# Fuse rating=Output power



## Jonathan (Oct 6, 2011)

I often read the "stupidest thing someone said in audio" thread, and to be honest I Think there are some people there that claim to know better, but havent really understood what they are talking about. Especially when it comes to amps. 

Fuses, are they magical devices that will blow at exactly their rating?
Can a 20A fuse only support 288W with 14.4V. No, not even Close.

There are at least two reasons why this is so.

1.
Fuses are rated with a I2t. So if you have high current but during a short time they wont blow. If you go over their current rating the time the fuse will survive is shortened. But that is it.

I Think the spec for a normal 20A fuse says it will normally handle 27A for a long time and 32A for a couple of minutes. Burping 40A for a couple of seconds should be ok. To make sure the fuse can handle 20A it has to have some margin for production variations and so on. The production variation of fuses are pretty bad, so if you get a bad 20A fuse it will be weak like a 10A fuse, but get a good one and it is like a 30A fuse.

I have tested fuses in amps quite a lot. One amp I tested a lot had 4kW output with a 175A fuse. This was when I tested the thermal protection circuitry. My lab supply didnt manage more than 14V, and I had 380A going in to the amp. This was a 2.7kW amp driven in to deep clipping. The thermal protection shut the amp down before the fuse had any problem. And that took 10-15 seconds, longer than necessary for burping. 

2.
The signal. Most of us listen to music, not sines. For music with a normal Crest factor (6dB), the average of the music is 6dB less than full Power when playing at onset of clipping. So an average 1000Wrms amp at the onset of clipping will have an average output of 250W with normal music. With 80% efficiency that will need 312W to do so. At 14.4V that is 22A. A 20A fuse should handle 22A for a long time as long as you play music with a normal Crest factor. 

All Power numbers above are RMS Power measured with an Audio precision.

So yes, fuse size matters, a 3000W Boss amp with a 20A fuse is not going to produce Power. We all know this from practical experience. But lecturing someone about amps and fuses can get a bit geeky, get your facts right if you dont want to embarrass yourself.


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## Victor_inox (Apr 27, 2012)

You think?


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## cajunner (Apr 13, 2007)

this information is valuable and I cannot stress enough, based on sounding like a broken record.

people that use fuse rating to judge alternator overload and capacitor efficacy are also far from "science" as well as those "BIG 3" proponents who insist, all the time...

can't be sliced thin enough, with a casual look-see and adding up fuse values.

I don't go on fadbook but I see enough of it here, supposedly a car audio (matured) board, misconceptions and posturing in place of good practices.


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## Sine Swept (Sep 3, 2010)

I always think of my 2150am bridged power rating of 600RMS. The amp comes stocked with 35 amps of fusing.


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## kyheng (Jan 31, 2007)

The amp's fuse are for short circuit protection. So under normal operation, it will be taking much lower than what is rated, maybe about 20-30% the most.


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## Beatbox (Jun 7, 2015)

Okay, I see

I should be able to smaller fuses in my amp than what it came with........

It came with 3- 30 amp fuses, but really only needs 3- good 10 amp fuses


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## Jepalan (Jun 27, 2013)

Beatbox said:


> Okay, I see
> 
> I should be able to smaller fuses in my amp than what it came with........
> 
> It came with 3- 30 amp fuses, but really only needs 3- good 10 amp fuses


Why would you want to do this? 
Just use the fuse that the manufacturer recommends. Simple.


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## TrickyRicky (Apr 5, 2009)

Jepalan said:


> Why would you want to do this?
> Just use the fuse that the manufacturer recommends. Simple.


Let him spend his money on fuses, as he will be replacing them every time he turns on the radio:laugh: If an amplifier has 90amp fusing then replacing that with 30amp is not a good idea as it will just pop them open with every dynamic note.

If for some reason the 30amp fuse don't pop/open then your 900watt amp is clearly a 300watt amplifier.


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## Victor_inox (Apr 27, 2012)

with 1/3 of fusing it might pop open on startup.


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## Kriszilla (Jul 1, 2013)

I think that OP needs to remember that if there's a short his amp is going to likely get fried no matter the fuse rating. The fuses in the amps really aren't there to protect the amps as much as they are there to protect the power wire.


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## Kriszilla (Jul 1, 2013)

Victor_inox said:


> with 1/3 of fusing it might pop open on startup.


Agreed. We should place bets.


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## Jepalan (Jun 27, 2013)

Kriszilla said:


> I think that OP needs to remember that if there's a short his amp is going to likely get fried no matter the fuse rating. The fuses in the amps really aren't there to protect the amps as much as they are there to protect the power wire.


No. 

The in-line fuse that is 18" from the battery is there to protect the power wire from catching fire in case it shorts to ground somewhere.

The fuses in the amp itself are basically there to prevent the amp from catching fire.


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## Victor_inox (Apr 27, 2012)

Kriszilla said:


> I think that OP needs to remember that if there's a short his amp is going to likely get fried no matter the fuse rating. The fuses in the amps really aren't there to protect the amps as much as they are there to protect the power wire.


Agreed, shorted output transistor will blow fuse, replacing fuse will not unshorten that transistor. it`s pointless.


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## Kriszilla (Jul 1, 2013)

Jepalan said:


> No.
> 
> The in-line fuse that is 18" from the battery is there to protect the power wire from catching fire in case it shorts to ground somewhere.
> 
> The fuses in the amp itself are basically there to prevent the amp from catching fire.


You're assuming that even backyard hack installers bother with fusing at the battery. The manufacturers don't take chances like that. All it would take is one yahoo to burn his car to the ground, blame it on the amp manufacturer, and drag them into court for months on end. 

It's cheaper to put the fuses on the amp as well. And, yes, I agree, that it will definitely help with keeping the amp from releasing its magic smoke.


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## Jepalan (Jun 27, 2013)

Kriszilla said:


> You're assuming that even backyard hack installers bother with fusing at the battery. The manufacturers don't take chances like that. All it would take is one yahoo to burn his car to the ground, blame it on the amp manufacturer, and drag them into court for months on end.
> 
> It's cheaper to put the fuses on the amp as well. And, yes, I agree, that it will definitely help with keeping the amp from releasing its magic smoke.


I guess I have better faith in my fellow hobbyists. Regardless, this prior statement is still wrong:


> "The fuses in the amps really aren't there to protect the amps as much as they are there to protect the power wire.


If the power wire shorts out anywhere between the battery and the input to the amp, the amp fuse will not blow and the power wire will melt and possibly flame. The amp fuse will only blow if something inside the amp or on the amp outputs creates an overload. Not the power wire. This was the only point of my prior post. 

FWIW - JL Audio amps are not fused.


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## cajunner (Apr 13, 2007)

the only fuses I know of that protect an amp, are those on the outputs. If a transistor in a bank of Darlington arrays goes thermal then shorts, the resultant surge of current that pops a fuse may indeed, save the amp from a full meltdown situation where the board gets so hot it becomes carbonized and is beyond salvage or repair.

Thinking that it will save the rest of the transistors in that amp's bank, is wishful thinking because any time a transistor blows, the only real correct way to repair is to replace all the transistors in the affected bank.


And, hopefully, matched so they operate efficiently, I think?


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## Jepalan (Jun 27, 2013)

cajunner said:


> the only fuses I know of that protect an amp, are those on the outputs. If a transistor in a bank of Darlington arrays goes thermal then shorts, the resultant surge of current that pops a fuse may indeed, save the amp from a full meltdown situation where the board gets so hot it becomes carbonized and is beyond salvage or repair.
> 
> Thinking that it will save the rest of the transistors in that amp's bank, is wishful thinking because any time a transistor blows, the only real correct way to repair is to replace all the transistors in the affected bank.
> 
> ...


Agreed. I think we were basically talking around one another and saying the same thing earlier in the thread.

My perception is:

1) Fuses on speaker outputs (does anyone still do this?) would protect the amp from overload in case of a VC or speaker wiring failure.

2) Fuses on the power input to the amp likely won't protect the amp, but they can prevent fire by preventing a failed amp from drawing too much current in the case of failure. The manufacturer puts these fuses in mostly for liability reasons. And there are some cases where an input power fuse will pop before the amp is damaged if the amp is overloaded due to VC or speaker wire failure.

3) An in-line fuse close to battery or distro block prevents possible car fires in the case that the power cable fails/shorts to ground.

I think Kris and I were just splitting hairs about #2. No worries though, it was just me being pedantic again.


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## Kriszilla (Jul 1, 2013)

Jepalan said:


> I think Kris and I were just splitting hairs about #2. No worries though, it was just me being pedantic again.


Yeah, we are splitting hairs. To be honest, you have raised some very valid points, so I'll definitely give credit where due, hombre.


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