850 Commando Negative Lead and Fuse

Still learning about my relatively new (to me) 850 Commando MK1. Bike is positive earth. Went to replace the battery today and, in the process, the fuse holder just broke off the lead wire. Must have been hanging by a thread. What I believe is likely the original Lucas 35 amp glass fuse was still in it. Fortunately, I had a couple of mini fuse holders on hand which I put in its place. But I am baffled by the skinny lead wire that is fused at 35 amps??? I could only find 30A mini fuses at my local auto parts store, so that’s what I put in. So, I have 2 questions:

1. How can the circuit get by with such a skinny wire but carry a 35 amp fuse?
2. Do you think I’m OK with a 30 amp fuse instead of a 35 amp fuse?

Hopefully those of you with more experience can enlighten me. Thanks!

"British and American standards for rating electrical fuses are different. Both provide an “Amp Rating” for
very similar looking fuses, but the two ratings mean very different things. The US system rates the fuse based
on the continuous load it can handle for a specified period of time w/o blowing, whilst the British system
expresses the load at which the fuse will immediately blow.
As an example, the 35 Amp fuse Btitish Lucas fuse is roughly equivalent to a US-spec BUSS AGC 17 Amp fuse "
The complete rating for the 35 amp Lucas fuse is - “17 AMPS CONTINUOUS / 35 AMP SURGE”
There is a excellent explanation at "Spokes" the MG Car Club Western New York Centre ,August 2012 .

Yep, what HRD said! You are correct in wondering how the skinny wire can support a 35A load - it can't! I wondered the same thing years ago before learning the difference between the US/British fuse rating on this site! I use a 20A fuse since I have never seen a 17A fuse. If there is concern that a 20 is more than the rating, a 15 would probably work OK on a totally stock Commando.

15A works fine in mine.

Guys, thanks a million, I knew it had to be something like this. At the moment I have a 30A mini fuse installed. Will replace with 15A and keep some spares on hand. I may eventually run some heated gloves but that will be on a separate circuit. You guys are great.

Yup , 15 A mini fuse here as well , have never burnt one .... simple wire job though brakes light , headlight , horn and ignition only .....

Can't be too careful around that high output charging system..... one could perhaps make an inopportune spark....
I don't figure I can complain though because it does run the lights and charge the battery... I run 15 also BTW.

On skinny wiring. I've read a long thread on the subject on a BSA forum. Where they made beautiful calculations on wiring heating versus wire gauge. The conclusion finally ended with that even quite heavy load would not cause the famous Lucas smoke. It is caused by some other faults in the system. A short circuit will blow a 30A fuse before wire harnass smoking. But with engine running, combining alternator output and current supplied from the battery, e.g. a shorted zener could produce a lot of smoke.

I had a short circuit between the blinker switch and ground, through the handlebars. That wire runs from the blinker relay in the battery compartment, into the wire harness, and all the way up to the handlebar switch cluster. My Shorai battery happily blew a 20 amp fuse. No smoke, nothing melted.

Joe Lucas was framed.

FWIW
Any fuse is better than no fuse. The proper fuse is still the best option. Not all issues are a dead shorts that will trip the wrong size fuse. Example being heated equipment or a high powered H-4 bulb, a continuous draw on the wrong size wire will overheat the wire but not trip the fuse.