I struggled to find a definitive answer on this, so thought I'd put out there.

I've recently fitted some Hella horns (2 off supertone red ones). In putting in the wiring you need to fuse from the battery +ve terminal. Helpfully (or otherwise!) the instructions are pretty cr*p and do not say what rating of fuse should be used so.... based on the data here: LINK each horn has a power rating of 66W, total = 2 x 66 = 132 watts. Battery voltage is 12V.

P = VI...

P = power in watts
V = voltage in volts
I = current in amps

I = P / V = (66*2) / 12 = 11 amps drawn by both horns

Based on what I've found, standard mini blade fuses come in: 5, 10, 15, 20, 25 and 30A variants.

The punchline being I'm fitting a 15 Amp mini blade fuse in circuit!

Hope this helps.

 

15A should be fine. I'd put a 20 in.

 

Thanks for the reply. I should have mentioned that I used the wires from some mains flex cable I had lying around. Typically 1.5mm^2 multicore flex is rated anywhere from 15 --> 18 amps depending, absolute max as 20 amps. Also std fuse rating is defined as 1.25 x current drawn in this case 1.25 x 11 = 13.75 amps fuse. (Ref this link: link

Since I did not know the exact rating of the wire I used I opted for the 15amp fuse as this is the only option inbetween the min. required fuse for the horns, and the max. (Worst case) capacity of the wire. 20A would be too high.