Not being very clued up on electrics, I am trying to find out what fuse I should use on my WD B40, it has a Boyer electronic ignition and, being ex-RAF, is fitted with indicators.
A new wiring harness I bought has a 35 amp glass fuse (the amp rate is printed on blue paper inside the glass tube) however, I have read somewhere that a modern glass tube fuses of 35 amp 'blow' fuse is equivalent to a 17 amp 'continuous' fuse.
Can someone please explain all this please and recommend which fuse I should fit. ???
You need to use the lowest fuse rating possible to give maximum protection to the wiring harness. Normally, on a 6V classic bike, 15/16A fuses should be ok with half that rating needed for a 12V system. The particular type of fuse used (glass, continental, blade etc) doesn't matter very much since you're basically just protecting the wiring harness from damage.
Sorry for being 'electrically dumb' but when you say "..on a 6V classic bike, 15/16A fuses should be ok with half that rating needed for a 12V system..." what amp fuse should I use?
Hi,
Try using this formula to calculate fuse ... Fuse Amps = Total Watts of equipment ÷ Voltage
As Arthur stated you are protecting the wiring so ensure the cable rating will be ok with the Watts consumed. Modern cables with more strands carry greater loads.
If the fuse is running close to max then it will heat, reducing its effectiveness hence blowing prematurely, glass fuses are delicate. I use automotive blade fuses as they are robust in situ.
Best wishes....
Quote from: DWS on 10 Mar, 2017, 10:38
Sorry for being 'electrically dumb' but when you say "..on a 6V classic bike, 15/16A fuses should be ok with half that rating needed for a 12V system..." what amp fuse should I use?
I would go for 15A fuse whether it is a 6V or a 12V electrical system since it is basically protecting the wiring which is rated the same for both.
Thanks for the responses,
I still have to wonder why the brand new WD B40 wiring harness I purchased recently from Burton Bike Bits has a 35 Amp fuse in it?
Handlebar warmers? lol
My cals say they are giving you 420 available watts and using 10 gauge or 2.6mm cable, hope nothing develops a fault.
Best advice is to total your consumption in watts use the formula add 10% and select the nearest fuse.
Hi,
For 12 volt my calc gives:
Boyer ignition - 21 Watt max (that's at fault level)
Coil -50 Watt
Headlight - 60/55 Watt max
Stop/Tail light -21/5 Watt
Indicators - 23 Watt each
Pilot Lights - 4 Watt x 2
Speedo/Rev - 2.2 Watt each
Total max 194.4 Watts. Assume horn and stop light straight from battery without fuse.
194.4Watt/12volts = 16.2 amps.. Maybe
You would think that someone like BBB would supply a harness for a specific motorcycle model to the correct specification?
Quote from: Bess on 10 Mar, 2017, 15:43
Hi,
For 12 volt my calc gives:
Boyer ignition - 21 Watt max (that's at fault level)
Coil -50 Watt
Headlight - 60/55 Watt max
Stop/Tail light -21/5 Watt
Indicators - 23 Watt each
Pilot Lights - 4 Watt x 2
Speedo/Rev - 2.2 Watt each
Total max 194.4 Watts. Assume horn and stop light straight from battery without fuse.
194.4Watt/12volts = 16.2 amps.. Maybe
And don't forget the Blue Flashing Light!
:)
lol, yep forgot that...
I guess if it was a 6 volt system it equates to 32.3 amps. However, usually headlamp for a 6 volt is less and indicators are possibly not used.
The fuse, as mentioned, is to stop wires burning out in the event of a short circuit. So a 35A glass fuse is OK for that. It's what I have in my bikes.
You can get all sophisticated and have fuses for various circuits and relays for the headlight for example but that's not how they came out of the factory.
As an indicator of electrical sophistication, my 1951 Land Rover has just one fuse. The workshop manual states that in event of fuses blowing, fit a higher rated fuse. Things have moved on since then.
Quote from: STAR TWIN on 18 Mar, 2017, 17:36
The fuse, as mentioned, is to stop wires burning out in the event of a short circuit. So a 35A glass fuse is OK for that. It's what I have in my bikes.
You can get all sophisticated and have fuses for various circuits and relays for the headlight for example but that's not how they came out of the factory.
As an indicator of electrical sophistication, my 1951 Land Rover has just one fuse. The workshop manual states that in event of fuses blowing, fit a higher rated fuse. Things have moved on since then.
Thanks for that Star Twin, I find it so annoying to be 'electrically challenged' :)
Hi, you wouldn't have wiring info for the indicators would you?? please??
Quote from: odbsmith on 28 Aug, 2025, 19:28 Hi, you wouldn't have wiring info for the indicators would you?? please??
No, but any late BSA model manual should have.
Just don't use DWS' 35A fuse recommendation!
Thanks V500. Its sod's law ...you look everywhere, post query on forum .... and then find it 30 minutes later!
!!
The 35A glass fuse was the standard for every vehicle from the 30's to the 60s and most cars would get by with one or two. I suspect one is fitted to the new harness as it is the correct "in period" part and will do the job. I've honestly no idea what a 35A glass fuse blows at but I'll bet its well over 35A if its a sudden surge since I've had wiring fires with the fuse intact.
I make my own harnesses and include a few changes for safety or usability. One of the hardest issues is what to fuse and why. There are good reasons not to fuse the ignition circuit for example, you don't want a blown bulb to cut the engine while you are at speed. I've recently gone over to all LED bulbs including my DIY indicators on the A10. This brings the current down a lot so I have fused the A10 with indicators at 20A and this seems more than enough. Horns are often not fused, they draw a lot of current and taking your hand off the button should stop the problem.
Indicators at 20A will see melted wires before the fuse even thinks of blowing.
The old standard 35A glass cartridge fuses will see most cables glowing like electric fires before it blows. The descrimination of those old fuses was so coarse that it probably took 45 or even 50A to blow it. BSA etc only fitted them to show how modern and up-to-date they were compared to the Japanese competition, but then they didn't want any chance of them blowing and more dealer returns and warranty claims.
Most old bikes will need the electronic reg fused (as recommended in the paperwork), the electronic ignition (if fitted) also, and an 8A or 10A fuse in the battery earth line.
The old "add up all the load and then choose a fuse above the total" as mentioned in the thread above couldn't be more wrong.
My posts reads wrong, the indicators draw under and amp as they are home made LED ones, its the whole bike that fused 20A on the main battery cable and I think that's too much so I'll probably drop that to 10 or 15. Sorry for the confusion.
Some interesting comments on fuses (and the usual smattering of mis-information thrown in). I was the Sales Manager for Littelfuse (yes that is the correct spelling) and we invented the ATO blade fuse, which is the fuse I would recommend for your automotive applications. The old "Lucas" 1 inch glass cartridge was indeed labelled at the "blow" rating rather than the rating it was designed to accommodate but I assure you a genuine Littelfuse fuse (which we manufactured at Washington, Tyne & Wear and marked LF) would not get to 45 or 50 amps as suggested. However, there were (maybe still are) fuses manufactured elsewhere of dubious quality that would operate nowhere near their designated current rating. You might as well use a nail; we had some fun testing some of these cheapo "fuses". If you buy cheap autojumble stall fuses or cheap eBay fuses, then more fool you.
We manufactured our ATO blade fuses in Switzerland and believe me, the quality controls and testing were stringent. I believe Littelfuse now manufacture elsewhere but you can be sure of the quality.
Also, bear in mind there are/were similar USA made glass cartridge fuses which were 1.25 inches but were labelled differently.
The ATO blade fuses (and Mini & Micro fuse equivalents) made by Littelfuse, Pacific and Bussman are all fast-acting and excellent quality. Fuses which have no manufacturer's markings should be avoided unless you like gambling.
Worth a watch....
https://youtu.be/nELwmfAOz_I?si=-DYrrqjXNmfP7MIK
Whatever fuse rating you decide don't use an inline fuseholder; they are unreliable. Better to use blade fuse holders. Then as Novocastrian says, fit a good quality fuse. Blade fuses are also more readily available if you have a problem miles from home.
If you go down the blade fuse route (I have) you can get mini resetalbe circuit breakers that fit a blade fuse holder. They are about £6 and while I can'y vouch for their accuracy they are really handy for fault finding since you can just keep resetting it until you find the short and great if you are out on the road.
Quote from: Novocastrian on 29 Aug, 2025, 22:56 Worth a watch....
https://youtu.be/nELwmfAOz_I?si=-DYrrqjXNmfP7MIK
It certainly was worth a watch! Thank you.