(Car Audio) Trying to figure out what fuse to pair with Amplifier

Bassblaster

Super Member
Ive heard we have a car audio forum for subscribers, but since im not a subscriber i'll have to post it here.
Anywho i got a new fuse holder for the subwoofer in my truck and im trying to figure out what value to put it. The amp is a mono 100W RMS at 4 ohm. Class AB. im thinking either a 20A or 25A would be perfect. but im not 100% sure
if it matters i'm running 10 Gauge OFC for 12V and GND
 
100 watts at 12 volts is about 8.5 amps of current. If the amplifier has 3 dB of head room that would be about 17 amps.

A 20 amp fuse would likely be okay, but without knowing the amount of head room for the amplifier and how hard you are going to drive it, it might need the 25 amp fuse.

It will also depend on the load on the amplifier. Will you be using a single 4 Ohm sub woofer? If you were to use 4 Ohm sub woofers in parallel that would drop the load to 2 Ohms and double the current needed.

Even a 50 amp fuse could be okay, although the maximum surge current during a short will be higher.
 
What ... no book with the amp? I'd go with what the manufacturer recommends. That should account for a wide impedance range and provide all the headroom protection you need.
 
Generally you fuse to keep the wire safe and wire is sized appropriately for the load and length to keep voltage drop in check.

25-30A is what I would use for 10ga.
 
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Thanks for the replies,
It will also depend on the load on the amplifier. Will you be using a single 4 Ohm sub woofer? If you were to use 4 Ohm sub woofers in parallel that would drop the load to 2 Ohms and double the current needed.
Yes, but this is a all in one deal with a single 10". so no way to add more drivers.

that would be about 17 amps
Is that including the efficiency loss with a class AB amp?

What ... no book with the amp?
Nope, friend of mine found it in his garage and asked me if i wanted it. it also doesnt have a fuse built into it with any markings.
 
i would use an ammeter to see max current then select a fuse at that figure . or maybe a little less depending on the peaks of current .
am guessing a slow blow fuse will be used so it will stand high current for an x amount of time and survive .
 
The correct fuse size is important for safety.

As an example, if there was a direct short and a connection has developed a resistance of 0.5 Ohms, not totally implausible. The amount of current would be about 28 amps and the power dissipated in the bad connection would be close to 400 watts. A 50 amp fuse would never go open.

This is enough power to cause heat, smoke and possibility fire. At times it is not just about wire gauge, which of course should be up to the task. This is why it can be important not to just throw a big fuse in the circuit. And keep all connections correct, clean and tight.

Is that including the efficiency loss with a class AB amp?

No it is not and it does not include power supply inefficiencies. Some times my meds make me a little fuzzy minded.

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The correct fuse size is important for safety.

As an example, if there was a direct short and a connection has developed a resistance of 0.5 Ohms, not totally implausible. The amount of current would be about 28 amps and the power dissipated in the bad connection would be close to 400 watts. A 50 amp fuse would never go open.

This is enough power to cause heat, smoke and possibility fire. At times it is not just about wire gauge, which of course should be up to the task.

Indeed. OCP (not OCD ;) ) is primarily about wire gauge/protection of the downstream wiring. The point is the OCP should not, generally, exceed the rating of the wire for the reason you mentioned.

In this example, 10ga wire is a reasonable size for a 100W car amp so long as the feed line isn't too long. And, a 30A fuse is appropriate to protect 10ga wire.

Of course, it isn't necessary to fuse a 10ga wire at 30A if the load is, for example, only 5 or 10A but then at that load there isn't really a need for 10ga unless it's a very long run.
 
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Thanks for the replies,

Yes, but this is a all in one deal with a single 10". so no way to add more drivers.


Is that including the efficiency loss with a class AB amp?


Nope, friend of mine found it in his garage and asked me if i wanted it. it also doesnt have a fuse built into it with any markings.

Forgive if you know, but I don't know what you know, so...


be sure your fuse is at the start of the wire run, where it connects to the power source. A fuse at the amp end is OK too if there is not one built in, but at the very beginning of the run is paramount in case a short develops somewhere downstream.

If it were mine, I would be putting a 30A fuse at the entry to that 10ga and if no fuse in the amp (maybe inside?) I'd be thinking a 20 or 25A at the amp end of the 10ga.
 
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fuse at source of current protects fire from the supply wire . fuse at other end of wire protects fires at other end in the unit , simple really . slow blow or time delay or motor rated fuses are basically same thing and will do over current for short term in normal use . in the case of a sudden dead short it wont last very long . not all faults are dead shorts and cause heat leading to fires . so as i see it a fuse should be working close to its limits . changing worn out fuses is better than fires in my book .
 
My point is that it is not necessary to fuse to the maximum rating of the wire if the load is smaller. Of course as mentioned it is not a good idea to use fuses larger than the capacity of the gauge of the wire used.

And of course I am applying this to situations where the load is fixed for a fixed, known device.

Fusing is like other safety measures, one should fuse for the worst case situation, because we can't predict when this might happen.
 
Well, maybe we can put a fork in it.

I used to play with car stereo a lot and still have most of my old Alpine amps. One of them is of similar power as the OPs.

The amp has a 25A fuse in it, and the manual recommends a 10ga supply wire fed via a 30A fuse.

My suggestions earlier in the thread were not "what if, cover my ass stuff". It's based on real use case.
 
I do not really want to get into a discussion of past experience, but I owned an electronics shop and we did a lot of professional installs in emergency vehicles, lights, sirens, terminals, two way radios and so on. And the specifications were very tight.

I do not believe that fusing for a worst case situation is CYA.
 
Hard to believe there is no fuse on the amp. I would use a small one. Car amps are notoriously overrated, I would use a 5 or 10 amp and then test at volume to see what happens.

If you use a smaller fuse than required, the worst thing that could happen is you blow it and replace it.
 
Okay lots of responses, i do have a 40A fuse at the battery, but thats just because thats what came with the kit. i would like to add a smaller more appropriate value at the amp. i had the amp pulled from its enclosure once and saw no fuse. im also fully aware how LOTSSS of cheaper car amps way over rate there gear cough cough boss audio cough. when i get new amps i always look for CEA-2006 ratings. But im pretty sure the one i got here is a real 100W. so when its not 15 degrees outside ill try the 20A. if it pops, ill use the 25A.

Here's a pic of the actual amp i took when i was looking for clearly burnt or blown parts, and no i'm not getting that "real 100W" just from the sticker on the PCB :p
IMG_1133.JPG
 
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