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Fuse sizes questions


DarkLeftArm

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DarkLeftArm

Hey everybody-

Getting pretty antsy about my trip to Ireland and my first meeting with my 02 RT. You all gave me a lot of great ideas for hooking my GPS. Today's question is about spare fuses. I added my spares to my repair kit, but today was looking them over again more closely, and started wondering what exactly I should have. What I currently have is a motley collection of 20, 25, 30 amps etcetera. I'm guessing they're mostly WAY too high. And honestly I'm hoping to never need to find out. But if I dont take some, I'll need them for sure! lol

 

Thank you for your expertise and time.

Cheers

Bert

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Hey everybody-

Getting pretty antsy about my trip to Ireland and my first meeting with my 02 RT. You all gave me a lot of great ideas for hooking my GPS. Today's question is about spare fuses. I added my spares to my repair kit, but today was looking them over again more closely, and started wondering what exactly I should have. What I currently have is a motley collection of 20, 25, 30 amps etcetera. I'm guessing they're mostly WAY too high. And honestly I'm hoping to never need to find out. But if I dont take some, I'll need them for sure! lol

 

Thank you for your expertise and time.

Cheers

Bert

 

Afternoon Bert

 

I used to carry 4 amp, 7.5 amp, 10 amp 15 amp & 20 amp but you can get by with just 10 & 15, as you can use the 10 in the lower slots. The bike doesn't use a 20 amp fuse but it is nice to have a few 20's in case you need one with a little higher rating.

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DarkLeftArm

Thanks for the rundown. I'll stop at a parts store.

The thought of using heavier than called for fuses gives me the willies. Brings back the days of gum wrappers and the smell of smoldering wiring!

Yep, I'm pretty old.

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I never like putting larger fuses in when one blows. If it blows it blows for a reason. If it has been running for years with whatever size fuse and then it blows, something is wrong if that same size fuse won't work anymore. Better that the fuse blow then the wire melts.

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Thanks for the rundown. I'll stop at a parts store.

The thought of using heavier than called for fuses gives me the willies. Brings back the days of gum wrappers and the smell of smoldering wiring!

Yep, I'm pretty old.

 

Evening Bert

 

Yes, you don't want to overdo the higher amp fuse thing but sometimes a slightly higher amperage fuse will allow you to continue a trip. Most BMW circuits are slightly under-fused for wire size & length.

 

You & your passenger could have a lot of heated gear on high & running on the 15 amp accessory outlet circuit at 14.5 amps with the battery at 13 volts. You stall the bike & hit the starter button & pull the battery voltage down to 10 volts so your heated gear draw just went over 18 amps (pop).

 

How big is the fuse between the ignition switch & the battery??

 

How big is the fuse between the ABS controller & the alternator??

 

How big is the fuse between the alternator & the battery??

_________________________

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Heh heh heh.

I wish I knew the answers to those questions. Thus the thread! lol

 

Evening Bert

 

Those are trick questions as BMW doesn't fuse those high amp circuits. Get a short & you get smoke.

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Heh heh heh.

I wish I knew the answers to those questions. Thus the thread! lol

 

F1 - 4A Instruments

F2 - 4A Tail, Side lights

F3 - 15A Rider Info

F4 - 7.5A Horn

F5 - 10A Motronic

F6 - 10A Fuel Pump

F7 - 4A Heated Grips

 

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How can the current go up when the voltage goes down on heated clothing?

 

Amps = Watts/Volts

 

100w/13v = 7.7A

100w/10v = 10A

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Power consumption is a result, not a constant, fixed value.

 

Re-stated, power is indeed Volts (aka potential) times amps (P=V*I). But changing the voltage applied to a fixed resistance also changes the current.

 

Ohms Law: Voltage equals current times resistance (V=R*I).

 

In this example, the circuit has a resistance of R=V/I= 13v/7.7a= 1.69 ohms.

 

Lowering the applied voltage to 10v reduces the current to 5.9a (10v/1.69ohm) so the power output is also reduced to 59 watts.

 

Manipulate the formulas a little and you see that power is the result of voltage squared divided by the resistance.

 

 

 

 

 

 

Edited by lkraus
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Just as stated above the wattage is not a constant. In this case the resistance is. The rated wattage of a pure resistance device such as heated clothing is at the rated voltage. Decrease the voltage and the current will also decrease.

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Wow, I never thought I would have occassion to make a statement contrary to DR.

 

But, think of the heater element(s) as a fixed resistance, so increased voltage results in increased current, and decreased voltage voltage results in decreased current.

 

Now, with electric motors, there the mechanical load (horse power, watts) is the constant, so decreased voltage results in increased amps (well, untill the motor stalls out).

 

Back to discussing fuses: SOmetimes fuses fail from fatique, so replace with same size. If a continues to blow from time to time, maybe try a slightly larger size, but should not need to unless have added additional devices to the circuit.

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Gravelpounder

I agree with Redman, fuses do fail from Fatique, I had this happen on a 75 model 750 Honda (way back when I was young), bike just lost power riding thru the central Texas hills. I pulled the main fuse, It looked ok until I held it up to the sky and could just detect that it has turned to ash right in the middle. Took a piece of foil gum wrapper and wrapped the fuse (I said it was way back when...) to get home.

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