AC Out wiring with Power Assist

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randy-s asked Apr 18, '23

AC Out wiring with Power Assist

As per my other post I am installing a 2nd MultiPlus 12/3000/120 with 50A transfer switch to add to my existing system (same). My shore power (and 13kW genset) is 240V 50A. I'm planning on installing the units in parallel split-phase mode. I have heavy demand all winter primarily due to heating, so input current limit is set to 50 amps. I am having trouble finding a definitive answer to my question. I understand Power Assist can provide an additional 25A of power for short periods of time. That says that for short periods of time I can supply 75 amps to my main breaker panel.

1) is this true?

2) exactly how long is "a short period of time"?

3) given the above, I need to run 6 ga from the MultiPluses to the panel with 75A breakers just at the output of the MultiPluses. This is no problem, but possibly conflicts with the stated need to keep the wiring as small as possible for balancing reasons. That suggests 8 ga wiring, however 8 ga is not really sufficient for 75 amps except for the very briefest periods of time, ergo question 2 above. So what gauge wiring should I be using?

Thanks....Randy

powerassist

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Johannes Boonstra (Victron Energy Staff) ♦♦ commented · Apr 21, 2023 at 11:14 AM

Hello,

Q1 yes that's true

Q2 Powerassist is effectifly inverter power so as low as your inverter gets power from the batteries and its not extreme warm, they both can do 3000VA each on top of each leg.

Q3 the output power is the inverter power plus the incoming power, so your cable needs to be able to handle that.

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Alexandra ♦ commented · Apr 18, 2023 at 06:11 AM

@Randy S

The gauge that takes the load is the size you use.

The inverters can power assist their inverting power to the incoming. But the factor used affects how the overloads work. Getting that right is the key.

For balancing you make the runs longer, it has a similar effect to going smaller. (resistance) This complicates the set up obviously if you have limited space.

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Answer 1 · 2023-04-18T06:32:28Z

randy-s answered · Apr 18, 2023 at 06:32 AM

Thanks for your reply. Unfortunately as I mentioned above about difficulty getting a definitive answer, that doesn’t exactly answer my questions. Is Power Assist indeed able to add 25A of current to my 50A of shore power that is passing through the MultiPlus? And if so, for how long? (eg until my batteries deplete to an unusable level?)

This might require a response from a knowledgeable Victron person and I wish they would clear this up. I have seen much written and advertised and documented using generalities, but nothing clear and specific. Thank-you.

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ejrossouw commented · Apr 18, 2023 at 07:19 AM

A quick search on the Victron site will have presented you with this and other useful resources. https://www.victronenergy.com/upload/documents/Technical-Information-Achieving-the-impossible-EN.pdf">https://www.victronenergy.com/upload/documents/Technical-Information-Achieving-the-impossible-EN.pdf

In essence, PowerAssist will kick in when the loads exceed the AC input current limit and stop when it drops below. The limitation will be the amount of storage capacity you have. Depending on the actual usage patterns, the system could end up in highly inefficient charging and discharging cycles, you could run out of battery being unable to service the additional power required. Just some of the many things you have to consider.

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Answer 2 · 2023-04-20T07:26:24Z

randy-s answered · Apr 20, 2023 at 07:26 AM

@Guy Stewart (Victron Community Manager) @mvader (Victron Energy)

Thanks for your reply @ejrossouw but I have read that document and many others. The answer to my questions are not in there. The document you linked to is quite old, it is using a 230V system (European I guess?) and discusses 16A and 32A current limits. I am questioning whether I can use the full 50A of passthru and still have Power Assist provide up to another 25A. And if so, for how long *exactly* because that will determine my Vout wiring size so I don't get caught with the oversize wiring problem with parallel split-phase systems.

I would add that this is not intended to be a long term power supplement. We live aboard our boat and for many months in the winter we pull pretty hard on the shore power. It is a very common occurrence (like once a week) where we are pushing the limit and the hot water heater kicks on, or my wife forgets and turns on the microwave and we blow the ship's main breaker, or shore power. This is very annoying going out into the rain or snow to reset breakers. Power assist would provide those few moments to discover the overuse and correct it. Or even just let it boost current for 3 minutes to boil the kettle or microwave the food.

Thanks for any help VIctron guys!

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ejrossouw commented · Apr 20, 2023 at 06:29 PM

Glad you have been through the Victron material ;) I will leave it at that, but remain perplexed as to why you think there is a specific time limit?

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Answer 3 · 2023-04-21T03:55:10Z

randy-s answered · Apr 21, 2023 at 03:55 AM

Thanks for your reply. I don't really think there is a time limit, I just don't know. And that is the point of my questions. It is not specified anywhere. I do see reference to "a short time" or "brief period" and that makes me uncomfortable. If the answer is that I can supply 25A of AC boost until my batteries run dead, that's great, I can plan around that. But all the unspecific comments and implied conclusions are not satisfactory. The words "probably" and "maybe" and "should be able to" have no place in a discussion where I'm engineering and planning and installing my electrical system. Thx.

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ejrossouw commented · Apr 21, 2023 at 07:20 AM

Depending on the actual context, "brief" for me refers to a short term requirement as opposed to a product limitation ;) Not seen "maybe" etc in this post or manuals. The installation manual is very specific on the AC-output and breakers on page 12 and the specifications very clear on inverter limitations at maximum power etc. Armed with this and understanding that PowerAssist in essence is just the inverter "kicking in" alongside with the passthru to serve the "overload", very much points back to my original answer. Key will be to ensure the peak requirements fall within the combined passthru and PowerAssist output with some room to spare or you will be tripping breakers on the AC output side or overload the inverter.

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