Wire size for victron 12v multiplus and 600ah lithium?

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woneill asked May 9, '21

Wire size for victron 12v multiplus and 600ah lithium?

I am installing a Victron 12v 3000 multiplus, 6 100 ah batteries and sterling 120a b2b charger. The batteries, inverter and b2b are as close to each other as possible while leaving required ventilation. The b2b and source are about 15 ft a part.

Getting different recommendations on wire size. 1/0 for b2b and single fused 2/0 between batteries and to inverter. this is the Recommendation from the Battleborn rep but he stated his reason for only using single 2/0 is a concern with using 4/0 because of inrush.

victron looks like it calls for 2 x 2/0 or 4/0 - mutiply web sites recommend 4/0 for batteries and inverter and 2/0 for b2b to source.

I am very new to this can anyone help clarify?

Second question - everything I am running is either 12v dc or 110 ac what are the advantages/disadvantages of switching to to 24v?

I understand there is loss when using an inverter to convert 12v to 110 - wouldnt there also be a loss using the inverter to covert from 24v back down to 12v? Or would going from 24v to 110 have less lose then 12 to 110 maybe making up for the lose from 24 to 12?

the advantage to increasing voltage appears to be a lowering amperage from 600 to 300 (reducing Resistance?) and requiring smaller gauge wire?

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Answer 1 · 2021-05-09T19:36:25Z

24 volt systems have 1/2 the DC current a 12 volt system. Most experts recommend 24 or 48 volt DC systems for high power situations. You sit on the fence with a single 3000 VA inverter.

Yes there are DC-DC (b2b) losses. Converters are between 90 and 96% efficient.

With 6 batteries in parallel or 3 strings of 2 batteries in series, a star connection is recommended. Each positive lead is fused close to the battery with equal length wiring to the star center. All DC loads then connect to the star via a busbar. Same length wires from the negative battery connections to the star center, but no fuse. The negative star center connects to the battery side of the battery monitor's shunt, then all loads and charging sources connect to the system negative of the shunt.

Inverter inrush is generally eliminated with a precharge circuit.

There is a huge current spike when the inverter is connected to the battery which can trip the BMS and also damage the disconnect switch or the connection studs/terminals. Inverter inrush is generally eliminated with a precharge circuit. The precharge is quite simply a resistor between the battery and the inverter positive terminal that gets connected first. The main DC disconnect switch is then closed a few seconds later. My precharge is manual, using a separate switch, but there are automatic ones, including one Battle Born sells. REC sells an automatic precharge unit but it requires an external contactor.

Victron's wire sizing to their inverters is not only ampacity, but also voltage drop and "ripple". The inverter's DC current is not constant, and follows the AC waveform (at twice the frequency). Any resistance in the DC path can create ripple.

You should size your wiring to meet or exceed the Victron specs. Total current divides between parallel paths, so the cross sectional area can be split up between paths. This current division is the reason all parallel paths must be the same length (same resistance -- same current).

Victron recommends 2 wires for their Multiplus DC connections to make it easier to route the wires into the unit.

For less than a 5 meters you need a total cross sectional area of 100 mm2. So 2 #1 or larger from the busbar to the inverter. Since your batteries are in parallel, you can use wires of 100/6 = 17 mm2 or #4 or larger from each battery to the star.

Lots of good information in "Energy Unlimited":

https://www.victronenergy.com/upload/documents/Book-Energy-Unlimited-EN.pdf

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