I have a fuse for this circuit, installed right at the battery (closest to the source is best), and it is sized at 1.25x the maximum current. So 50A x 1.25 yields 62.5A. I used a 60A fuse since they don't make a 62.5A as far as I am aware.
This piece of advice needs to be clarified. If you're fusing for a 50A circuit you use a 50A. Fuses are sized based on the current they will hold indefinitely. They will open at some current higher than their rating, which is going to be based on a time-current curve. So the higher above their holding current the faster it will open.
Let's use an example, the fairly common MRBF that Eaton/Bussman offer. If you used their 60A this is the time-current curve for it.
You'll see at 100% (60A) it approaches asymptotically to the point they don't even give a data point. In the data sheet they say it will hold 100% for
greater than 100 hours.
It will only start to reliably open at around 135% of rated value, which is their lowest data point. This will take no more than 900 seconds but could be less. They can't say for sure exactly so it's given as a maximum in the specs, although their chart indicates the range might be from 3 second to 2000 seconds.
Their next data point is 200% of rated value. A 60A MRBF will open at 120 amps in no more than 60 seconds. That's still pretty long, enough that you need to make sure your device or wire can handle it. Here again the chart indicates it could be as quick as 0.2 seconds. At 60 seconds you might need to think about insulation material depending on heating of the wire at 60 seconds. By that I mean you might want to use 125°C EPDM rather than 75°C PVC.
The next data point is 350%. Here they can say a bit more precisely that it will open between 0.1 min to 1 second max. That's 210 amps for a 60A MRBF. This is usually sufficient to prevent wire melting through insulation but is a lot of current from a device or load perspective.
The highest data point they give is 600%, where they state it will open in less than 0.2 seconds.
Now all that said, there are reasons you may have selected to derate, e.g. using 125% of the expected 50A circuit current. One is due to elevated ambient temperatures you find under the hood. Fuses are typically rated for an environment of 25°C but 40°C is pretty typically used for a baseline under hood or on the chassis. That's part of the underlying reason for the range of opening times, but Eaton doesn't give temperature derating guidance to my knowledge for MRBF but for other types 25% is IMO excessive for 40°C. Some require no adjustment at this temperature, some might be 5% or 10%.
When a fuse (or circuit breaker, which have similar trip characteristics) does give this it'll look like this. This being for an Eaton Series 22X ATC shaped circuit breaker. You have to potentially adjust over current protection in application that are exceptionally warmer or colder than what we know as normal ranges. It should be made clear that a fuse will operate over a very wide range, perhaps -40°C to +85°C, but what the designer needs to verify is what value to select to achieve the opening current desired. So a 60A in this example will open at +85°C but it might occur at 30 amps almost instanteously. Vice versa it will also open at -40°C but in that case it could be at 90A after an hour.
I don't mean to second guess your 125% value but you may need to explain why you chose this. Normally inrush may require it (but this is typically implicit in the fuse time-current, e.g. an exceptionally harsh 350% in-rush might last long enough to blow it) or complex current as you might find on abnormally inductive AC loads.
In some cases you may have to derate down in value, too (e.g. use a 40A in a 50A since a fuse will only open with real current, not complex). You may see in your example going down 25% to get the fuse opening to nearly immediate at 50A but using a 40A (or nearest) to achieve that.
I have a Victron 18A DC-DC and don't find that either of those cases are severe enough to justify a 25% derating but that's only a sample of one example. Not all DC-DC chargers, or indeed any 12V load or device, necessarily implements in-rush protection and could see large turn on current if they have a lot of capacitance or inductance that may require balancing ratings between sufficient protection while avoiding nuisance tripping (e.g. the fuse blows randomly when you power up).
The main thing I would note, though, is you shouldn't be using the DC-DC rating to size fuses. You would use the wire branch size. If you're using 8 AWG wire then you fuse for that regardless of the load you hang on it. So your first design choice is to pick a wire size you need based on load current. Then you pick the source-side fuse size for that wire. You can use a smaller value if you wish, but the fuse indicated for wire size is absolute maximum, of course subject to temperature adjustment if you find it necessary.
The device itself should have it's own fuses required for protecting it. You can put a 50A DC-DC on a 1/0 AWG feeder protected with a 120A fuses. That doesn't imply the DC-DC will see 120A. If the device does need external fuses on it's supply (or output potentially, too) then you should put those at the input of the device, not the source end of the branch. The manufacturer will probably suggest a specific brand, type and value.
