Fuse ratings

[JulianS]

Fuse goes between battery terminal and the feeds.

[Phil C]

So let me check I understand: I should have one wire coming from the battery. This has a fuse, and then joins with two wires, one of which goes off to the headlamp etc, the other to the horn etc. Is that right? Phil (novice)

[AWJDThumper]

Thanks very much for this AWJDT- very useful. Not to look a gift horse in the mouth, and as part of my learning, please could you tell me: why count the horn and stop light, as they are on a separate circuit? Should they not have their own, separate, fuse? Phil (novice)

If you still have two separate feeds from the battery then a better solution would be to fuse each circuit separately, with each of these circuits taking a max of about 10A. A 10A continuous fuse in each would probably be ok and they would each blow at about 20A rather than the 35A for the single fuse. That said, the single 35A fuse should give more than enough protection in most circumstances.

The stop light definitely needs to be included in the calculation of the max continuous current because people sometimes sit in stationary traffic with the foot on the back break. You could certainly argue that the horn is never normally used for more than a few seconds but, worst case, it could be used for much longer than that (or jam on) and then produce the heating effect within the fuse of a continuous current.

[Phil C]

Yes, thanks, I do quite like the idea of keeping the two separate brown and blue wires coming from the battery as per the wiring diagram.

[Phil C]

Not doubting your expertise, AWJDT, but you say a 10A fuse for each. But won't 10A be not quite enough for the stop light/ horn circuit, and a bit borderline for the headlights etc circuit? From your figures which you kindly sent me, the horn and stop light add up to 11.5 A, and the headlight etc nearly 9A?  I'd better use a 15A fuse, hadn't I?  I'm thinking maybe 15 A for each? Phil (novice)

[Bess]

Hi,
    Realistically the fuse protects the cable. I rewired my bike using the smallest cable for neatness. Because modern cables have greater current carrying capabilities I calculated the bike load, fitted the smallest cable possible, then protected the cable with a fuse with a value below the max cable current capability.

I suggest you check the size and number of cores of your bike cable research the max current for the cable, then fit a fuse below the cables max current capabilities. All should be well for all your electrical items including the stop light, horn and coil inrush current.

Best wishes....

[AWJDThumper]

The max current in the headlight circuit is 8.9A. However, I didn't include the dwell time of about 50% for the points which reduces the coil current by about 1.25A. If this is taken into acount, the total current drops to about 7.65A which is at about the correct 75% max operating current for the 10A fuse.

The max current for the other circuit is 11.5A. Strictly speaking a 15A fuse is required but in practice it depends on how you use your horn. I rarely use a horn on a motorbike and probably a 5A fuse would be ok for me. If you sound the horn regularly and for many seconds at a time then perhaps a 15A fuse would be best otherwise I would use a 10A fuse.

[Blackthou]

Thanks one and all. Most informative and filled in the gaps I had.

[AdrianS]

As said before, the fuse is there usually to protect a direct short and protect the battery.
Most old bikes never had fuses fitted. I have always stuck a 15 amp fuse close to the battery terminal and never had problems. The only time I have blown a fuse is if I have connected something the wrong way ( rectifier once on a Bantam) or inadvertently shorted a circuit out with a screwdriver or a loose wire and each time the fuse blew and no damage done. I would think a direct short would blow fuses of any rating up to the maximum output of the battery so 15 amp is good for me. I wouldn't use a fuse near to the current drain in a circuit in case the fuse failed while out on a ride. Always give yourself a bit of a margin.

[Blackthou]

Sorted, went for 15A.

Reconnected the battery tonight with my new solid state regulator rectifier and following a  general tidy up of the electrics - replacing crappy connectors, decent earths and am pleased to report the fuse didn't blow and everything electrical appepared correct. Hope to recommission her at the weekend.

[Phil C]

Pleased to hear it, Rick.
Let us know how it goes.
Phil

[mashermick]

 i am fitting led head ,tail /stop  bulbs to my ss8o 6 volt  wont carry much load ,fitted fuse on power side 25 amp should be ok