How many amps to a fuse?

[REW]

Got the G3 out of hibernation, new battery, bit of air in the tyres, lots of tickle and kicking (new indoor game, innit?) and "bam!" away it goes with a strong tickover. Took an age for the oil to sputter at the return which is hard to endure however often you watch it.

Good 50 miles round the dales in splendid isolation (word of the moment) and all good, apart from the ammeter which has split in two Cheap plastic body I guess.

Point of all this ramble: I have treated the new battery to an in line fuse 'cos it's a good thing apparently. My 6v system probably swings 60W so 10 amp fuse is suggested. However, that's a bit close to the 10 amp current draw isn't it? What would people suggest?

[SPRIDDLER]

The fuse is there to protect the wiring from a short circuit.
I doubt you'll get 60W from a 1951 dynamo.
The 'short' dynamo will at 6V produce about 42 Watts max when in perfect condition which is 7 Amps.
A 10 Amp fuse will be fine.
EDIT.
Did you mean a 60W draw when all light are on? I doubt it's that much.

[Mick D]

Hi

60W seems a little high unless you brake a lot at night whilst hitting the horn

I'm using a 10A fuse in my alternator G3 with standard filaments - not blown one yet.

I use 28/0.30 cable as it is the closest match to the original wiring and is good for 17.5A so I could up the fuse a tad if necessary - what cable have you used?

Regards Mick

[Rob Harknett]

With all fuses, in particular those of my PA gear, although I knew what fuse to fit, if not available I would go lower. I'd sooner replace a fuse than blow up gear.
If you decide on 10 amp Ron, I am sure you will buy a couple, also get one a bit higher. Tape them to the wire inside the battery as spares. A fuse will blow when out on the bike not in the garage.
Why I say get one higher ? I got a new battery that had a fuse in the wire. I then went to get an M o T.
I had no lights. They did work, as I had checked just using the brake light. M o T man said you must have blown the fuse.
He fitted a new much higher fuse.

[SPRIDDLER]

I am sure you will buy a couple, also get one a bit higher. Tape them to the wire inside the battery as spares.

A 10 amp fuse in my '54 G3LS hasn't blown, and I have had a spare 10A fuse taped beside it for donkey's years.

2019-11-20 Burlen 6V 5Ah battery 009 (640x480).jpg

[DM2]

Look at table 1 and figure 4, 10A is a suitable fuse.

https://www.pecj.co.jp/en/fuse/outline/p3.html

[les ward]

I use a 10amp circuit breaker.

In my experience, if you have a problem you put in the spare fuse and that blows as well. With a circuit breaker you can disconnect one part of the circuit at a time and see if the circuit breaker still trips. This mean you can isolate the problem without wasting lots of fuses.

I used to get them from Maplins, but they have closed. If you go on ebay and search "10amp push button circuit breaker" you should get something suitable. Leave out "push button" and you will get domestic ones.

[REW]

Well thanks everyone, that is an informative set of useful info. In no particular order:

Yes, 60w is probably an optimistic max possible current draw, so 10 amp should do. I recognise the wisdom of protecting the wiring despite ignoring it for decades.

Spare fuse always carried and if the problem is solved having used all fuses, I can disconnect the fused line bullets and wire direct to get home.

The push button circuit breaker is a new idea (to me); to be explored.

Sprid: that Burlen "Cyclon" battery was the type I just replaced after 10 or more years so it has been great. However I never felt happy charging it even with a modern charger though I am not sure why. The new one is a Motobatt sealed AGM which is the make I have used most successfully for years on my more modern bikes. It lights up the bulbs far better than I ever remember the Burlen doing even after attempting to charge it. Again, no proven reason why.

[SPRIDDLER]

Sprid: that Burlen "Cyclon" battery was the type I just replaced after 10 or more years so it has been great. However I never felt happy charging it even with a modern charger though I am not sure why.

Yep, they're good. Mine's 14 yrs old and still going strong. Under 20 quid from Burlen. 6v and 5Ah, maintenance free, which has been adequate even for night riding with standard filament bulbs on my mag ign bike. On the rare occasions I've charged it (only after faffing about in the shed with wiring and lights) I've used an ordinary, 40 yrs old basic charger. After my accident it was unused for 2 years and had kept its charge.

2019-11-19 004 (480x640).jpg

Note, 600A short circuit current. That would melt any loom without a fuse!

2019-11-19 005 (640x480).jpg

[Rob Harknett]

Just a thought. I guess the fuses are all the quick blow type. How would an anti surge fuse suit us??