How to determine necessary fuse ratings ?

Gerry77

Gerry77

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Hymer Exsis 504 2014
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I understand that fuse ratings are to protect the cable not the appliance etc but how to calculate what that should be.
I have now assembled the gear I need to up grade to lithium (I hope),
Roamer 100ah lithium battery,
Victron MPPT 30A solar controller ( already installed)
Victron IP 22 mains battery charger,
Victron Orion -Tr Smart DC-DC Charger 1212 18amp( + 2 number 60mm diam fans)
All cable Hiflex 16mm2, lugs, busbars etc to suit.
I plan to fit a fuse close to the leisure battery and engine battery and from dc-dc charger output but what sizes ?
Victron say 60 amp for both batteries, is that correct for the above equipment please ?
Also is it best to connect a D+ supply to the Victron B2B ?
Apologies for having to ask but cant afford to wreck anything !
 
I am not going to gives specifics because I have had a beer but generally speaking.

Check what the cable can carry based on cross sectional area and length. This defines your maximum fuse size.
Then look at what your device requires at startup/surge. Add 20% and this should be your bare minimum fuse size. If your bare minimum is bigger than your maximum then you got the wrong cable size.
But if it is good, choose any fuse between.

This is my personal method, others may have more to add or a different view.
 
Firstly, thanks for replying.
My concern is that Victron only spec a 60amp fuse for the dc-dc chargers for the Orion range. Mine is the smallest at 18amps and I am using 16mm2 throughout so 60 amp seems disproportionate. Max cable runs will be 2/2.5m.
Thanks again Gromett for your guidelines.
 
The 60A fuse is to protect the cable and nothing more. 60A seems fine to me off the top of my head.
Remember the fuse should be as close to the battery as is humanly possible and on the red cable (I have seen some people install on the black).

My personal way of speccing would be to double the demand so 18 x 2 = 36A so a 40A fuse. Please note If this exceeds the capabilities of the cable I would drop it back down but I prefer not to run a cable so close to max where this would an issue.
For the 30A B2B I would use a 60A fuse. That said I see nothing wrong with a 60A fuse for an 18A device as the fuse is for the cable not the device.

So to be honest you could go with either a 40A or a 60A and it will make absolutely no difference to the safety of the system. Both size fuses will blow if any cable damage occurs and you have a short.
Neither fuse will have any impact on the safety of the Victron itself which has it's own internal fuses to protect itself from any internal fault.

Many on here who can confirm. Raul springs to mind, I have just got up and my brain is not ticking on all four just yet.
 
If you fuse near the source, the 40A will be plenty, oversized cable it’s a good thing, oversized fuse not so good. The type of fuse does indeed matter, if it’s a midi strip rest assured will not blow at rated current. So 40A will be more than enough. Only fast blow fuses like T class or HRC will open at rated current, strip fuses will have at least 1.6 times more before it blows.

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I just want to add. For cables running power directly from the batteries I ALWAYS add additional physical protection. My main power cables always run in Kopex flexible conduit and these are always clipped.
 
Gromett said:
I just want to add. For cables running power directly from the batteries I ALWAYS add additional physical protection. My main power cables always run in Kopex flexible conduit and these are always clipped.
Click to expand...
That goes without saying Gromett, any chaffing should be avoided at all costs. Either copex, or any mechanical fasteners to secure the cable so it does not move with vibrations. I use P clips, zip ties, and copex where it goes into cavities or spaces where it may touch the chassis.
 
Raul said:
That goes without saying Gromett, any chaffing should be avoided at all costs. Either copex, or any mechanical fasteners to secure the cable so it does not move with vibrations. I use P clips, zip ties, and copex where it goes into cavities or spaces where it may touch the chassis.
Click to expand...
I have seen many DIY installs where the cable is left loose in cavities or behind cupboards etc. One of my big bugbears.
 
One more tip for the DIYer. Look out for Tesa tape (51608) rather than electrician insulation tape. Much better for automotive use.
 
Raul any more tips you can add? We do not know the things we don't know until someone tells us. I only found out about Tesa tape a few years ago.

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This is what I used until I discovered Tesa tape :D :p. Showing my age.

1704030527885.png
 
Sorry I hit the whiskey pretty well, we started celebrating new year, hope to make it to the morning.
Happy new year to all.
 
Raul said:
Sorry I hit the whiskey pretty well, we started celebrating new year, hope to make it to the morning.
Happy new year to all.
Click to expand...
You too. Just about to start on the beer as well. Have a great night and happy new year to you all.
 
Happy new year to you guys that help me on my way.
I am just working my way down a nice bottle of Merlot, washing down a leftover Christmas pudd !
Hope you all have a good night!

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