Fuse Rating

I’ve seen several threads about fuses and the confusion regarding fuse rating. I came across the following information from the August 2012 issue of Spokes, the official publication of the MG Car Club Western New York. Hopefully, I’m not violating any copyright laws by quoting from the article but I found it helpful and thought I’d pass it along.
“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 without blowing, while the British system expresses the load at which the fuse will immediately blow.
That’s a significant difference! As an example, the 35 amp fuse called out for many vintage Britcars [and motorcycles] is roughly equivalent to a US-spec BUSS AGC 17 amp fuse. Install a US 35 amp fuse into your fuse box and the wiring harness will melt down before the fuse blows.
Do not use a USA rated 35 amp fuse in your vintage British car! [emphasis by original author]
The complete rating for the 35 amp Lucas fuse is “17 AMPS CONTINUOUS / 35 AMP SURGE”
Early British fuses were simply marked with the quick blow rating, but later fuses are given two ratings in an effort to minimize the confusion. For instance, the 35 amp fuse is often marked as “17 continuous / 35 blow”, or simply 17/35. In that case, use the lower “continuous” rating as the one to which you try to match an American BUSS fuse rating.”
Hope this information is useful.

WEM, thanks for sharing this! Nice to have publication as a source to validate this issue. In the past, I have purchased glass fuse holders with proper color coded wires that came with a 35A (USA) fuse! I have gone to using blade style fuse holders and run 15A. I always carry a spare, glass or blade. The advantage of blade style is availability. The convenience store gas station will more likely have blade style in the case of an emergency.

Use SFE 20 if using glass original type fuses. These are the closest available to the British 35-amp fuses.
AGC 20 is too "heavy" whether glass or blade
AGC 15 is generally OK whether glass or blade
I use ACC 5 or 10 amp while wiring and testing bikes and switch to SFE 20 when done. If you don't use the original horn or never push the button, the AGC 10 will run the bike fine. If using a working original horn, then AGC 5 will blow if you press the horn button more than momentarily! A non-working original horn will usually blow even an AGC 10 - the horn current spikes VERY high if its point makes contact and don't open and even when blowing the average current is often more than the rest of the bike put together!

Notes:
SFE 20 is the only SFE that fits the original fuse holder. Unlike AGC, their length varies as their capacity varies.

AFAIK, blade fuses are all equivalent to AGC so don't go over 15 amps - even that may be overkill but AFAIK, they jump from 10 to 15 amps - 12.5 would be better if it exists.

The original wire in British bikes is 14-strand which is very close to 18ga (16 thinner strands usually). Depending on where you look, either can continuously carry 7 or 8 amps. That's fine for almost all the wires, However the ground wires on a Norton and the Brown/Blue (and Brown/White if used) really should be 28-strand (14 ga). For the battery to the master switch, there is only one wire. It splits to four wires from the switch to various circuits (in most harnesses with the master switch by the air cleaner). So, those wires from the master switch have an easy time of it, but the ones mentioned must carry the full load of the bike. I've taken apart many harnesses with melted insulation on the ground wires and/or Brown/Blue wires.