Derating fuse and wiring

[SolaroSsaurius]

Hi all.
I need some insights!

I'm building a system around a 24v 5000kVA Multiplus II - but this is applicable to any situation, I guess.

Instructions state to use a 400A fuse and 2x50mm2 cables (under 5 meters) to battery (LiFePo4 24v 280Ah , EVE LF280k based, with a JK 200A BMS)

Spec sheet:
Cont. output power at 25°C: 4000 W / 5000 VA
Peak power: 9000 W
Max. battery charge current: 120A
Maximum AC input current: 50A

If everything goes according to plan, I'll never use more than 140A (including inverter's 96% efficiency for AC loads + DC loads)

My understanding (which could very well be wrong) is that this 400A fuse and this brutal cabling is aligned with the peak power:
9000w / 24v / 96% efficiency = 390A, next aprox fuse would be 400A

May I go down to a 175A or 200A fuse and wire it accordingly?
200A at 24v, using 2% voltage drop for 2m (1m length and return) would be 2x 25mm2

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Thanks in advance for your opinions!

Regards

 

[SolaroSsaurius]

@FilterGuy , I understand that you've been doing a path from breakers to fuses... do you have any advice on this particular situation?
Reducing wire size and fuse due to real needs, even if equipments handle a lot more...

Thanks in advance!

 

[yabert]

My understanding (which could very well be wrong) is that this 400A fuse and this brutal cabling is aligned with the peak power

Yes, 400A is overkill. In fact such an high amps fuse can cause more problem than help.
Better to fuse for continues power with a bit of buffer (4000w/24v = 167A). So a 200A fuse seem fine.
For the peak power, look at the rating of the fuse you plan to use, but most fuse can take 3-5x her nominal amps for 10 sec., so no problem with 375A for few sec.

 

[FilterGuy]

Yes, you can always size down, but if you ask the inverter for more power it will try to do it and blow the fuse.

 

[Q-Dog]

How many amps can the multiplus pull if the batteries are low? The MultiPlus can surge for some crazy amount of time. 30 seconds ... a couple of minutes?

So, If your batteries are low or begin to sag during a long surge the amp draw will go up. Redo your math with 20v at the battery instead of 24v nominal ... how many amps do you get?

If the multiplus ever tries to pull maximum capacity you will want to use the recommended fuse and cable size.

 

[740GLE]

At 20v wouldn’t the BMS clear on under voltage protect?

Is this a Victron document or a third party document? Either way does it state what type of fuse at 400a?

That might be a mega fuse commonly found in their lynx products which may heat differently than say an ANL or class T fuse. A 200a fuse even at 160a may get pretty warm.

Is your desire for using smaller conductor to save on money?

 

[Q-Dog]

At 20v wouldn’t the BMS clear on under voltage protect?

Is this a Victron document or a third party document? Either way does it state what type of fuse at 400a?

That might be a mega fuse commonly found in their lynx products which may heat differently than say an ANL or class T fuse. A 200a fuse even at 160a may get pretty warm.

Is your desire for using smaller conductor to save on money?

I guess it depends on your BMS settings. But, if your BMS cuts off at 20v, you should do the math for 20v, because the inverter will keep drawing until the battery cuts off.

 

[yabert]

I guess it depends on your BMS settings. But, if your BMS cuts off at 20v, you should do the math for 20v, because the inverter will keep drawing until the battery cuts off.

For a 8S LFP battery, there is very little energy left below 24V and almost nothing between 23V and 20V.
So, except if you plan to alway drain your battery at 100%, 24V as nominal seem fine.

Of course we can talk about voltage sag, but Solarossaurius plan is to pull 140A out of 280Ah cells, so no important sag here at 0.5C.

 

[740GLE]

Any LiFePo cells that have 20% voltage sag for loads under 1c aren’t worth messing with.

If you are planning on using 90-100% of the capacity some more stage is in order ?