Author Topic: Fuse ratings  (Read 470 times)

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Phil C

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Fuse ratings
« on: 19 March, 2018, 21:59:57 »
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)

AWJDThumper

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Re: Fuse ratings
« Reply #1 on: 19 March, 2018, 23:14:28 »
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.

Phil C

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Re: Fuse ratings
« Reply #2 on: 25 March, 2018, 17:28:05 »
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)

AWJDThumper

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Re: Fuse ratings
« Reply #3 on: 25 March, 2018, 18:25:20 »
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!


Phil C

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Re: Fuse ratings
« Reply #4 on: 25 March, 2018, 18:38:32 »
Okay, thanks. But, just out of interest, should the coil be included in amps calculations, given that it is fed from the battery?

Bess

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Re: Fuse ratings
« Reply #5 on: 25 March, 2018, 19:12:31 »
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....

DEAN SOUTHALL

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Re: Fuse ratings
« Reply #6 on: 25 March, 2018, 19:32:21 »
I've always run a 10amp fuse on the A10s

AWJDThumper

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Re: Fuse ratings
« Reply #7 on: 25 March, 2018, 19:35:28 »
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!

AWJDThumper

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Re: Fuse ratings
« Reply #8 on: 25 March, 2018, 19:46:40 »
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 ;)

Phil C

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Re: Fuse ratings
« Reply #9 on: 25 March, 2018, 19:57:22 »
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) 

DEAN SOUTHALL

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Re: Fuse ratings
« Reply #10 on: 25 March, 2018, 20:53:54 »
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  ::)
« Last Edit: 25 March, 2018, 20:55:54 by DEAN SOUTHALL »

AWJDThumper

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Re: Fuse ratings
« Reply #11 on: 25 March, 2018, 23:20:58 »
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.


Phil C

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Re: Fuse ratings
« Reply #12 on: 26 March, 2018, 08:17:06 »
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)

JulianS

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Re: Fuse ratings
« Reply #13 on: 26 March, 2018, 09:26:03 »
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.

Phil C

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Re: Fuse ratings
« Reply #14 on: 26 March, 2018, 09:31:14 »
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)