Fusible Link wire length

[Sparepart]

I am stripping out all the unused wires from the Sierra loom that has been used in the Exmo. I now realise that when I fitted the loom back in 1998, in my haste to finish the build, I cut out some of the length of the fusible link wire that connects directly to the battery positive terminal. At that time I was ignorant of it's importance and just thought it looked a bit flimsy. I now see (from the bit that remains) that it has "16GA FUSE LINK" written on it. I wonder if anyone know how long it should be ? as I would like to replace it. Alternatively has anyone changed it for something like a Maxi Fuse, and knows what rating the fuse should be ?. I looked at web sites that discuss this and they seem a bit vague about drawing an exact comparison between a length of fuse link wire and a fuse rating.

[IanS]

Length is not what matters very much.

Cross sectional area and material matter, a fuse works by melting and the molten material shrinking back. Material gives restivity and CSA gives current density and hence how hot the wire gets. Material also sets the melting point.

I think that only a short length of the cable is actually the fuse and the copper ends are designed with a small cross section to limit the cables ability to take heat away from the fuse section.

The electrical loads can be calculated.

Headlights 60W main beam 5A each 10A total (dipped is less)

Side lights 5W each so 5 off (include numberpalte light) 2.5A

Brake lights 21W so 3.5A total.

Ignition under 4A

Electric fan 5 to 10A

So total load arround 30A

Now for the other part what is the alternator output? as nearly full output can flow back to the battery to replace cjarge used by the starter shortly after the engine starts.

The alternator output should be greater than the total of all normal loads so as to have some left over to charge the battery so worst case with the above 30A total normal load a 40A alternator would have 10 over to charge the battery with everything switched on and 36A to charge the battery with only the ignition as a load. These charge rates will only be for a short time before going into a trickle charge rate as the battery regains full charge.

So with a 40A alternator I would use a 50A fuse and with a 60A alternator I would use a 70A fuse.

[Sparepart]

Thank you for your reply and the calculations and advice. My focus was on the current given out by the battery, I had not considered the alternator. I have done a bit more digging around, and am 99 percent in agreement with your notes. The only aspect that I think might matter is that of how long it takes the fuse/fuse link to "blow". Here, I think that the length of the link or type of fuse has an effect. I was thinking about two rediculous extreemes. If I used a very long fuse link it would not blow, just get hot and the high resistance would severely limit the available current in normal use. If I used a tiny tiny length of fuse link, then it would be able to support the accessories, but then blow when the alternator kicks in after a start. So the puzzle is that if a fuse rated to take the maximum possible current is used then it will be too high to protect the "normal" wiring for the accessories. That's why I think that the length of the fuse link matters, it is in effect a "slow blow" type of fuse. It probably gets a bit hot when the alternator kicks in but won't blow right away, it lasts intact until the the max current peak passes. I assume that this must be a short time ?, a few seconds/ (no Idea).  Anyway that is why I would like to replace my fuselink with a piece of the correct length. The wire  I need is available from the USA but I don't know what length to use. If I go over to using a blade or cartridge or other sort of fuse I really aught to use a fuse that mimics the "slow blow" characteristics of the original fuse link. Perhaps I am being too pernickety about this, after all I have several spare bottles of Lucas Loom Smoke in the garage in case any should leak from the loom. :-)

[IanS]

If you want a bit more information.

https://www.littelfuse.com/~/media/automotive/catalogs/littelfuse_fuseology.pdf

 

[Sparepart]

This is great, just the sort of detail to keep me happy for a long time. Just for the "fun" of it, I am thinking of making a crude high current ammeter by having a coil of thin wire around a thick wire through which the high current will pass. Depending on the number of turns in the coil, I would hope to generate a voltage in the 0-5 v range that I can measure using a Raspberry Pi. This might then have sufficient granularity to generate the graphs of current against time for the spikes. A sort of cheap oscilliscope. Of course this will have to wait to the next lockdown.  Anyway thanks for the document link, I now plan to replace the fusible link with a fuse with some sense of competence.