I notice that in my Haynes workshop manual, at the start of chapter 6, Electrical Equipment, the fuse loading is given as "35 amps - all models except the earlier Star series." I'm not sure whether my 1965 B40F is an "earlier Star series" - I guess so. The Haynes book doesn't say what the fuse for them should be. But I was wondering: why 35 amps for some models? Seems a lot to me. I was thinking of maybe about 15 amps for mine. Obviously I'd need to add up all my wattages before finally working out the rating. But 35 amps seems a lot. Is it because the horn is included perhaps? Phil (novice)
Fuses are rated in two different ways: either continuous rated or blow rated. The old style glass fuses tend to be blow rated whereas modern fuses tend to be continuous rated. For many classic bikes a 17.5A continuous rated fuse will provide adequate margin relative to the maximum current provided by the battery and this equates to a blow rating of 35A which is the figure normally quoted.
When adding up how many amps maximum might flow, in order to work out the fuse rating, do I need to include anything for the ignition coil? Otherwise I guess its just headlight, tail light, stop light, speedo light (horn is on another circuit). My bike is a 1965 B40F and I'm thinking the amps add up to just under 10, so I guess a 15 amp fuse would be okay. I note however that AWJDT says 17.5A is suitable for many classic bikes, having a blow rating of about 35A. So maybe that would be more appropriate? Also, I believe glass fuses have both the continuous and the blow ratings marked on them, to avoid confusion? Phil (novice)
The main thing you are protecting against is a short circuit of a wire from the battery which could cause the wiring to glow red hot and then catch fire. In the event of a short circuit, the current that will flow will be much greater than 35A (unless the battery is knackered) and so a 35A blow rated fuse will be ok. In principle, you could use a slightly lower rated fuse but it won't really provide you with that much greater protection. The danger is that if you were to fit a 25A blow fuse, then the inevitable current spikes generated when you switch on the headlights or use the horn could end up regularly blowing the fuse, which would be extremely annoying!
Okay, thanks. But, just out of interest, should the coil be included in amps calculations, given that it is fed from the battery?
Hi Phil,
I did a calc awhile back to reduce cable size and came up with this.
Boyer ignition 21 Watt max (that's at fault level)
Coil 50 Watt
Headlight 60/55 Watt max
Stop/Tail light 21/5 Watt
Pilot Light 4 Watt
Speedo/Rev 2.2 Watt each
Total max 165.4 Watts. Assume horn rarely used and only intermittently so I didn't include it, I have never had a problem.
165.4 Watts/12 Volts = 13.8 amps.
Best wishes....
I've always run a 10amp fuse on the A10s
Quote from: Phil C on 25 Mar, 2018, 18:38
Okay, thanks. But, just out of interest, should the coil be included in amps calculations, given that it is fed from the battery?
Yes. It takes ~2.5A although that's only when the contact breaker points are closed!
Quote from: DEAN SOUTHALL on 25 Mar, 2018, 19:32
I've always run a 10amp fuse on the A10s
The A10 electrical system is not really the same as that of a B40 since you have both a dynamo and a magneto fitted rather than an alternator, rectifier and coil ignition ;)
Okay, so if the coil only takes about 2.5A, as AWJDT says, then I guess it doesn't make much difference to the fuse rating, even though my battery is 6v. But Bess says 50 watts for the coil? With 6v that would be 9 - 10 A? I'm slightly confused! Phil (novice)
Quote from: AWJDThumper on 25 Mar, 2018, 19:46
Quote from: DEAN SOUTHALL on 25 Mar, 2018, 19:32
I've always run a 10amp fuse on the A10s
The A10 electrical system is not really the same as that of a B40 since you have both a dynamo and a magneto fitted rather than an alternator, rectifier and coil ignition ;)
Don't have a dynamo or mag on my A10. Its alternator and electronic ignition but I take the point that this may not be comparable to the B40. Sometimes I just have to butt in ::)
Here is the max current usage of a B40:
Mainbeam: 30W / 5A
Tail Light: 6W/ 1A
Speedo Light: 2.2W / 0.4A
Stop Light: 18W / 3A
Horn: 51W / 8.5A
Coil: 15W/ 2.5A
Total: 122W / 20.4A
The above total of 20.4A is the max current possible and so the fuse should be chosen to cope with it. On this basis, the 35A blow rated fuse is probably the sensible choice.
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)
Phil - the horn and stop light should both be included in the fused circuit, they are not separate from it. You could feed each from the battery via separate fuse but I think that would be an unnecessary complication.
Some owners replace the standard tail/stop bulb with an LED type which takes significantly less current, probably less than one amp.
I don't understand. Two wires come from the battery -ve (live) terminal: one goes to the ammeter and then the headlight, ignition switch, etc; the other goes to the horn and stop lamp switch. Are they not therefore separate circuits? Am I being dim? Phil (novice)
Fuse goes between battery terminal and the feeds.
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)
Quote from: Phil C on 26 Mar, 2018, 08:17
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.
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.
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)
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....
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.
Thanks one and all. Most informative and filled in the gaps I had.
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.
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.
Pleased to hear it, Rick.
Let us know how it goes.
Phil
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