Topic: Fuse rating?

[SOHC4ever]

Bike is a '76 750F.

I blew my headlight fuse the other night, swapped in the spare and was on my way. The fuse box says that it's supposed to be a 7A fuse, but the burnt fuse that I removed is 10A.

Is the 3A difference significant? Should I replace it with the correct 7A, or with another 10A?

[TwoTired]

Is your headlight stock?

It's a sub fuse.  Meaning they should blow before the main if there is a problem on the branch.

If the main fuse is 15 amps, all sub fuses should total less than what the main can carry.

A 7 and a 7 total 14 which is less than the main's 15.
A 7 and 10 total 17.  Which means a 15 could blow before the branch fuses do.

Assuming you are reading the fuse box labels correctly, right now you know there is a problem with the headlight branch and the bike still runs.  If the main blew, you wouldn't know where to look and you'd have a dead bike.

Cheers,

[SOHC4ever]

Is your headlight stock?

It's a sub fuse.  Meaning they should blow before the main if there is a problem on the branch.

If the main fuse is 15 amps, all sub fuses should total less than what the main can carry.

A 7 and a 7 total 14 which is less than the main's 15.
A 7 and 10 total 17.  Which means a 15 could blow before the branch fuses do.

Assuming you are reading the fuse box labels correctly, right now you know there is a problem with the headlight branch and the bike still runs.  If the main blew, you wouldn't know where to look and you'd have a dead bike.

Cheers,

Thanks, TT. This is helpful as usual.

First off, I think that the HL is a more powerful aftermarket lamp but it's unknown.

If I'm reading you correctly, the aftermarket headlight could use more amps than the stock 7 and, consequently, a previous owner has upped the rating on the main, say to 20, in order to run a 10A HL fuse with a stock 7A TL fuse - 17 on the branch and 20 on the main.

If that's kosher, I'll check the main to see if it's in fact 20, and replace the HL with a 10?

If in fact I have a problem on the branch, do I just follow along the branch measuring amperage to see where I'm over-drawing?

[TwoTired]

The headlight is often stamped with it's wattage rating.  Find that.
Use the formula I = W/12.8 to find the current it consumes.

Add about 2 amps (10-15%) for a margin.

I trust the Honda engineers to install the correct size fuse.  PO behavior is often random with no evidence of thought beyond recognition of having a part on hand.

It is also possible that the fuse clip resistance increased over 30 years and the clip heating is/was melting fuses in the holder.  A higher rated fuse melts at a higher temperature, and a higher wattage headlight makes them run hotter. 
A fuse that melts from over current melts near the center of the glass window.  A fuse that is having clip heating issues melts near the end cap.
A frustrated PO may have simply installed larger fuses cause he found some, or that he didn't know that the fuse clips need periodic burnishing/polishing to remove oxidation, be it transparent or visible.

Cheers,

[flybox1]

A fuse that is having clip heating issues melts near the end cap.
A frustrated PO may have simply installed larger fuses cause he found some, or that he didn't know that the fuse clips need periodic burnishing/polishing to remove oxidation, be it transparent or visible.

yep, seen that a few times!

[RAWNOG]

Hey I doubt this is going to be answered but Im replacing my sub fuses both with 7amp fuses, from what I've read here I can do that without messing anything up....?