Fuses with custom wiring

swooshdave · Jun 22, 2010

I have 6 circuits. I'm taking guesses at which blade fuses to use. Let me know if I'm ok.

*updated with more research*

Ignition - 5 amp (Boyer pulls 1.5amp max so I might stick with the 5amp, or maybe go to 3 amp fuse)
Headlight - 15 amp (My math had the headlight pulling 5amp with a 60w bulb so maybe 10 amp fuse)
Turnsignals (LED) - 1 amp (I think I'm safe here)
Horn - 15 amp (Fiamms pull 5 amps so I think I'll try a 10amp fuse)
Front Brake/Horn Button (with relay) - 5 amp (This circuit will light the rear brake light and trigger the horn relay, which I believe is negligible).
Rear Brake/Tail Light - 5 amp (Same as above)

I also need a fuse from the Podtronic to the battery. I think 30 amp here. (Going to drop this to 20 amp)

Coco · Jun 22, 2010

Not sure of the others you mentioned, but I'm running a 20A fuse between my regulator and battery, but that is the only fuse I have and I think you have a lot more going on than I do for electrics.

swooshdave · Jun 22, 2010

So I'm going to throw out some math. You have been warned.

Assuming 12v and an alternator that puts out at most 180watts:

180w/12v = 15amp

So I guess the biggest fuse I would need is 20amp for between the Podtronic and battery?

Headlight is at most 60w?

Does that mean that:

60w/12v = 5amp

Sound low for a headlight but I'm just guessing.

Am I on the right track for speccing out the fuses? Figure out the max amps and the get the next bigger fuse?

Nice thread here that helped me a little.
http://forum.miata.net/vb/archive/index ... 46372.html

And the horn draws 5 amps so I think a 10amp fuse will work?
http://www.fiammamerica.com/Product.asp?ProductID=155

Looks like the Boyer pulls a max of 1.5 amps.
http://www.boyerbransden.com/html/our_story.html

Trumpeteer · Jun 22, 2010

Would these be continuous or blow ratings?

swooshdave · Jun 22, 2010

Trumpeteer said:
Would these be continuous or blow ratings?

Sorry, blade (ATC/ATO) fuses. I meant to write that earlier.

Scooter62 · Jun 22, 2010

swooshdave

Although i'm more used to dealing with much higher voltage than 12v, the general theory of fuses is that they are used to protect the wiring and are sized to the wire rather than the draw. When a short occurs the full load of the battery or charging system is dumped to ground, if the wire is of sufficient gauge; no problem, but if the wire can not take the load heat and smoke and possibly fire give you the notification something ain't right. I'm not knowledgeable enough of low voltage wiring to tell you what the correct amperage for each gauge should be but I'm sure there is something on the internet that would give you that information.
Hope this helps,

Scooter
"Norton Commandos, an exercise in love and patience"

Jeandr · Jun 22, 2010

I posted the current ratings of the different wire sizes in a reply to this post wiring-diagrams-t4725.html?hilit=%20diagrams Fuses are sized to protect the wires and a fuse is designed to carry full rated current indefinitely, the time it takes to blow is proportional to the current, for example, a 20 amp fuse may carry 21 amps for many seconds, 22 amps for less time... and blow immediately with 40 amps.

Jean

swooshdave · Jun 22, 2010

Jeandr said:
I posted the current ratings of the different wire sizes in a reply to this post wiring-diagrams-t4725.html?hilit=%20diagrams Fuses are sized to protect the wires and a fuse is designed to carry full rated current indefinitely, the time it takes to blow is proportional to the current, for example, a 20 amp fuse may carry 21 amps for many seconds, 22 amps for less time... and blow immediately with 40 amps.

Jean

Jeandr said:
Here goes. Heavy wire, #14 is good for 17 amps, #16 for 13 amps, #18 10 amps or 204 watts, 156 watts and 120 watts respectively at 12 volts

This does not help me.

Jeandr · Jun 22, 2010

Just put in 15 amp fuses everywhere and don't worry about it, if there is a short, they will blow, and it is very unlikely that the wiring will go up in smoke due to an overloaded circuit unless you use the same wire to power the whole bike and it is small caliber wire (#18 or smaller gauge {larger number} )

Jean

Trumpeteer · Jun 23, 2010

Came across this which may be of some use

http://www.rbeelectronics.com/wtable.htm

swooshdave · Jun 23, 2010

Jeandr said:
Just put in 15 amp fuses everywhere and don't worry about it, if there is a short, they will blow, and it is very unlikely that the wiring will go up in smoke due to an overloaded circuit unless you use the same wire to power the whole bike and it is small caliber wire (#18 or smaller gauge {larger number} )

Jean

The thinnest wire is for the tail light, everything else is either the stock thickness or greater.

dazarooney · Mar 7, 2011

I had a similar question but this thread has helped me greatly, thanks.

grandpaul · Mar 7, 2011

I'd size the fuse to protect the device being served, or else why even have a fuse?

Certainly you'll protect the wire at the lower amperage fuse rating.

You could burn up every device and bulb on the bike before burning any wires or blowing any fuses.

bluto · Mar 7, 2011

grandpaul said:
I'd size the fuse to protect the device being served, or else why even have a fuse?

Certainly you'll protect the wire at the lower amperage fuse rating.

You could burn up every device and bulb on the bike before burning any wires or blowing any fuses.

In my experience when a regulator or zener diode fails nothing burns out right away and fuses don't blow (though bulb life will be greatly reduced) but the high voltage does boil the battery which will eventually ruin it and of course if acid boils out that can do plenty of damage. Fuses don't help here but a voltmeter or LED voltage monitor will indicate the trouble and FWIW leaving the headlight on w/ a stock Lucas alternator will probably keep the voltage down to a safe level until the zener or regulator is replaced.

Hobot · Mar 8, 2011

Maybe Norton fuses are like the oil levels in manual, a bit over the best level. A shade tree like me discovered the best fuses to use on Ms Peel's various circuits by starting too low and working up till held. I've had some wires melt and smoke on single 20A fuse and fire start on too big a fuse stuck in to get fast 'repair' done on the road. Next time I'll just use single 20A and relay lights and horn, as about any short that matters will blow fuse and ignition is so little current don't really need relay with decent wire. there is a story where red necks put .22 bullet to fix a fuse in PU truck and got shot in leg off the road : ) That would be a neat double take to use somewhere, with the powder removed.

I see plenty of those tiny plastic fuses but never the fuse holders for them.

swooshdave · Mar 8, 2011

hobot said:
there is a story where red necks put .22 bullet to fix a fuse in PU truck and got shot in leg off the road : )

http://kwc.org/mythbusters/2004/01/myth ... ompre.html

Busted. :mrgreen:

caddy chris · Mar 8, 2011

grandpaul said:
I'd size the fuse to protect the device being served, or else why even have a fuse?

Certainly you'll protect the wire at the lower amperage fuse rating.

You could burn up every device and bulb on the bike before burning any wires or blowing any fuses.

I respectfully disagree. Only devices wired in series with the short will be effected. Parallel circuits will not see the current generated by a short. Fuses protect wires. If a device pulls enough current to blow its fuse chances are its toast and the point is to clear the fault to protect the rest of the system and get you home or to the side of the road. That said, I only run a single 20amp and keep spares in my kit. So if I blow a fuse im screwed until I find the short. Whereas seperate fused systems allow you to isolate the short and move on or repair.

Fuses with custom wiring

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