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Fuse size for stand alone, 12 volt, LED turn signals


gregleck
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I am adding turn signals to my 1937 Buick Coupe. It is a 6 volt system, which I am keeping.

The turn signals will be a stand alone, 12 volt system, powered by a small, 12 volt SLA battery with 2.9 AH.

The lights themselves will be 12 volt LED - two amber lights for rear turn signals, and two switchback lights

in the fender light housings for front white running lights and when the turn signal is activated, amber directional signals.

I would like to install an inline fuse between the battery, and the LED three prong flasher.

How do I determine what size fuse to use?

I have basic knowledge of electricity and can use a multimeter.

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You could hook up one light to your voltage source and put a DC ammeter in series with it to see the amount of current draw it has. Multiply by 4 to get the system draw. That should at least give you an idea on the minimum value the fuse has to be.

Terry

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I would base it on the size wire you are protecting and not worry much about the draw.

I would use 14ga wire, it is capable of handling 20 amps in a 12v system and I would protect it with a 15 amp fuse.

Edited by Rp1967 (see edit history)
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14 AWG wire is rated for 15 amps only, not 20. This LED system probably only draws a couple of amps if that. If it's not on the spec sheet, make a mock-up on the bench and measure the worst case current draw (four way flasher if equipped), which I predict to be about 3-4 amps. So a 5 amp fuse should suffice.

Just looked it up to be sure , 14 ga wire is good for 20 amps in a 12 v circut up to 15 feet long which wont be exceded in an automobile flasher circut.

You could also run 16 ga with a 10 amp or 18 ga with a 7.5 and so on.

The draw is irrelevant if you use wire large enough to handle it.

you are using a fuse to protect the wiring from shorting out and burning, you are not protecting the turn signal.

i would run 14ga just in the case that if I decided I didn't like the LEDs I could change to filaments later with no wiring change.

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  • 2 weeks later...

i would run 14ga just in the case that if I decided I didn't like the LEDs I could change to filaments later with no wiring change.

Great idea.

However, using a pair of wire strippers, it looks like the wire leads on the flasher, bulb sockets, and turn signal are actually 16 or 18 gauge. Is there a more accurate

way of measuring stranded wire? The wire can be stripped using 14, 16, or 18 gauge holes on the stripper, although the 18 gauge hole does end up cutting off

one copper strand.

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Hi Greg,

I build (upfit) police and fire equipment for a living, so fusing wiring is kinda close to home for me...

Go with a 7.5 or 10 amp fuse. As Rp1967 says 'You are protecting the wiring not the flasher or bulbs'. Another thing is that the short smaller wire on the flasher and sockets will have no (little) effect as the real thing is the resistance of the wire which goes up with length or smaller size of the wire, so a short piece of smaller wire has little effrect in this case. Now just to clarify before I get flamed... you have to be reasonable with wire size. You cannot use a 18 gauge wire, even a very short one, in a 100 amp circuit. It will burn out very quickly, that is how a FUSE works. Look at a fuse and you will see a very small wire inside the glass, plastic housing to keep the burning fuse element from starting a fire.

Now one thing you might want to consider is adding the turn signals to the existing 6 volt system. This is what I am going to do to the 37 Roadmaster when it gets out of the shop this spring. The Buicks have dual element tail lights (tail and brake) which are easy to add the wiring for turn signals and the front fender lights can either be used as is for turn signals or modified for dual element if you really want. I am leaving mine single element and just adding a turn signal switch kit and some wiring. Now I am having a wiring harness made to keep everything in a loom as I need new wiring for the restoration anyway, but adding a couple of hidden wires would be easy and look fine. Another thing, there are LED bulbs available for 6 volt systems so either way you might look into that to eliminate the extra battery and hastle of charging it...

Good luck and let us know how it turns out.

R

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Another thing, there are LED bulbs available for 6 volt systems so either way you might look into that to eliminate the extra battery and hastle of charging it..

R

I purchased 6 volt red LED dual contact "bulb" for the tail and brake lights - just plugged them in and they work.

Tried 6 volt white LED light replacement for the size 55 bulb in the map light - the incandescent bulb gave better light.

Have tried, and was told, that there are NO 6 volt Amber LEDs out there. That is what convinced me to use a 12 volt stand alone system.

Also there are no switchback 6 volt LEDs either.

The main reason for LEDs is visibility and safety. Too many people texting or talking on cell phones out there.

I don't want to alter the originality of the car permanently, either. Will be using small, rear facing amber lights mounted to the bumber

for rear turn signals.

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