Rear/brake light, wire gauge?

[Ianwilko13]

Hi all, the picture shows my recent "short to earth" I have rewired using the same gauge of wire (oval twin 0.75mm) as was fitted when I acquired the bike (1961 Matchless G3) but; 1, I'm not sure what would have been originally fitted?
2, Is this gauge adequate?

I ask this because the 6 volt system will still blow a 5 amp blade fuse if the lights are on and the brake light is used
but not a 7.5 amp fuse. All wiring has been traced and checked wire by wire, all good, thanks for any suggestions, Ian.

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[MalcW]

0.75mm wire is typically rated at around 6 amps, but the type of wire itself and the type of insulation can affect that. So 6 amps at 6 volts would be 36 watts. Since I doubt that either your brake or tail lights are that high you should be fine. If your rear light is a typical 5W/21W bulb, that's 26W total which would be about 4.67amps, right on the limit of a 5amp fuse, assuming that fuse is powering both brake and tail.

Malc

[Ianwilko13]

Thanks for your knowledgeable reply Malc, the fuse (blade type & holder) was fitted by myself to stop a short & fire scenario, it is fitted between the positive terminal and earth. Therefore I assume that it has to carry the full electrical load, which is; Full beam 30 Watts pilot light 4 Watts Speedo back-light 4 Watts Rear light 5 Watts & Stoplight 21 Watts. This would be 64 Watts through the earth protection fuse, so I believe my wire gauge choice is marginal (I'm an Engineer) I am also running a 20 amp fuse, could I get away with a 15amp fuse?? Kind regards, Ian.

[MalcW]

Hi Ian

Based on those figures your should be pulling a maximum of 10.67 amps, so a 15 amp fuse should be fine. There is a tiny risk that you could exceed 15 amps if you switched from dip to main beam or vice versa, both bulb elements came on for a split second and you were braking at the time. I don't know what current a horn pulls, but it's something to consider. I would be inclined to go with the 15 amp fuse but take your 20 amp along, just in case. To be honest if something shorts out it's likely to blow either fuse size.
My bike runs 12V so I can use lower fuse ratings. I have a halogen headlamp bulb but everything else bar the indicators are LED. I have a 10 amp main fuse and a separate 10 amp fuse for the indicators.

Malc

[Ianwilko13]

Thanks for all the advice Malc, when the above short occurred it blew my 15 amp fuse, and the output from the alternator (without the battery in circuit) blew all my vintage (good) bulbs, so I think 20 amp to be safe and
1.5 mm flex (if available) and I should be safe, thanks again & kind regards, Ian.

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[jackstringer]

I used 2mm² wire on my loom. Partly as the wire wasn't that expensive and I do tend to do most things in overkill mode.

V × A = W
W ÷ A = V
W ÷ V = A

Handy way to check how much current draw there will be is to measure the Voltage of the battery as the bike sits there and then do it again with the engine running as the voltage changes. This will allow you to work out the current (A) worse case (battery) and typical (running). Most wire and fused will accept short moments of mild over load but they will blow if there is a short.

The main thing is the fuse needs to be rated lower than the wiring as you want the fuse to blow before it melts the wiring loom.

[Ianwilko13]

Thanks for your input Jack, wire gauge seems to be an issue now that " Wilko" have closed Electrical wholesalers want me to have 50 meters, all I've found is the wife's Hairdryer cable, but she would notice in the morning flyaway tend to sell Chinese wire any supply details would be helpful thank you and best regards, Ian.

[Ianwilko13]

Flyaway was a spell checker fleabay.

[Groily]

You could try these people for wires cut to length, in pretty much any gauge. Also, for info on current capacity etc. I find their stuff very satisfactory:
https://www.vehiclewiringproducts.co.uk ... -pvc-cable

When thinking about fuses, don't forget that the current required to illuminate a cold bulb is double or even more what it draws once it's lit up. So it's not just a Q of calculating the max power requirement based on the wattage of the bulbs plus something for ignition, you need more than that. And horns are commonly current-hogs too.

Typically, I'd go for a 20A main fuse on 6v machines, and 10A or a tad more on 12v, ensuring that all loads go through it, including horn and brake light if they are wired directly off the battery. Ancillary fuses rated to suit individual circuits aren't a bad idea either, of course, as they pin-point any fault that may arise subsequently.

[Ianwilko13]

Thanks Bill, I will check them out, it's nice to see you put some numbers on the fuse ratings, as an Engineer I have built many engines, fabricated many structures and assembled a few tank transporters, I'm not electrically stupid but looking at all the calculations involved, I kept putting fuses in until I could get 3 of the same rating to blow whoever said Engineers are daft, kind regards, Ian.