neilbridgeman avatar image

Critical Loads


Is the critical Loads AC Out (AC Out 2) terminal bi-directional?

If I want to keep my complete house AC distribution board powered during a power cut but also fully utilising the solar and battery power stored when grid is available do I just wire one circuit from my house DB to the Mulit I AC Out AC 2?

If so are there any restrictions to the Multiplus that I can expect from wiring the complete house DB through AC Out 2?

Multiplus-IIac out 2
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2 Answers
Alexandra avatar image
Alexandra answered ·


Some things for consideration are:

1) You would need to make sure that the full load of you house is not above what the transfer ability of the Victron is. The transfer ability is usually the full load of the inverting capability plus charging amps, don't forget that charging current is also a load.

2) What surges there would be through the system for high start current items like pumps.

3) Would there be an overload if the power goes out and the full load of the house is on?

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neilbridgeman avatar image
neilbridgeman answered ·

Thank you @Alexandra,

I hear you. I think the answer is to run two circuits? One to AC IN (protected by a 45A MCB) to handle normal operation and one to AC 2 (protected by a 20A MCB). The cable returning from AC 2 I can then wire through a changeover switch if I want to use the 20A AC 2 output and then isolate high demand circuits prior to turning the changeover switch over.

Powercuts will be an extremely rare occurrence but if I am going to run cables from the ground floor to the loft I may as well try and get the full use from the Mulitplus ii.

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@NeilBridgeman Am I understanding the idea of bi directional correctly? You want to feed back into the multi through the AC2 output? Or are you concerned it can happen? Yes it can happen with poor wiring (have seen it) but is not designed for that not good for the health of the inverter.

The answer to the question on wiring, usually if the Multi is able to handle the whole household, we take the feed from the mains switch to the AC in of the multi and split the DB over the two different AC out feeds based on what the customer determines as critical and non critical loads.

But if the Multi is too small in capacity to handle the entire household then we leave grid feed to the circuits that we know the multi cannot handle, splitting the DB board into three. That way mistakes cannot be made.

We have used a changeover switch on an inverter output so that when the householder wants to run higher loads, the loads can be powered from Grid directly, but it was not a Victron, hence the creative wiring. If there was no power they could choose their loads then switch it over to inverter, requires good household occupant communication though to prevent overloads.


yes you are interpreting my definition of bi-directional correctly and now that you have made it clear I can see it is not a sensible or practical idea.

My concern in taking a cable from the main switch to the Multi would be over current protection. I am pretty sure the terminals of AC In will not accept 16mm or 25mm cables so I would need to fuse the cables down to 50A and this may then be a 6mm cable (subject to cable calculations).

In this case I would then need 3 DBs? DB 1 will be power to AC In on the Multi and 2 more outgoing circuits to feed DB 2 (Essential) and DB 3 (Non-Essential) with both these DBs fed from AC Out 1 and 2.

Seeing as the largest single Multi is 5kw which is about 20 amps this is quite small and whether it could cope or not would be dependant on when the power cut occurs. A single Multi does peak at 9000kw (40A) which should be enough for me to assess loads in the rare time a power cut occurs.

With this in mind I think it is probably best to at first wire all circuits from the main AC Output and treat the whole house as 'essential'. If I see an issue then I can:

A) look to split circuits in my DB or;

B) install an additional Multi in parallel to increase the max Kw available

If I do run everything through the main AC Output do I still need a circuit to AC In or will AC Out be bi-directional? Assuming all loads are within limits of course?

Have you thought of using and Easy Solar instead of a multi? It has a mini DB built onto it.