Hi, can anyone explain the fuse ratings for this model? The manual says a 60A fuse should be installed as close as possible to the input source positive connection. For some reason I’ve got a 40A midi fuse there which I think was as a result of a lot of research at time of installation. Relooking at this, it seems people use 40A, 50A, and 60A.
I’m thinking of changing to a 60A circuit breaker fuse on the input side to be able to isolate the unit and prevent it draining the van battery during unused periods, and then installing a 30A output side fuse to be able to turn off the supply to the van itself should that be required.
You should never run a fuse for long periods close to its rated capacity, it will get hot, rules of thumb are typically 75% to 80% of rated capacity, but of course the wiring should be rated the same or higher than the fuse. The Victron recommendations will take account of any other factors of importance.
Thanks for the info and to be clear, I’m a well researched DIY’er, but that doesn’t make me close to an expert so I’m treading cautiously. The reason I’m looking at this again is because my van starter battery died a few days ago, and I’m trying to establish if it’s my setup of the dc dc unit that has caused it. It’s disconnected for now!
I’ve followed the wiring precisely for distance and gauge etc. to ensure the wire doesn’t get hot and/or I get voltage drop.
I believe the fuse is different in that whilst both wire and fuse undersized will lead to undesired issues, oversizing the wire should not, but oversizing the fuse will if it doesn’t trip when it needs to and prevent an issue.
So with a 30A unit, the 75% rule would require a 40A fuse. A 60A fuse is 50% and the only reason I can see for this is that it is the exact same value as the short circuit rating of 60A.
So if the only two situations are running normally, and short circuiting at 60A, then the 60A fuse makes sense, but I would prefer not to allow 45A (75% of 60A) being drawn without issue from my starter battery.
Also the Victron minimum cabling and fuse recommendations, are in conflict with each other.
A 0.5 meter run recommends 6mm cable, which will be rated around 40A to 50A, which is well under the 60A fuse rating and so the cable will fail first.
So to my mind, the fuse should be rated less than the thinnest cable rating and above the known spikes rating for the unit.
I have 16mm because of my 3 meter run length, so a 60A fuse is fine for my cable, but seems overkill for the installation, and is definitely not erring on the side of caution - unless Victron know something we don’t about the spikes.
6mm2 = ~10AWG, which -assuming you’re using the specified high-strand DC wire- is good for 60A continuous as long as it’s unbundled and not installed inside an engine space or some other high-heat area, so the 60A fuse rating is appropriate if being installed correctly and within applicable electrical codes.
Editing to add: the input side of a 30A DC-DC will, obviously, pull more power than 30A due to efficiency losses. In order to deliver 30A output, the unit -depending on a plethora of other conditions- may pull anywhere from 36A up to (observed) 45A from the input side, which is why a 60A fuse would often be recommended on the input side (so long as the appropriate wire is used) to avoid nuisance-trips of the fuse or breaker.