which fuse to use

bombardier · Post by **bombardier** » Tue Mar 15, 2011 7:28 pm

hi all a quicky !!! i have a 1960 g3 with a 12v lucas rita system fitted and i have just fitted a 12v 9amp battery to the bike and now i would like to know which size fuse i should be using would a 20amp one be ok ?

Rob Harknett · Post by **Rob Harknett** » Tue Mar 15, 2011 8:40 pm

As its best to be on the safe side, and not chance it to guess work, try starting at say 15amp, if it soon blows go to 20 and so on.
I had 30 in my 12V battery, it blew when the rectifier fell off, hanging by the wire & earthing. When I fitted a 20amp fuse as I never had a 30, the 20amp fuse soon blew.
So I would not dare suggest you fit a 30 amp fuse, and you burnt out wiring, or worse, insted of a fuse.

bombardier · Post by **bombardier** » Tue Mar 15, 2011 8:45 pm

cheers i thought that was the way to go

g80csp11 · Post by **g80csp11** » Tue Mar 15, 2011 11:00 pm

I'd recommend a maximum of a 15A fuse on 12v system , especially if you are running a zennor diode

On the way to Italy, i'd fitted a 20A as i'd run out of 15A fuses , and as a result of a failed zennor (short to earth) in France I lost the complete wiring loom as every path to earth burnt out before finally the 20A fuse blew.
Luckly , 4hrs later and a reel of cable i managed to rewire the bike.

Eric · Post by **Eric** » Tue Mar 15, 2011 11:33 pm

The one thing you must not do is keep increasing the value of the fuse until it never blows any more, otherwise why fit a fuse?

The thing to do is calculate what load the fuse has to protect. So never being any good with electrics but still remembering: -

Divide the Watts by the Volts so a 35 watt headlight bulb will be 35 divided by 12 lets say the answer is 3 and that will be amps.

Add up all the wattage of everything you could have on at any one time to be protected by the fuse e.g. Headlight main beam, rear sidelight, and perhaps brake light bulb unless its a separate circuit with the horn. Don't forget the speedo light and anything else you may have fitted.

So lets guess the answer is about 58 watts that would be 6 amps, the same wattage in a 6 volt system would give an answer of 12 amps so you can see there is a big difference.

Lets allow a working margin of 50% over the maximum load so the 6 amps of a 12 volt system would need 9 amps as a fuse the nearest is 10 so that's what to use.

Others more expert may say my 50% margin is too low and that maybe true.

SPRIDDLER · Post by **SPRIDDLER** » Wed Mar 16, 2011 1:42 am

quote:
I'd recommend a maximum of a 15A fuse on 12v system , especially if you are running a zennor diode

On the way to Italy, i'd fitted a 20A as i'd run out of 15A fuses , and as a result of a failed zennor (short to earth) in France I lost the complete wiring loom as every path to earth burnt out before finally the 20A fuse blew.
Luckly , 4hrs later and a reel of cable i managed to rewire the bike.

And Sprid was there with his camera - see my post under 'General topics'Edited by - SPRIDDLER on 16 Mar 2011 01:44:00 AM

Groily · Post by **Groily** » Wed Mar 16, 2011 7:52 am

FWIW I use max 15A on 12v systems and 20A on 6v. Enough margin per Eric's point never to blow absent a fault, but will blow in a heartbeat if there's a dead short. 13A household fuses have worked very well on 12v systems for me as well, in old-fashioned fuse holders.
A reason for a good margin over the maximum rated continuous loads on the system is to cope with the short-term extra current draw as a headlight filament warms up.

Eric · Post by **Eric** » Wed Mar 16, 2011 9:04 am

If you were feeling more adventurous when doing a bike rewire you could included two or even three fuses.

The most important thing is one fuse is easy to do and with the correct rating could protect you from loosing your bike in a fire.

alfie · Post by **alfie** » Thu Mar 24, 2011 3:51 am

True, i couldnt believe it , no fuse on my ms16,
I have fitted. One on the pos ,and one on the neg side of the battery,
I was glad i did when my horn had a dead short !

Don Madden · Post by **Don Madden** » Thu Mar 24, 2011 4:17 pm

I remember reading a review of the new AMC bikes where the maker stated that with their high quality manufacture, no fuses were necessary! Most 12v British bikes of the era fitted a 20A fuse. Cheers, Don.