and is this a serious mistake? what if you put in a higher amperage fuse in a fuse slot that requires a lower amperage? say a 30-amp fuse in a 20-amp slot? let's say it's for the radio. is the 10-amp difference considered significant enough to damage the radio or any other components not related to the blown fuse (can it affect say the headlights)? or does it take a very big differencem say 15 to 20-amp?

Over-fusing is a fire hazard. The circuit could be getting more juice than it can safely handle, overheats, starts fire. It is the fire that could hurt your radio, and many other parts.

No, never. This could be a very damaging and costly mistake. Like Dean said, it can cause a fire. What reason would you need to put in a higher amp fuse? Does the required fuse keep blowing? If so, there is a problem with that circuit and installing a higher amp fuse could destroy your electrical wiring on that circiut and even start a blaze. This is a very big "no-no".

That is why everyone is warned not to install aluminum foil in place of a fuse. This is a recipe for a roaring blaze.

Fuse size does not dictate how much current a radio or any other device uses, but it limits the current that can flow in a circuit.

A 20A fuse will 'blow' (actually a metal filament or strip melts when it gets too hot from too much current) if more than 20 amperes flow in the circuit, and generally a 20A fuse will be used to protect a circuit that draws less than 20 amperes, say 15-16 amperes.

The wire is sized for acceptable voltage drop with this current. If the circuit draws more current, there is more more voltage loss and more heating of the wire. In extreme cases, say a 50A fuse and 10 ampere wire, it could get hot enough to start a fire and the fuse would not blow.

If nothing goes wrong in the circuit, a 20A, 30A, or 50A fuse won't matter. If something does go wrong, the larger fuse could allow dangerous overheating, and as pointed out, could allow a fire to start.

If you have a choice between being stranded, or using a larger fuse temporarily to get home, use the larger fuse, then go back to the correct size as soon as you can.

Oh, and fuses only 'blow' when too much current causes the strip to melt. If a fuse has blown, there has been an overload. Replacing the fuse may mean the new fuse will also blow, unless the overload has been removed.

M-B fuses can also corrode, usually the contact ends, so they don't pass current either, but that's a different story.


Best Regards,
Jim

thanks for all the info, guys! I think I understand the issue and consequences now. say that a 30-amp fuse was placed in a slot requiring a 20-amp fuse. and say that fuse blew. what is the possibility that it could fry wires/electric components and/or cause a fire because too much current passed through before the fuse could break in time? are these "risks" a high possibility and one or both scenarios are likely to result. or when you guys talk about the possibility of fires and damaged components, you guys mean that's what could happen but not very often except under extreme conditions. thanks again.

If it keeps blowing a fuse, it is very possible that it could eventually result in a blaze. Jus wondering- do you have a fuse that keeps blowing out? What kind of car do you own?

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One thing that is being overlooked here is time. If there is a dead short, the fuse blows instantly because the instantaneous current is way over 30 amps. It might even be 500 amps for a fraction of a second. This cannot start a fire and it cannot damage any wires.

Now, if the circuit is overloaded with devices (you added a stereo and some additional wattage on the headlights, etc.) and the current draw is 28 amps or so, then you are looking at some problems. The wires in the circuit are designed for a maximum of 20 amps and you are now running 28 amps. This will eventually overheat the wires and/or connections, but, it won't happen instantly. It might take five or ten minutes for the temperature to climb up to dangerous levels.

In most cases, the larger fuse will not cause any problems, although it is not adviseable to keep it there.

There is a "Darwin" legend about a guy who went hunting and upon returning to his pickup truck that night and starting, that the headlight circuit blew a fuse.

Not having a spare, he opted for a 22-caliber bullet, which apparently fit the fuse slot.

The overheated circuit eventually got hot enough to discharge the bullet, the trajectory of which was directed to his groin!

Actually, they tried this in "Mythbusters" and they got the same results!

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"Are you thinking, what would happen if you do such a thing by a MISTAKE or are you thinking of placing some load on existing wire?"

tkamiya, I'm thinking about what would happen if one did that by mistake. thanks for the info, and brian brought up an interesting point about the risk/time factor.

another reason for asking is that looking through the archives I hear some people putting in a higher-amp fuse for a fuse slot that requires a fuse with a lower amperage either diagnose a problem or just that the lower-amp fuse was insufficient.