the time so this will obviously be an issue when boondocking. I read there is a work around but haven’t seen any concrete information. I asked the question to my Victron dealer but haven’t heard a response. Can someone please advise?

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4 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

It depends on the output configuration of your generator. If it is 120 volts, then the Multi will see it as such and switch the output to two legs of 120 volts with 0 volts between them. Generally OK in an RV unless there are 240 volt loads.

This is also what happens when there is no shore power or generator.

If the generator output is 120/240 split phase (180 degrees between legs), this would work also with the Multi. L2 will pass through the Multi. L1 will benefit from the inverter assist.

If your generator has two 120 volt outputs that are in phase, then you would only be able to connect one of these outputs to the Multi. I seem to recall from another post that there might be a modification to the Onan 5500 generator to change it’s output to something more compatible.

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shaner07 avatar image shaner07 commented ·
Yes, It is a single phase configuration and I understand the functionality of the MP when it sees the single phase coming in. Hoping someone can tell me if it’s possible to work around this and how to achieve it.

Have read through other posts regarding it, but no resolution.

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klim8skeptic avatar image klim8skeptic ♦ shaner07 commented ·
I have not actually seen your question.

What do you need the multi ll 2 x 120 to achieve when connected to your genset?

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shaner07 avatar image shaner07 klim8skeptic ♦ commented ·
Ive seen reports that when the generator is running that the multiplus doesn’t like the fact that both L1 and L2 are in phase and it drops L2.

Is there a setting in the mp2 that allows both legs of power to pass through when running on generator power that has 2 lines of 120 in phase?

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exmoso avatar image exmoso shaner07 commented ·

Could you put a multimeter on your generator output while its running and see if there is a voltage between line 1 and line 2? The red and black wire.

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ripnc avatar image ripnc shaner07 commented ·
I have the 5500 too. The 2 30A circuits are 120V with matching sine waves so they should produce 0 volts when tested L1 to L2. If so you can parallel them by putting both generator lines on L1 input of your transfer switch. The MP2 drops L2 when the waves match because it assumes you are using a 30/50A dogbone adapter (which bridges L1/L2 to send the single 30A circuit to both). By combining on L1 of the transfer switch the inverter can use the entire 46A output from the Onan. You may need to then jump L1/L2 on the generator input side of your transfer switch so that it makes the contact.

I'm waiting to get back to my home RV site to do mine, but check out Changing Lanes' youtube on this. We had communication about this issue via email but Chad beat me to getting it done.
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ripnc avatar image ripnc ripnc commented ·
PS - the generator cicuits on my RV share a neautral that is heavier than the hots to carry the load of both circuits - the neautral stays as wired.
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rvsolarconnections avatar image
rvsolarconnections answered ·

The MultiPlus ll 2x120 can do standard split-phase 120/240VAC, with 180 deg phase angle between L1 and L2 input. This is commonly 240V exactly.

This can range to 180-280VAC between L1 and L2. This is approx. a 120 deg phase angle of 120V lines , which is two legs of a 3ph supply, but accommodates for individual line voltages in the range of 90-140VAC.

If the MP 2x120 senses only 90-140VAC on input, it will only allow L1 to connect (The min/max input is adjustable to a finer range with software), as this is considered a 1ph supply voltage range.

The blocking of L2 when a 1ph supply voltage range is detected is intentional, as the MP 2x120 measures the phase shift in order to calculate the current passing through the neutral.

When the phases are in sync, this can no longer be measured, which could potentially be dangerous, as more current could pass through the unit than it was designed to handle.

Theoretically, allowing in phase lines to pass through the MP 2x120 could be possible through a software update, but only if there can be a guarantee that the unit would not see current above what the relays and internal wiring are designed to handle.

Victron does not seem to have a solution at this time, but they are aware of the issue here and brainstorming ways to resolve this.


Caza G.

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ripnc avatar image
ripnc answered ·

For my Onan 5500 I just took the L1 and L2 hots and combined (parallel) them into L1 on my ATS. My ATS will work with only L1 hot. You'll note that L1/L2 from the genset are really just 2 - 30A circuits in phase and share the neautral. Neutral should alread have been sized to carry both circuits which will max out at 46A on the 5500 (not 30A + 30A of the breakers). But with the 25A boost from MP2 you'll achieve +/- 70A shared by L1/L2. The finished product is actually producing more available amperage than the genset alone.

This works great for RV panels that use no 220v distribution breakers.

PS - if your ATS doesn't work w/o power to L2, just add a jumper from L1 to L2 (I'd recommend removing the jumper on the L2 output side of the contactor that jumps L2 to L2 on the output side of shore and genset contractors).

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philm avatar image philm commented ·
Do you have Power Assist activated?
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ripnc avatar image
ripnc answered ·


My answer above is based on running shore power and genset to ATS and then the ATS output to the MP2 2x120. That is my configuration (alomg with the changes in my other post.

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