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rwcran asked

Multiplus 3000 maximum peak Inverter DC input amperage?

Assume a multiplus 3000 12/3000/120 with an inverter specification of 6000 watts AC peak power connected to a fully charged 400 amp hour 12 VDC lithium battery bank. The battery and multiplus connecting cables are 4/0 welding cable with 2062/30 stranding and the cable length is 3 feet. Given a 120 vac 6000 watt peak load event what would be the expected peak 12 vdc input current?

Victron recommends fusing the multiplus operated at 12 vdc with a 400 amp fuse. The multiplus has a published inverter efficiency specification of 93%. If the 6000 peak watts of AC output specification is true and given an inverter efficiency of 93% my calculations indicate a 400 amp fuse is under the momentary peak DC input current. Does anyone know in actual practice what is the highest possible peak 12 vdc DC input current drawn from the battery bank by the multiplus inverter?

When might such an event happen? Assume the AC input current limit of the multi 12/3000 is set to 20 amps and the multi is being supplied 120 vac shore power. The multi is supplying a 4400 watt AC load and configured for power assist and actively operating in power assist mode. An additional 1600 watts of AC load is then energized. Shore power drops off line when the additional 1600 watt load comes on line and the multi attempts to supply 100% of the 6000 watts AC load. Will the 400 amp fuse between the battery bank and the multi handle the momentary DC load until the multi software turns off the inverter?

MultiPlus Quattro Inverter Charger
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2 Answers
wkirby avatar image
wkirby answered ·

All of this really depends upon the type of fuse. Also consider the ambient temperature. Some fuses are designed to respond quicker than others.

A 400A Littelfuse Mega, a fuse type that is commonly used in inverter - battery circuits is rated to carry 200% (800A) for between 1 and 15 seconds.
You'll also need to de-rate with increasing ambient temperatures. Continuous current will be lower than the nameplate rating. This is usually fine unless you really thrash your equipment at full load full time.

For more information check out the Littefuse Mega datasheet, lots of pretty graphs and tables to help you decide: https://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/bolt-down-fuses/littelfuse_mega_datasheet.pdf

Also consider a read of the book Wiring unlimited, there is a section in there about fuses:
https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

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rwcran avatar image
rwcran answered ·

To clarify, my question was not about fuses. It was "what is the inverter's peak input DC current that can be expected with the multiplus 3000 given the 6000 watt peak AC specification?"

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·

P=U*I -> I=P/U -> I=6000W/12V -> I=500A

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rwcran avatar image rwcran Matthias Lange - DE ♦ commented ·

Thank you Matthias. I agree with your calculation. Given the efficency of the multi inverter is 93% the max input current is actually higher than 500A and will be about 500/.93 = 538 amps. The question is how long will this current last before the Multi software shuts down the inverter? I don't know the answer? It's clear if one uses the Victron suggested fuse size of 400 amps it must have the correct blow delay characteristics or the fuse will open on the peak current of 538 amps resulting in nusance outages. I cannot find anywhere Victron specifies a suggested blow delay timing characteristic and the installer must select the correct blow delay timing. Often installers use T class fuses or Littelfuse Megas for this application and both seem to be effective.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ rwcran commented ·

The peak power you can get only for a few seconds, I think less than 5s.

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