Circuit Breaker Sizing Multiplus II 48 / 5000

[PeterWA]

I am in the process of putting together a battery system 48V 314Ah LFP with a Victron Multiplus II 48/5000.

Victron, in their manual, suggest a 200A DC fuse. Following local regulations I need to provide a means to disconnect under load, so instead of a fuse I plan to use a DC rated circuit breaker.

Looking what's currently in stock at a reasonable price, 125A would be a lot better value. 160A and 200A appear to be mostly out of stock. I might have to go to 250A, which then would be way too close to the cable current rating for my liking...

Now let's do the maths. The Multiplus 48/5000 has a max continuous output of 5000VA. Assuming ~90% efficiency it would draw only about 107A (@52V) from the battery. That is well below what a 125A breaker can handle.

I realise that the Multiplus can deliver peak power of 9000W (translating into 190A DC @52V), but from what I read it would only be able to sustain that for a couple of seconds. Even 7000W (translating into 150A DC @52V) it can only sustain for a couple of minutes.

Looking at how circuit breakers work, tripping very quickly with high overload currents (i.e. 10 times rated), and very slowly with lower over-currents it should never trip during any of those normal operating conditions. Even on a hot day: the Multiplus should de-rate quicker than the breaker.

What's everyone's opinion? 125A worth a try?

 

[sunshine_eggo]

When using LFP batteries, you need to use an AIC rating of at least 20,000A, so any breaker you choose needs to be equivalent to a Class-T fuse or better.

Many jurisdiction require equipment to be installed in accordance with manufacturer's instructions. If you're working to local regulations, you may not have a choice here.

Using breaker as on/off switches may wear them out prematurely.

Fuses and breakers protect wires. They should be sized to do that. In the U.S. you size them for 1.25X the wire to prevent nuisance triggering of overcurrent protection as running any resistance based component above 80% of its rating ensures it will wear out.

There are charts for the MP. At peak output, the efficiency is closer to 85%, so you're talking about 115A.

Do I think this will work? Yep. Do I think it's that simple? Probably not.

 

[PeterWA]

When using LFP batteries, you need to use an AIC rating of at least 20,000A, so any breaker you choose needs to be equivalent to a Class-T fuse or better.

The breaker I would like to use is specified with 15 kA for both Icu and Ics. It is advertised for battery systems (and most of them these days are lithium). An alternative (with slightly better availabiltiy in 200A, but at twice the price) specifies 25 kA.

Where did you get the 20 KA requirement from? In a totally different context someone took the 3.2V and 17mOhm internal resistance of a LFP cell to get to that value as a possible fault current. What they neglected though is that lots more resistance keeps adding up in the system. Starting with the busbars that connect the cells, to the BMS, to the wire that connects the battery to the circuit breaker (even if that is kept as short as possible). I very much doubt that the breaker would see any more than 5 kA fault current from any fault that happens further downstream (i.e. in the Multiplus).

Of course it would be possible to add a class T-fuse in addition. High enough A rating that it does not blow unless something major has gone wrong. I don't like changing fuses...

Using breaker as on/off switches may wear them out prematurely.

I have no intention of using the breaker under load. Although it is rated for 2000 cycles (electrical) and 10,000 cycles (mechanical). Seem the manufacturer assumes 1 in 5 operations happens under load.

There are charts for the MP. At peak output, the efficiency is closer to 85%, so you're talking about 115A.

I had previously found the attached table in the Victron Community, can't remember if it came from a staff member of someone who measured it. 89% at 5kW. Drops a lot when it goes into overload though.

The question: how often will 5 kVA be drawn? Rarely. My house is very energy efficient. Typical energy consumption, if I don't include the EVs that are charged from surplus solar power, is less than 10 kWh/day.

The main risk comes from the kitchen. If the girlfriend turns on the coffee machine (1100W), kettle (2000W), toaster (1400W) and microwave (1100W) all at the same time, we go into overload. For about 1 minute, until the kettle boils, toaster pops or microwave is done and power drops right down below 5kW again, on the way back to the 400W average.

 

[stromy]

there is no risk in using a lower rated fuse, e.g. if you ever only need 150A you can use a 150A fuse instead of the suggested 200A.
However you cannot use a lower rating for the open circuit voltage or the short circuit current. That has to fit to your battery. And you never ever can use an AC fuse for DC!

 

[robbob2112]

Lower rated fuses have more resistance and will run hotter, nothing wrong except the time vs current curve gets adjusted and it will blow sooner than if running cool.

I would add the class T of the same size or slightly larger than the breaker. Time vs current curve means it will run for around 10 minutes at double the rating before blowing. Tbis may be long enough to melt the wire sheath depending on the rating. The breaker should trip sooner.

Fuses protect wires, a dropped wrench or slip with a screwdriver can result in thousands of amps for a split second before the BMS cuts off or is damaged and acts as a short allowing all possible current to flow. The MOSFET in the BMS fail by shorting before they are destroyed and typically blow apart open.

With a class T at 6x rated current it blows in 0.01 to 0.1 seconds...so think of it as a last line of defense.

 

[Siamac]

Would this work?

https://www.amazon.com/dp/B0CLQPC886/ref=sspa_mw_detail_1?ie=UTF8&psc=1&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9kZXRhaWwp13NParams