question

Battery cables/fuse for Victron LifePO4 Smart and Multiplus 12/3000

Just getting started here, so apologies in advance if the answer is obvious. The Multiplus manual suggests double 50mm cables and a 400A fuse on the main battery connection. If I calculate my maximum current draw from the battery due to both DC and A/C loads, it's way less than this. I'm assuming that the charging current to the 12V 200AH LifePo4 battery is less than or equal to the recommended max of 100A. So why such large cables and fuse? Could one not size the wire and fuse based on what's actually going to be used? In my case, I would think 150A would be more than enough.

But I'm probably missing something...

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Keep in mind also that the battery current isn't constant when inverting. The current drawn from the battery will roughly follow the AC current waveform. There are capacitors in the inverter that help smooth this but there will still be current variation at twice the line frequency (assuming single phase inverter system). I'm not sure how big this variation is but probably can't be ignored because Victron makes an issue of ripple battery voltage. Unfiltered, the peak to average would be about 50%. Average 12 volt current with a 3000 watt load with 95% inverter efficiency would be approximately 3000 / 12 / 95% = 260 amps and peak current could be 50% larger than that: 390 amps. The peak is certainly much less due to the inverter's internal capacitors. Keeping resistance in the battery path to a minimum is the goal to minimize battery voltage ripple as much as making sure the average current doesn't cause an excessive temperature increase in the cables.

So Victron's cable sizing addresses all these issues. If your system loads are less than the 3000 watts, then things (including wire sizing) could be scaled back. But in the grand scheme of things wire is relatively inexpensive and overdesign will insure the system works in the worst case conditions.

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Thank you for that link. It answers the how, but not the why, unless I missed something. If my expected draw is never more than 150A and the charging current is never more than 150A, what would cause more current than that to be present in the battery cable?

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Maths,

Your 3000w inverter running at 12 volts will draw 250a from the battery (12 x 250 = 3000) ignoring losses/inefficiencies.

Your inverter can also handle peak loads of 6000w. < you can do the maths.

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I did a install in my RV and tried what was recommended by the RV dealership. Basically 2/0 cable was recommended. I quickly found that I was getting overload issues. The assist function had trouble with generator and air conditioner.

I then went to the 4/0 cable and have had zero issues with inverter function. Spend the extra money and install the cable that is recommended. You don't want to have to go back like I did.

You will be so happy with having a system that you can at least add more load if you want to.

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