Fuse Rating

[bikerbob]

Hi there. How do you work out what rating fuse to use on my BSA A65. I have 12v electrics. Electronic ignition and indicators also the usual lights. The fuse that is in at present is the one that came with the fuse holder that is 10amps but I have now fitted indicators do I need to increase the rating. Thanks.

[L.A.B.]

Watts divided by Volts = Amps.

     

[twolitre]

L.A.B. is spot on. But fuses should not be!

It is best to fit fuses about 25-50% higher rating than the normal current in the circuit.
Two reasons for this:-
Many items (such as bulbs) actually pass more current as they warm up.
And, fuses operating at near blowing capacity are likely to fail prematurely due to heat fatigue.
Jim.

[L.A.B.]

And, fuses operating at near blowing capacity are likely to fail prematurely due to heat fatigue.

It often depends on the type of of fuse, because modern auto fuses are usually continuous rated, so a "10A" fuse does not blow until 20A which does leave quite a large safety margin for those odd times when everything is working at once (including the horn).

http://www.vehicle-wiring-products.eu/VWP-onlinestore/fuses/fuses.php

Only the old style British [Edit]1-5/32" (30mm) glass fuses are still listed by blow rating (but not 1", 25mm glass) which often causes some confusion as the "35A" fuse commonly specified in British motorcycle handbooks and manuals is approximately equivalent to a 17A continuous fuse and many British bike owners have been known to fit a 35A continuous fuse often with disatrous consequences for the wiring when a short occurred. If a blow rated fuse is used then it should be one of at least 25A (12.5A continuous).





   

     

[bikerbob]

Hi there. Thanks for the quick reply but I am still a little confused the fuse holder that I bought actually takes the fuses that you fit in your home appliance plugs the one that was in when I bought it was 10amps it blew accidentally when I was wiring up the indicators so I fitted a standard 13amp fuse from a household plug. Now do they fail at anything over  13amps load or will they not fail until 26amps load. As I am understanding it if I fit a seperate fuse for the indicators which have a total wattage of about 45 watts that's 2indicator bulbs and a warning light plus the relay which I do not know the wattage for, then that equates roughly to a 5amp load so am I correct in fitting a10 amp fuse or should it be 5amps. Thankyou.

[L.A.B.]

the fuse holder that I bought actually takes the fuses that you fit in your home appliance plugs the one that was in when I bought it was 10amps

I think your fuse holder is probably meant to take either the 1 inch (25mm) glass fuses or the ceramic Bosch type fuses, as 10A isn't a domestic fuse rating (at least not in the UK) and I'm not exactly sure that using domestic fuses in automotive applications is a good idea (unless perhaps in an emergency)?


http://en.wikipedia.org/wiki/Fuse_(automotive)_

"Automotive fuses are a class of fuses used to protect the wiring and electrical equipment for vehicles. They are generally rated for circuits no higher than 24 volts direct current, but some types are rated for 42-volt electrical systems."

A continuous fuse of 15A rating should be adequate to protect the average 12V motorcycle electrical system with indicators, however specific electrical items can always be protected with separate sub-fuses of a sufficient rating rather than just relying on a single "main" fuse to protect all the circuits that will shut down the complete electrical system if it blows.     

[twolitre]

L.A.B. You have introduced an aspect of fuses of which I was not aware. I did not realise that most fuses were rated by continuous current rather than blowing current now.
I left school nearly 60 years ago. Perhaps it is time I went back?
Jim.

[Rex]

60 years ago would have been just the same, except the fuses then were generally less sensitive; rewireable fuse carriers are/were notoriously variable in the current carrying capacity. Cheap fuses now are just the same. The "rule of thumb" used to be the rupture current was 2.5X the rated current.
Incidentally there are 10A fuses available in the "domestic" plug size, just as there are 1A, 3A 5A 10A and 13A, and probably more besides.

[L.A.B.]

Incidentally there are 10A fuses available in the "domestic" plug size, just as there are 1A, 3A 5A 10A and 13A, and probably more besides.

Yes, I stand corrected on that (I should have checked first) 3A, 5A & 13A just being the ones most commonly used.

http://www.tlc-direct.co.uk/Main_Index/Necessities_Index/Plug_Top_Fuses/index.html?source=adwords&kw=electric+fuses&gclid=CIL9reSXma0CFYQLfAodnWyeoQ