Fuse question

[Blackthou]

I've just fitted a 6v solid state regulator / rectifier to my 1961 C15. I've also added a fuse holder as the bike was unfused. Should I fit a 5A or 10A fuse?

[MGI]

 The most popular function of a fuse in most systems (overall -  cars, houses, static machinery etc) is to protect against overload but my belief is the most common reason for any fuse blowing is usually a dead short, and in most applications it doesn't make much difference what size the fuse is (within reason) as a dead short will blow a 10A fuse just as quick as a 5A unit.

[Blackthou]

Thanks,

I totally agree but I was thinking more of load i.e. if I fit a 5A fuse will the load blow it purely by demand and a 10A is required?

[courtney]

Not sure if it makes much difference, but I converted my C15 to 12v with solid state reg/rec and fitted a 7.5 amp fuse.

I used the information in the below link, which seems to be quite good although it's for a M@t!#less.
https://www.matchlessclueless.com/electrical/general/installing-a-fuse/

[Mike40M]

It's a common practice to fit an in-line blade fuse holder fitted directly to the battery. Primary function is to protect when shorts occur. So if your ammeter goes to 10 Amps, a 10A fuse will be fine.
But since the horn and brake lamp is connected prior to the ammeter, more current might occasionally be drawn. So a 15 Amps might be needed (probably not). In case of a short, a 15 Amps fuse will blow as well.
The secondary function of a fuse is to protect wiring from overheating. The original wire gauge was selected to keep voltage loss low. Which makes wire heating low. But if you have fitted heated grips, extra lamps etc. it could be a problem. Fitting indicators don't affect wire heating much as current is intermittent and only for short times.
So if you are a belt and braces type, an additional smaller fuse could be fitted on the outgoing side of the ammeter. But as a fuse have a resistance so that excessive current melts it, it gives a small voltage loss to the lamps.
My advice is that you fit a 10 A fuse and have a 15 A fuse in your pocket. If it works you could try with a smaller one.
Keep in mind that that if your battery is almost flat, the charging system will push as much current as it can into the battery. Probably enough to blow a 5 Amps fuse.

[Phil C]

Looking for advice on what fuses to fit, on19 March 2018 on this forum I began a discussion entitled " Fuse Ratings". I mention it in case you're interested.   Phil

[Blackthou]

Thank you @Mike40M and @Phil. Much appreciated
Rick